The stability of human acidic beta-crystallin oligomers and hetero-oligomers

Cholesterol and Q147E Deamidation Modulates αA-Crystallin Membrane Binding Elucidating Protective Role of Lens Membrane Composition Changes With Aging

Purpose

The αA-Crystallin (αAc) binding with lens membranes increases with age and cataract formation. However, the role of lipids and cholesterol (Chol) in Q147E-αAc membrane binding remains unclear, which we aim to elucidate in this study.

Methods

We have used the electron paramagnetic resonance spin-labeling method to probe the Chol/ 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and Chol/ sphingomyelin (SM) membranes binding with wild-type (WT) and Q147E-αAc.

Results

Compared to WT-αAc, the Q147E mutant had increased binding to POPC and decreased binding to SM membranes without Chol. Adding 33 mol% Chol to the POPC and SM membranes decreased the binding of WT and, to a lesser degree, decreased the binding of Q147E-αAc to the membranes. Adding 60 mol% Chol completely inhibited Q147E mutant and WT binding to POPC membranes. However, 33 and 60 mol% Chol completely inhibited WT and Q147E mutant binding to SM membranes, respectively. WT and Q147E-αAc membrane binding decreased membrane mobility while increasing order and hydrophobicity near the headgroup.

Conclusions

In Chol-free membranes, the deamidated Q147E-αAc binds significantly more to the POPC membranes compared to WT, whereas WT binds significantly more to the SM membranes compared to Q147E-αAc. In contrast, for 33 mol% Chol-containing membranes, the deamidated Q147E-αAc binds significantly more to POPC and SM membranes than WT. Conversely, 60 mol% Chol-containing membranes completely inhibit WT and deamidated Q147E-αAc binding to POPC and SM membranes. These results suggest that increased Chol content of the lens membranes during aging protects against accumulation of modified proteins on the membrane associated with cataracts.

Measurement of absolute abundance of crystallins in human and αA N101D transgenic mouse lenses using 15N-labeled crystallin standards

Stable isotope labeled standards of all major human lens crystallins were created to measure the abundance of lens endogenous crystallins from birth to adulthood. All major human crystallins (αA, αB, βA2, βA3/A1, βA4, βB1, βB2, βB3, γA, γB, γC, γD, γS) were cloned with N-terminal 6 x His tagged SUMO for ease of purification and the ability to generate natural N-termini by SUMO protease cleavage when producing crystallins for structure/function studies. They were then expressed in 15N-enriched media, quantified by mass spectrometry, and mixed in proportions found in young human lens to act as an artificial lens standard. The absolute quantification method was tested using soluble protein from 5-day, 23-day, 18-month, and 18-year-old human lenses spiked with the 15N artificial lens standard. Proteins were trypsinized, relative ratios of light and heavy labeled peptides determined using high-resolution precursor and data independent MS2 scans, and data analysis performed using Skyline software. Crystallin abundances were measured in both human donor lenses and in transgenic mouse αA N101D cataract lenses. Technical replicates of human crystallin abundance measurements were performed with average coefficients of variation of approximately 2% across all 13 crystallins. αA crystallin comprised 27% of the soluble protein of 5-day-old lens and decreased to 16% by 18-years of age. Over this time period αB increased from 6% to 9% and the αA/αB ratio decreased from 4.5/1 to 2/1. γS-crystallin also increased nearly 2-fold from 7% to 12%, becoming the 3rd most abundant protein in adult lens, while βB1 increased from 14% to 20%, becoming the most abundant crystallin of adult lens. Minor crystallins βA2, βB3, and γA comprised only about 1% each of the newborn lens soluble protein, and their abundance dropped precipitously by adulthood. While 9 of the SUMO tagged crystallins were useful for purification of crystallins for structural studies, γA, γB, γC, and γD were resistant to cleavage by SUMO protease. The abundance of WT and N101D human αA in transgenic mouse lenses was approximately 40-fold lower than endogenous mouse αA, but the deamidation mimic human αA N101D was less soluble than human WT αA. The high content of αA and the transient abundance of βA2, βB3, and γA in young lens suggest these crystallins play a role in early lens development and growth. βB1 becoming the most abundant crystallin may result from its role in promoting higher order β-crystallin oligomerization in mature lens. The full set of human crystallin expression vectors in the Addgene repository should be a useful resource for future crystallin studies. 15N labeling of these crystallins will be useful to accurately quantify crystallins in lens anatomic regions, as well as measure the composition of insoluble light scattering crystallin aggregates. The standards will also be useful to measure the abundance of crystallins expressed in transgenic animal models.

The Functional Significance of High Cysteine Content in Eye Lens γ-Crystallins

Cataract disease is strongly associated with progressively accumulating oxidative damage to the extremely long-lived crystallin proteins of the lens. Cysteine oxidation affects crystallin folding, interactions, and light-scattering aggregation especially strongly due to the formation of disulfide bridges. Minimizing crystallin aggregation is crucial for lifelong lens transparency, so one might expect the ubiquitous lens crystallin superfamilies (α and βγ) to contain little cysteine. Yet, the Cys content of γ-crystallins is well above the average for human proteins. We review literature relevant to this longstanding puzzle and take advantage of expanding genomic databases and improved machine learning tools for protein structure prediction to investigate it further. We observe remarkably low Cys conservation in the βγ-crystallin superfamily; however, in γ-crystallin, the spatial positioning of Cys residues is clearly fine-tuned by evolution. We propose that the requirements of long-term lens transparency and high lens optical power impose competing evolutionary pressures on lens βγ-crystallins, leading to distinct adaptations: high Cys content in γ-crystallins but low in βB-crystallins. Aquatic species need more powerful lenses than terrestrial ones, which explains the high methionine content of many fish γ- (and even β-) crystallins. Finally, we discuss synergies between sulfur-containing and aromatic residues in crystallins and suggest future experimental directions.

Eye lens β-crystallins are predicted by native ion mobility-mass spectrometry and computations to form compact higher-ordered heterooligomers

Eye lens α- and β-/γ-crystallin proteins are not replaced after fiber cell denucleation and maintain lens transparency and refractive properties. The exceptionally high (∼400-500 mg/mL) concentration of crystallins in mature lens tissue and multiple other factors impede precise characterization of β-crystallin interactions, oligomer composition, size, and topology. Native ion mobility-mass spectrometry is used here to probe β-crystallin association and provide insight into homo- and heterooligomerization kinetics for these proteins. These experiments include separation and characterization of higher-order β-crystallin oligomers and illustrate the unique advantages of native IM-MS. Recombinantly expressed βB1, βB2, and βA3 isoforms are found to have different homodimerization propensities, and only βA3 forms larger homooligomers. Heterodimerization of βB2 with βA3 occurs ∼3 times as fast as that of βB1 with βA3, and βB1 and βB2 heterodimerize less readily. Ion mobility experiments, molecular dynamics simulations, and PISA analysis together reveal that observed oligomers are consistent with predominantly compact, ring-like topologies.

Cataract-causing variant Q70P damages structural stability of βB1-crystallin and increases its tendency to form insoluble aggregates

Congenital cataract is the primary cause of childhood blindness worldwide. As the predominant structural protein, βB1-crystallin plays an important role in maintaining lens transparency and cellular homeostasis. Numerous cataract-causing mutations of βB1-crystallin have been identified with unclear pathogenic mechanism. We previously identified the mutation Q70P (Q to P at residue position 70) of βB1-crystallin linked to congenital cataract in a Chinese family. In this work, we investigated the potential molecular mechanism of βB1-Q70P in the congenital cataract at the molecular, protein, and cellular levels. We purified recombinant βB1 wild-type (WT) and Q70P proteins and compared their structural characteristics and biophysical properties by spectroscopic experiments under physiological temperature and environmental stresses (ultraviolet irradiation, heat stress, oxidative stress). Notably, βB1-Q70P significantly changed the structures of βB1-crystallin and exhibited lower solubility at physiological temperature. Meanwhile, βB1-Q70P was prone to aggregation in eukaryotic and prokaryotic cells, and was more sensitive to environmental stresses, along with impaired cellular viability. Furthermore, the molecular dynamics simulation indicated that the mutation Q70P damaged secondary structures and hydrogen bond network of βB1-crystallin, which were essential for the first Greek-key motif. This study delineated the pathological mechanism of βB1-Q70P and provided novel insights into treatment and prevention strategies for cataract-associated βB1 mutations.

Insight into Pathogenic Mechanism Underlying the Hereditary Cataract Caused by βB2-G149V Mutation

Congenital cataracts account for approximately 5-20% of childhood blindness worldwide and 22-30% of childhood blindness in developing countries. Genetic disorders are the primary cause of congenital cataracts. In this work, we investigated the underlying molecular mechanism of G149V point missense mutation in βB2-crystallin, which was first identified in a three-generation Chinese family with two affected members diagnosed with congenital cataracts. Spectroscopic experiments were performed to determine the structural differences between the wild type (WT) and the G149V mutant of βB2-crystallin. The results showed that the G149V mutation significantly changed the secondary and tertiary structure of βB2-crystallin. The polarity of the tryptophan microenvironment and the hydrophobicity of the mutant protein increased. The G149V mutation made the protein structure loose and the interaction between oligomers was reduced, which decreased the stability of the protein. Furthermore, we compared βB2-crystallin WT and the G149V mutant with their biophysical properties under environmental stress. We found that the G149V mutation makes βB2-crystallin more sensitive to environmental stresses (oxidative stress, UV irradiation, and heat shock) and more likely to aggregate and form precipitation. These features might be important to the pathogenesis of βB2-crystallin G149V mutant related to congenital cataracts.

A novel missense variant c.71G > T (p.Gly24Val) of the CRYBA4 gene contributes to autosomal-dominant congenital cataract in a Chinese family

PURPOSE

To investigate the potential genetic defects in a five-generation Chinese family with autosomal dominant congenital cataract (ADCC).

METHODS

Whole exome sequencing was performed to search the variants in the candidate genes associated with congenital cataract. Sanger sequencing was used to validate the variants and examine their co-segregation in the patients and their relatives. The potential effect of the variants was analyzed using several bioinformatic methods and further examined through Western blotting and co-immunoprecipitation.

RESULTS

A missense variant c. 71 G > T (p. Gly24Val) in the CRYBA4 gene, a known ADCC candidate gene, was identified to be heterozygously present in the patients and co-segregate with cataract in the family. The mutation was absent in all of the searched databases, including our in-house exome sequences of 10,000 Chinese. The alignments of the amino acid sequences of CRYBA4 in a variety of species revealed that the amino acid residue Gly24 was evolutionarily highly conserved, and the in silico analysis predicted that the missense mutation of Gly24Val was damaging for the protein structure and function of CRYBA4. Then, the in vitro expression analysis further revealed that the Gly24Val mutation in CRYBA4 inhibited its binding with CRYBB1. The impaired interaction of β-crystallin proteins may affect their water-solubility and contribute to the formation of precipitates in lens fiber cells.

CONCLUSION

We identified a novel missense variant in the CRYBA4 gene as a pathogenic mutation of ADCC in a Chinese family. Our finding expanded the CRYBA4 variation spectrum associated with congenital cataracts.

Zinc and Copper Ions Induce Aggregation of Human β-Crystallins

Cataracts are defined as the clouding of the lens due to the formation of insoluble protein aggregates. Metal ions exposure has been recognized as a risk factor in the cataract formation process. The γ and β crystallins are members of a larger family and share several structural features. Several studies have shown that copper and zinc ions induce the formation of γ-crystallins aggregates. However, the interaction of metal ions with β-crystallins, some of the most abundant crystallins in the lens, has not been explored until now. Here, we evaluate the effect of Cu(II) and Zn(II) ions on the aggregation of HβA1, as a representative of the acidic form, and HβB2, as a representative of the basic β-crystallins. We used several biophysical techniques and computational methods to show that Cu(II) and Zn(II) induce aggregation following different pathways. Both metal ions destabilize the proteins and impact protein folding. Copper induced a small conformational change in HβA1, leading to high-molecular-weight light-scattering aggregates, while zinc is more aggressive towards HβB2 and induces a larger conformational change. Our work provides information on the mechanisms of metal-induced aggregation of β-crystallins.

Human γS-Crystallin Resists Unfolding Despite Extensive Chemical Modification from Exposure to Ionizing Radiation

Ionizing radiation has dramatic effects on living organisms, causing damage to proteins, DNA, and other cellular components. γ radiation produces reactive oxygen species (ROS) that damage biological macromolecules. Protein modification due to interactions with hydroxyl radical is one of the most common deleterious effects of radiation. The human eye lens is particularly vulnerable to the effects of ionizing radiation, as it is metabolically inactive and its proteins are not recycled after early development. Therefore, radiation damage accumulates and eventually can lead to cataract formation. Here we explore the impact of γ radiation on a long-lived structural protein. We exposed the human eye lens protein γS-crystallin (HγS) to high doses of γ radiation and investigated the chemical and structural effects. HγS accumulated many post-translational modifications (PTMs), appearing to gain significant oxidative damage. Biochemical assays suggested that cysteines were affected, with the concentration of free thiol reduced with increasing γ radiation exposure. SDS-PAGE analysis showed that irradiated samples form protein-protein cross-links, including nondisulfide covalent bonds. Tandem mass spectrometry on proteolytic digests of irradiated samples revealed that lysine, methionine, tryptophan, leucine, and cysteine were oxidized. Despite these chemical modifications, HγS remained folded past 10.8 kGy of γ irradiation as evidenced by circular dichroism and intrinsic tryptophan fluorescence spectroscopy.

Cataract-causing G91del mutant destabilised βA3 heteromers formation linking with structural stability and cellular viability

BACKGROUND/AIMS

Congenital cataracts, which are genetically heterogeneous eye disorders, result in visual loss in childhood around the world. CRYBA1/BA3 serves as an abundant structural protein in the lens, and forms homomers and heteromers to maintain lens transparency. In previous study, we identified a common cataract-causing mutation, βA3-glycine at codon 91 (G91del) (c.271-273delGAG), which deleted a highly conserved G91del and led to perinuclear zonular cataract. In this study, we aimed to explore the underlying pathogenic mechanism of G91del mutation.

METHODS

Protein purification, size-exclusion chromatography, spectroscopy and molecular dynamics simulation assays were used to investigate the effects on the heteromers formation and the protein structural properties of βA3-crystallin caused by G91del mutation. Intracellular βA3-G91del overexpression, MTT (3-(4,5)-dimethylthiahiazo (-z-y1)-3,5-di-phenytetrazoliumromide) and cell apoptosis were used to investigate the cellular functions of βA3-G91del.

RESULTS

βA3-crystallin and βB2-crystallin could form heteromers, which have much more stable structures than βA3 homomers. Interestingly, βA3/βB2 heteromers improved their resistance against the thermal stress and the guanidine hydrochloride treatment. However, the pathogenic mutation βA3-G91del destroyed the interaction with βB2, and thereby decreased its structural stability as well as the resistance of thermal or chemical stress. What's more, the βA3-G91del mutation induced cell apoptosis and escaped from the protection of βB2-crystallin.

CONCLUSIONS

βA3/βB2 heteromers play an indispensable role in maintaining lens transparency, while the βA3-G91del mutation destabilises heteromers formation with βB2-crystallin, impairs cellular viability and induces cellular apoptosis. These all might contribute to cataract development.

Chemical Properties Determine Solubility and Stability in βγ-Crystallins of the Eye Lens

βγ-Crystallins are the primary structural and refractive proteins found in the vertebrate eye lens. Because crystallins are not replaced after early eye development, their solubility and stability must be maintained for a lifetime, which is even more remarkable given the high protein concentration in the lens. Aggregation of crystallins caused by mutations or post-translational modifications can reduce crystallin protein stability and alter intermolecular interactions. Common post-translational modifications that can cause age-related cataracts include deamidation, oxidation, and tryptophan derivatization. Metal ion binding can also trigger reduced crystallin solubility through a variety of mechanisms. Interprotein interactions are critical to maintaining lens transparency: crystallins can undergo domain swapping, disulfide bonding, and liquid-liquid phase separation, all of which can cause opacity depending on the context. Important experimental techniques for assessing crystallin conformation in the absence of a high-resolution structure include dye-binding assays, circular dichroism, fluorescence, light scattering, and transition metal FRET.

Crystallins and Their Complexes

The crystallins (α, β and γ), major constituent proteins of eye lens fiber cells play their critical role in maintaining the transparency and refractive index of the lens. Under different stress factors and with aging, β- and γ-crystallins start to unfold partially leading to their aggregation. Protein aggregation in lens basically enhances light scattering and causes the vision problem, commonly known as cataract. α-crystallin as a molecular chaperone forms complexes with its substrates (β- and γ-crystallins) to prevent such aggregation. In this chapter, the structural features of β- and γ-crystallins have been discussed. Detailed structural information linked with the high stability of γC-, γD- and γS-crystallins have been incorporated. The nature of homologous and heterologous interactions among crystallins has been deciphered, which are involved in their molecular association and complex formation.

A Novel Mutation p.S93R in CRYBB1 Associated with Dominant Congenital Cataract and Microphthalmia

Purpose: To identify the pathogenetic mutations in a four-generation Chinese family with dominant congenital cataracts and microphthalmia.Methods: A four-generation Chinese family with dominant congenital cataracts were recruited. Genomic DNAs were collected from their peripheral blood leukocytes and subjected to whole exome sequencing. The genetic mutations were identified by bioinformatic analyses and verified by Sanger sequencing.Results: Whole exome sequencing revealed a c.279C>G point mutation in the CRYBB1 gene which was further verified by Sanger sequencing. The nucleotide replacement results in a novel mutation p.S93R in a conserved residue of βB1 crystallin which is predicted to disrupt normal βB1 structure and function.Conclusions: We identified a novel missense mutation p.S93R in CRYBB1 in a Chinese family with autosomal dominant congenital cataracts and microphthalmia. This serine residue is extremely conserved evolutionarily in more than 50 βγ-crystallins of many species. These data will be very helpful to further understand the structural and functional features of crystallins.

Biochemical characterization of G64W mutant of acidic beta-crystallin 4

Crystallins are structural proteins in the lens that last a lifetime with little turnover. Deviant in crystallins can cause rare but severe visual impairment, namely, congenital cataracts. It is reported that several mutations in the acidic β-crystallin 4 (CRYBA4) are related to congenital cataracts. However, the pathogenesis of these mutants is not well understood at molecular level. Here we evaluate the biochemical properties of wild type CRYBA4 (CRYBA4^WT) and a pathogenic G64W mutant (CRYBA4^G64W) including protein folding, polymerization state and protein stability. Furthermore, we explore the differences in their interactions with α-crystallin A (CRYAA) and basic β-crystallin 1 (CRYBB1) via yeast two-hybrid and pull-down assay in vitro, through which we find that G64W mutation leads to protein misfolding, decreases protein stability, blocks its interaction with CRYBB1 but maintains its interaction with CRYAA. Our results deepen our understanding of the pathogenesis of congenital cataracts.

Sequence specific 1H, 13C and 15N resonance assignments of the C-terminal domain of human γS-crystallin

The high solubility and stability of crystallins present in the human eye lens maintains its transparency and refractive index with negligible protein turnover. Monomeric γ-crystallins and oligomeric β-crystallins are made up of highly homologous double Greek key domains. These domains are symmetric and possess higher stability as a result of the complex topology of individual Greek key motifs. γS-crystallin is one of the most abundant structural βγ-crystallins present in the human eye lens. In order to understand the structural stability of individual domains of human γS-crystallin in isolation vis-à-vis full length protein, we set out to structurally characterize its C-terminal domain (abbreviated hereafter as γS-CTD) by solution NMR. In this direction, we have cloned, over-expressed, isolated and purified the γS-CTD. The 2D [¹⁵N-¹H] HSQC recorded with uniformly ¹³C/¹⁵N labeled γS-CTD showed a highly dispersed spectrum indicating the protein to adopt an ordered conformation. In this paper, we report almost complete sequence-specific ¹H, ¹³C and ¹⁵N resonance assignments of γS-CTD using a suite of heteronuclear 3D NMR experiments.

Deficiency of Yes-Associated Protein Induces Cataract in Mice

Cataract is a major cause of blindness worldwide, its complicated and unclear etiopathogenesis limit effective therapy. Here, we found that Yap, a downstream effector of the Hippo pathway, is specifically expressed in lens epithelial cells and Yap conditional knockout (cKO) in the lens leads to cataract. Histologically, Yap deficient lens show fewer epithelial cells, retention of nuclei and accumulation of morgagnian globules in the transitional zone and the posterior area. Mechanistically, GFAP-mediated Yap cKO leads to the reduced proliferation of epithelial cells, delayed fiber cell denucleation and increased cellular senescence in lens. Further RNA profiling analysis reveals Yap cKO results in a significant alteration in gene transcription that is involved in eye development, lens structure, inflammation, cellular proliferation and polarity. Collectively, our data reveal a novel function of Yap in the lens and links Yap deficiency with the development of cataract, making Yap a promising target for cataract therapy.

Human βB2-Crystallin Forms a Face-en-Face Dimer in Solution: An Integrated NMR and SAXS Study

βγ-Crystallins are long-lived eye lens proteins that are crucial for lens transparency and refractive power. Each βγ-crystallin comprises two homologous domains, which are connected by a short linker. γ-Crystallins are monomeric, while β-crystallins crystallize as dimers and multimers. In the crystal, human βB2-crystallin is a domain-swapped dimer while the N-terminally truncated βB1-crystallin forms a face-en-face dimer. Combining and integrating data from multi-angle light scattering, nuclear magnetic resonance, and small-angle X-ray scattering of full-length and terminally truncated human βB2-crystallin in solution, we show that both these βB2-crystallin proteins are dimeric, possess C2 symmetry, and are more compact than domain-swapped dimers. Importantly, no inter-molecular paramagnetic relaxation enhancement effects compatible with domain swapping were detected. Our collective experimental results unambiguously demonstrate that, in solution, human βB2-crystallin is not domain swapped and exhibits a face-en-face dimer structure similar to the crystal structure of truncated βB1-crystallin.

Cataract-causing mutation S228P promotes βB1-crystallin aggregation and degradation by separating two interacting loops in C-terminal domain

β/γ-Crystallins are predominant structural proteins in the cytoplasm of lens fiber cells and share a similar fold composing of four Greek-key motifs divided into two domains. Numerous cataract-causing mutations have been identified in various β/γ-crystallins, but the mechanisms underlying cataract caused by most mutations remains uncharacterized. The S228P mutation in βB1-crystallin has been linked to autosomal dominant congenital nuclear cataract. Here we found that the S228P mutant was prone to aggregate and degrade in both of the human and E. coli cells. The intracellular S228P aggregates could be redissolved by lanosterol. The S228P mutation modified the refolding pathway of βB1-crystallin by affecting the formation of the dimeric intermediate but not the monomeric intermediate. Compared with native βB1-crystallin, the refolded S228P protein had less packed structures, unquenched Trp fluorophores and increased hydrophobic exposure. The refolded S228P protein was prone to aggregate at the physiological temperature and decreased the protective effect of βB1-crystallin on βA3-crystallin. Molecular dynamic simulation studies indicated that the mutation decreased the subunit binding energy and modified the distribution of surface electrostatic potentials. More importantly, the mutation separated two interacting loops in the C-terminal domain, which shielded the hydrophobic core from solvent in native βB1-crystallin. These two interacting loops are highly conserved in both of the N- and C-terminal domains of all β/γ-crystallins. We propose that these two interacting loops play an important role in the folding and structural stability of β/γ-crystallin domains by protecting the hydrophobic core from solvent access.

Deamidation of N76 in human γS-crystallin promotes dimer formation

BACKGROUND

Cataract formation is often attributed to the build-up of post-translational modifications in the crystallin proteins of the eye lens. One such modification, the deamidation of N76 in human γS-crystallin to D76, is highly correlated with age-related cataract (Hooi et al. Invest. Ophthalmol. Vis. Sci. 53 (2012) 3554-3561). In the current work, this modification has been extensively characterised in vitro.

METHODS

Biophysical characterisation was performed on wild type and N76D γS-crystallins using turbidity measurements to monitor aggregation, intrinsic fluorescence and circular dichroism spectroscopy to determine the folded state and NMR spectroscopy for identifying local changes in structure. Protein mass was determined using SEC-MALLS and analytical ultracentrifugation methods.

RESULTS

Relative to the wild type protein, deamidation at N76 in γS-crystallin causes an increase in the thermal stability and resistance to thermally induced aggregation alongside a decrease in stability to denaturants, a propensity to aggregate rapidly once destabilised and a tendency to form a dimer. We ascribe the apparent increase in thermal stability upon deamidation to the formation of dimer which prevents the unfolding of the inherently less stable monomer.

CONCLUSIONS

Deamidation causes a decrease in stability of γS-crystallin but this is offset by an increased tendency for dimer formation.

GENERAL SIGNIFICANCE

Deamidation at N76 in human γS-crystallin likely has a combinatorial effect with other post-translational crystallin modifications to induce age-related cataract. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Biophysical chemistry of the ageing eye lens

This review examines both recent and historical literature related to the biophysical chemistry of the proteins in the ageing eye, with a particular focus on cataract development. The lens is a vital component of the eye, acting as an optical focusing device to form clear images on the retina. The lens maintains the necessary high transparency and refractive index by expressing crystallin proteins in high concentration and eliminating all large cellular structures that may cause light scattering. This has the consequence of eliminating lens fibre cell metabolism and results in mature lens fibre cells having no mechanism for protein expression and a complete absence of protein recycling or turnover. As a result, the crystallins are some of the oldest proteins in the human body. Lack of protein repair or recycling means the lens tends to accumulate damage with age in the form of protein post-translational modifications. The crystallins can be subject to a wide range of age-related changes, including isomerisation, deamidation and racemisation. Many of these modification are highly correlated with cataract formation and represent a biochemical mechanism for age-related blindness.

Differences in solution dynamics between lens β-crystallin homodimers and heterodimers probed by hydrogen-deuterium exchange and deamidation

BACKGROUND

Lens transparency is due to the ordered arrangement of the major structural proteins, called crystallins. βB2 crystallin in the lens of the eye readily forms dimers with other β-crystallin subunits, but the resulting heterodimer structures are not known and were investigated in this study.

METHODS

Structures of βA3 and βB2 crystallin homodimers and the βA3/βB2 crystallin heterodimers were probed by measuring changes in solvent accessibility using hydrogen-deuterium exchange with mass spectrometry. We further mimicked deamidation in βB2 and probed the effect on the βA3/βB2 heterodimer. Results were confirmed with chemical crosslinking and NMR.

RESULTS

Both βA3 and βB2 had significantly decreased deuterium levels in the heterodimer compared to their respective homodimers, suggesting that they had less solvent accessibility and were more compact in the heterodimer. The compact structure of βB2 was supported by the identification of chemical crosslinks between lysines in βB2 within the heterodimer that were inconsistent with βB2's extended homodimeric structure. The compact structure of βA3 was supported by an overall decrease in mobility of βA3 in the heterodimer detected by NMR. In βB2, peptides 70-84 and 121-134 were exposed in the homodimer, but buried in the heterodimer with ≥50% decreases in deuterium levels. Homologous peptides in βA3, 97-109 and 134-149, had 25-50% decreases in deuterium levels in the heterodimer. These peptides are probable sites of interaction between βB2 and βA3 and are located at the predicted interface between subunits with bent linkers. Deamidation at Q184 in βB2 at this predicted interface led to a less compact βB2 in the heterodimer. The more compact structure of the βA3/βB2 heterodimer was also more heat stable than either of the homodimers.

CONCLUSIONS

The major structural proteins in the lens, the β-crystallins, are not static, but dynamic in solution, with differences in accessibility between the homo-and hetero-dimers. This structural flexibility, particularly of βB2, may facilitate formation of different size higher-ordered structures found in the transparent lens.

GENERAL SIGNIFICANCE

Understanding complex hetero-oligomer interactions between β-crystallins in normal lens and how these interactions change during aging is fundamental to understanding the cause of cataracts. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Gamma crystallins of the human eye lens

BACKGROUND

Protein crystallins co me in three types (α, β and γ) and are found predominantly in the eye, and particularly in the lens, where they are packed into a compact, plastic, elastic, and transparent globule of proper refractive power range that aids in focusing incoming light on to the retina. Of these, the γ-crystallins are found largely in the nuclear region of the lens at very high concentrations (>400 mg/ml). The connection between their structure and inter-molecular interactions and lens transparency is an issue of particular interest.

SCOPE OF REVIEW

We review the origin and phylogeny of the gamma crystallins, their special structure involving the use of Greek key supersecondary structural motif, and how they aid in offering the appropriate refractive index gradient, intermolecular short range attractive interactions (aiding in packing them into a transparent ball), the role that several of the constituent amino acid residues play in this process, the thermodynamic and kinetic stability and how even single point mutations can upset this delicate balance and lead to intermolecular aggregation, forming light-scattering particles which compromise transparency. We cite several examples of this, and illustrate this by cloning, expressing, isolating and comparing the properties of the mutant protein S39C of human γS-crystallin (associated with congenital cataract-microcornea), with those of the wild type molecule. In addition, we note that human γ-crystallins are also present in other parts of the eye (e.g., retina), where their functions are yet to be understood.

MAJOR CONCLUSIONS

There are several 'crucial' residues in and around the Greek key motifs which are essential to maintain the compact architecture of the crystallin molecules. We find that a mutation that replaces even one of these residues can lead to reduction in solubility, formation of light-scattering particles and loss of transparency in the molecular assembly.

GENERAL SIGNIFICANCE

Such a molecular understanding of the process helps us construct the continuum of genotype-molecular structural phenotype-clinical (pathological) phenotype. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

βA3/A1-crystallin: more than a lens protein

Crystallins, the highly abundant proteins of the ocular lens, are essential determinants of the transparency and refractivity required for lens function. Initially thought to be lens-specific and to have evolved as lens proteins, it is now clear that crystallins were recruited to the lens from proteins that existed before lenses evolved. Crystallins are expressed outside of the lens and most have been shown to have cellular functions distinct from their roles as structural elements in the lens. For one major crystallin group, the β/γ-crystallin superfamily, no such functions have yet been established. We have explored possible functions for the polypeptides (βA3-and βA1-crystallins) encoded by Cryba1, one of the 6 β-crystallin genes, using a spontaneous rat mutant and genetically engineered mouse models. βA3-and βA1-crystallins are expressed in retinal astrocytes and retinal pigment epithelial (RPE) cells. In both cell types, these proteins appear to be required for the proper acidification of the lysosomes. In RPE cells, elevated pH in the lysosomes is shown to impair the critical processes of phagocytosis and autophagy, leading to accumulation of undigested cargo in (auto) phagolysosomes. We postulate that this accumulation may cause pathological changes in the cells resembling some of those characteristic of age-related macular degeneration (AMD). Our studies suggest an important regulatory function of βA3/A1-crystallin in astrocytes. We provide evidence that the cellular function of βA3/A1-crystallin involves its interaction with V-ATPase, the proton pump responsible for acidification of the endolysosomal system.

Functions of crystallins in and out of lens: roles in elongated and post-mitotic cells

The vertebrate lens evolved to collect light and focus it onto the retina. In development, the lens grows through massive elongation of epithelial cells possibly recapitulating the evolutionary origins of the lens. The refractive index of the lens is largely dependent on high concentrations of soluble proteins called crystallins. All vertebrate lenses share a common set of crystallins from two superfamilies (although other lineage specific crystallins exist). The α-crystallins are small heat shock proteins while the β- and γ-crystallins belong to a superfamily that contains structural proteins of uncertain function. The crystallins are expressed at very high levels in lens but are also found at lower levels in other cells, particularly in retina and brain. All these proteins have plausible connections to maintenance of cytoplasmic order and chaperoning of the complex molecular machines involved in the architecture and function of cells, particularly elongated and post-mitotic cells. They may represent a suite of proteins that help maintain homeostasis in such cells that are at risk from stress or from the accumulated insults of aging.

The N-terminal extension of βB1-crystallin chaperones β-crystallin folding and cooperates with αA-crystallin

β/γ-Crystallins are the major structural proteins in mammalian lens. The N-terminal truncation of βB1-crystallin has been associated with the regulation of β-crystallin size distributions in human lens. Herein we studied the roles of βB1 N-terminal extension in protein structure and folding by constructing five N-terminal truncated forms. The truncations did not affect the secondary and tertiary structures of the main body as well as stability against denaturation. Truncations with more than 28 residues off the N-terminus promoted the dissociation of the dimeric βB1 into monomers in diluted solutions. Interestingly, the N-terminal extension facilitated βB1 to adopt the correct folding pathway, while truncated proteins were prone to undergo the misfolding/aggregation pathway during kinetic refolding. The N-terminal extension of βB1 acted as an intramolecular chaperone (IMC) to regulate the kinetic partitioning between folding and misfolding. The IMC function of the N-terminal extension was also critical to the correct refolding of β-crystallin heteromer and the action of the lens-specific molecular chaperone αA-crystallin. The cooperation between IMC and molecular chaperones produced a much stronger chaperoning effect than if they acted separately. To our knowledge, this is the first report showing the cooperation between IMC and molecular chaperones.

Lens β-crystallins: the role of deamidation and related modifications in aging and cataract

Crystallins are the major proteins in the lens of the eye and function to maintain transparency of the lens. Of the human crystallins, α, β, and γ, the β-crystallins remain the most elusive in their structural significance due to their greater number of subunits and possible oligomer formations. The β-crystallins are also heavily modified during aging. This review focuses on the functional significance of deamidation and the related modifications of racemization and isomerization, the major modifications in β-crystallins of the aged human lens. Elucidating the role of these modifications in cataract formation has been slow, because they are analytically among the most difficult post-translational modifications to study. Recent results suggest that many amides deamidate to similar extent in normal aged and cataractous lenses, while others may undergo greater deamidation in cataract. Mimicking deamidation at critical structural regions induces structural changes that disrupt the stability of the β-crystallins and lead to their aggregation in vitro. Deamidations at the surface disrupt interactions with other crystallins. Additionally, the α-crystallin chaperone is unable to completely prevent deamidated β-crystallins from insolubilization. Therefore, deamidation of β-crystallins may enhance their precipitation and light scattering in vivo contributing to cataract formation. Future experiments are needed to quantify differences in deamidation rates at all Asn and Gln residues within crystallins from aged and cataractous lenses, as well as racemization and isomerization which potentially perturb protein structure greater than deamidation alone. Quantitative data is greatly needed to investigate the importance of these major age-related modifications in cataract formation.

Tyrosine/cysteine cluster sensitizing human γD-crystallin to ultraviolet radiation-induced photoaggregation in vitro

Ultraviolet radiation (UVR) exposure is a major risk factor for age-related cataract, a protein-aggregation disease of the human lens often involving the major proteins of the lens, the crystallins. γD-Crystallin (HγD-Crys) is abundant in the nucleus of the human lens, and its folding and aggregation have been extensively studied. Previous work showed that HγD-Crys photoaggregates in vitro upon exposure to UVA/UVB light and that its conserved tryptophans are not required for aggregation. Surprisingly, the tryptophan residues play a photoprotective role because of a distinctive energy-transfer mechanism. HγD-Crys also contains 14 tyrosine residues, 12 of which are organized as six pairs. We investigated the role of the tyrosines of HγD-Crys by replacing pairs with alanines and monitoring photoaggregation using light scattering and SDS-PAGE. Mutating both tyrosines in the Y16/Y28 pair to alanine slowed the formation of light-scattering aggregates. Further mutant studies implicated Y16 as important for photoaggregation. Mass spectrometry revealed that C18, in contact with Y16, is heavily oxidized during UVR exposure. Analysis of multiple mutant proteins by mass spectrometry suggested that Y16 and C18 likely participate in the same photochemical process. The data suggest an initial photoaggregation pathway for HγD-Crys in which excited-state Y16 interacts with C18, initiating radical polymerization.

Intrinsic lens forming potential of mouse lens epithelial versus newt iris pigment epithelial cells in three-dimensional culture

Adult newts (Notophthalmus viridescens) are capable of complete lens regeneration that is mediated through dorsal iris pigment epithelial (IPE) cells transdifferentiation. In contrast, higher vertebrates such as mice demonstrate only limited lens regeneration in the presence of an intact lens capsule with remaining lens epithelial cells. To compare the intrinsic lens regeneration potential of newt IPE versus mouse lens epithelial cells (MLE), we have established a novel culture method that uses cell aggregation before culture in growth factor-reduced Matrigel. Dorsal newt IPE aggregates demonstrated complete lens formation within 1 to 2 weeks of Matrigel culture without basic fibroblast growth factor (bFGF) supplementation, including the establishment of a peripheral cuboidal epithelial cell layer, and the appearance of central lens fibers that were positive for αA-crystallin. In contrast, the lens-forming potential of MLE cell aggregates cultured in Matrigel was incomplete and resulted in the formation of defined-size lentoids with partial optical transparency. While the peripheral cell layers of MLE aggregates were nucleated, cells in the center of aggregates demonstrated a nonapoptotic nuclear loss over a time period of 3 weeks that was representative of lens fiber formation. Matrigel culture supplementation with bFGF resulted in higher transparent bigger-size MLE aggregates that demonstrated increased appearance of βB1-crystallin expression. Our study demonstrates that bFGF is not required for induction of newt IPE aggregate-dependent lens formation in Matrigel, while the addition of bFGF seems to be beneficial for the formation of MLE aggregate-derived lens-like structures. In conclusion, the three-dimensional aggregate culture of IPE and MLE in Matrigel allows to a higher extent than older models the indepth study of the intrinsic lens-forming potential and the corresponding identification of lentogenic factors.

Evolution of crystallins for a role in the vertebrate eye lens

The camera eye lens of vertebrates is a classic example of the re-engineering of existing protein components to fashion a new device. The bulk of the lens is formed from proteins belonging to two superfamilies, the α-crystallins and the βγ-crystallins. Tracing their ancestry may throw light on the origin of the optics of the lens. The α-crystallins belong to the ubiquitous small heat shock proteins family that plays a protective role in cellular homeostasis. They form enormous polydisperse oligomers that challenge modern biophysical methods to uncover the molecular basis of their assembly structure and chaperone-like protein binding function. It is argued that a molecular phenotype of a dynamic assembly suits a chaperone function as well as a structural role in the eye lens where the constraint of preventing protein condensation is paramount. The main cellular partners of α-crystallins, the β- and γ-crystallins, have largely been lost from the animal kingdom but the superfamily is hugely expanded in the vertebrate eye lens. Their structures show how a simple Greek key motif can evolve rapidly to form a complex array of monomers and oligomers. Apart from remaining transparent, a major role of the partnership of α-crystallins with β- and γ-crystallins in the lens is to form a refractive index gradient. Here, we show some of the structural and genetic features of these two protein superfamilies that enable the rapid creation of different assembly states, to match the rapidly changing optical needs among the various vertebrates.

Association of high myopia with crystallin beta A4 (CRYBA4) gene polymorphisms in the linkage-identified MYP6 locus

Background

Myopia is the most common ocular disorder worldwide and imposes tremendous burden on the society. It is a complex disease. The MYP6 locus at 22 q12 is of particular interest because many studies have detected linkage signals at this interval. The MYP6 locus is likely to contain susceptibility gene(s) for myopia, but none has yet been identified.

Methodology/Principal Findings

Two independent subject groups of southern Chinese in Hong Kong participated in the study an initial study using a discovery sample set of 342 cases and 342 controls, and a follow-up study using a replication sample set of 316 cases and 313 controls. Cases with high myopia were defined by spherical equivalent ≤ -8 dioptres and emmetropic controls by spherical equivalent within ±1.00 dioptre for both eyes. Manual candidate gene selection from the MYP6 locus was supported by objective in silico prioritization. DNA samples of discovery sample set were genotyped for 178 tagging single nucleotide polymorphisms (SNPs) from 26 genes. For replication, 25 SNPs (tagging or located at predicted transcription factor or microRNA binding sites) from 4 genes were subsequently examined using the replication sample set. Fisher P value was calculated for all SNPs and overall association results were summarized by meta-analysis. Based on initial and replication studies, rs2009066 located in the crystallin beta A4 (CRYBA4) gene was identified to be the most significantly associated with high myopia (initial study: P = 0.02; replication study: P = 1.88e-4; meta-analysis: P = 1.54e-5) among all the SNPs tested. The association result survived correction for multiple comparisons. Under the allelic genetic model for the combined sample set, the odds ratio of the minor allele G was 1.41 (95% confidence intervals, 1.21-1.64).

Conclusions/Significance

A novel susceptibility gene (CRYBA4) was discovered for high myopia. Our study also signified the potential importance of appropriate gene prioritization in candidate selection.

βB1-crystallin: thermodynamic profiles of molecular interactions

Background

β-Crystallins are structural proteins maintaining eye lens transparency and opacification. Previous work demonstrated that dimerization of both βA3 and βB2 crystallins (βA3 and βB2) involves endothermic enthalpy of association (∼8 kcal/mol) mediated by hydrophobic interactions.

Methodology/Principal Findings

Thermodynamic profiles of the associations of dimeric βA3 and βB1 and tetrameric βB1/βA3 were measured using sedimentation equilibrium. The homo- and heteromolecular associations of βB1 crystallin are dominated by exothermic enthalpy (−13.3 and −24.5 kcal/mol, respectively).

Conclusions/Significance

Global thermodynamics of βB1 interactions suggest a role in the formation of stable protein complexes in the lens via specific van der Waals contacts, hydrogen bonds and salt bridges whereas those β-crystallins which associate by predominately hydrophobic forces participate in a weaker protein associations.

Aggregation of deamidated human betaB2-crystallin and incomplete rescue by alpha-crystallin chaperone

Aging of the lens is accompanied by extensive deamidation of the lens specific proteins, the crystallins. Deamidated crystallins are increased in the insoluble proteins and may contribute to cataracts. Deamidation has been shown in vitro to alter the structure and decrease the stability of human lens betaB1, betaB2 and betaA3-crystallin. Of particular interest, betaB2 mutants were constructed to mimic the effect of in vivo deamidations at the interacting interface between domains, at Q70 in the N terminal domain and at Q162, its C-terminal homologue. The double mutant was also constructed. We previously reported that deamidation at the critical interface sites decreased stability, while preserving the dimeric 3D structure. In the present study, dynamic light scattering, differential scanning calorimetry and small angle X-ray scattering were used to investigate the effect of deamidation on stability, thermal unfolding and aggregation. The bovine betaLb fraction was used for comparative analysis. The chaperone requirements of the various samples were determined using bovine alpha-crystallins as the chaperone. Deamidation at both interface Gln residues or at Q70, but not Q162, significantly lowered the temperature for unfolding and aggregation, which was rapidly followed by precipitation. This deamidation-induced aggregation and precipitation was not completely prevented by alpha-crystallin chaperone. A potential mechanism for cataract formation in vivo involving accumulation of deamidated beta-crystallin aggregates is discussed.

N-terminal extension of beta B1-crystallin: identification of a critical region that modulates protein interaction with beta A3-crystallin

The human lens proteins beta-crystallins are subdivided into acidic (betaA1-betaA4) and basic (betaB1-betaB3) subunit groups. These structural proteins exist at extremely high concentrations and associate into oligomers under physiological conditions. Crystallin acidic-basic pairs tend to form strong heteromolecular associations. The long N-terminal extensions of beta-crystallins may influence both homo- and heteromolecular interactions. However, identification of the critical regions of the extensions mediating protein associations has not been previously addressed. This was studied by comparing the self-association and heteromolecular associations of wild-type recombinant betaA3- and betaB1-crystallins and their N-terminally truncated counterparts (betaA3DeltaN30 and betaB1DeltaN56) using several biophysical techniques, including analytical ultracentrifugation and fluorescence spectroscopy. Removal of the N-terminal extension of betaA3 had no effect on dimerization or heteromolecular tetramer formation with betaB1. In contrast, the level of self-association of betaB1DeltaN56 increased, resulting in homotetramer formation, and heteromolecular association with betaA3 was blocked. Limited proteolysis of betaB1 produced betaB1DeltaN47, which is similar to intact protein formed dimers but in contrast showed enhanced heteromolecular tetramer formation with betaA3. The tryptic digestion was physiologically significant, corresponding to protease processing sites observed in vivo. Molecular modeling of the N-terminal betaB1 extension indicates structural features that position a mobile loop in the vicinity of these processing sites. The loop is derived from residues 48-56 which appear to be critical for mediating protein interactions with betaA3-crystallin.

Truncated human betaB1-crystallin shows altered structural properties and interaction with human betaA3-crystallin

The purpose of the study was to determine the effects of truncation of various regions of betaB1-crystallin on its structural properties and stability of heterooligomers formed by wild-type (WT) betaB1 or its deletion mutants with WT betaA3-crystallin. For these analyses, seven deletion mutants of betaB1-crystallin were generated with the following sequential deletions of either N-terminal arm [betaB1(59-252)], N-terminal arm + motif I [betaB1(99-252)], N-terminal arm + motif I + motif II [betaB1(144-252)], N-terminal arm + motif I + motif II + connecting peptide [betaB1(149-252)], C-terminal extension [betaB1(1-234)], C-terminal extension plus motif IV [betaB1(1-190)], or C-terminal extension + motif III + motif IV [betaB1(1-148)]. The betaB1-crystallin became water insoluble on the deletion of C-terminal extension and subsequent deletions of the C-terminal domain (C-terminal extension plus motifs III and IV) while it remained partially soluble on the deletion of the N-terminal domain (N-terminal arm plus motifs I and II). However, circular dichroism spectral analysis showed that the deletion of the N-terminal domain but not the C-terminal domain exhibited relatively greater structural changes in the crystallin. The deletion of the C-terminal domain resulted in a greater exposure and disturbance in the microenvironment of Trp-100, Trp-123, and Trp-126 (localized in the motif II), suggesting a relatively greater role of the C-terminal domain than the N-terminal domain in the structural stability of the crystallin. The deletion of the N-terminal extension in betaB1 resulted in maximum exposure of hydrophobic patches and compact structure and in a maximum loss of subunit exchange with WT betaA3-crystallin compared to deletion of either the C-terminal extension, the N-terminal domain, or the C-terminal domain. The thermal stability results of the heterooligomer of betaB1- plus betaA3-crystallins suggested that oligomers lose their stability on deletion of the C-terminal domain. Together, the results suggested that the N-terminal arm of betaB1-crystallin plays a major role in interaction with betaA3-crystallin during heterooligomer formation, and the solubility of betaB1-crystallin per se and that of the heterooligomer with betaA3-crystallin are dependent on the intact C-terminal domain of betaB1-crystallin.

Mechanism of the very efficient quenching of tryptophan fluorescence in human gamma D- and gamma S-crystallins: the gamma-crystallin fold may have evolved to protect tryptophan residues from ultraviolet photodamage

Proteins exposed to UV radiation are subject to irreversible photodamage through covalent modification of tryptophans (Trps) and other UV-absorbing amino acids. Crystallins, the major protein components of the vertebrate eye lens that maintain lens transparency, are exposed to ambient UV radiation throughout life. The duplicated beta-sheet Greek key domains of beta- and gamma-crystallins in humans and all other vertebrates each have two conserved buried Trps. Experiments and computation showed that the fluorescence of these Trps in human gammaD-crystallin is very efficiently quenched in the native state by electrostatically enabled electron transfer to a backbone amide [Chen et al. (2006) Biochemistry 45, 11552-11563]. This dispersal of the excited state energy would be expected to minimize protein damage from covalent scission of the excited Trp ring. We report here both experiments and computation showing that the same fast electron transfer mechanism is operating in a different crystallin, human gammaS-crystallin. Examination of solved structures of other crystallins reveals that the Trp conformation, as well as favorably oriented bound waters, and the proximity of the backbone carbonyl oxygen of the n - 3 residues before the quenched Trps (residue n), are conserved in most crystallins. These results indicate that fast charge transfer quenching is an evolved property of this protein fold, probably protecting it from UV-induced photodamage. This UV resistance may have contributed to the selection of the Greek key fold as the major lens protein in all vertebrates.

Association properties of betaB1- and betaA3-crystallins: ability to form heterotetramers

As major constituents of the mammalian lens, beta-crystallins associate into dimers, tetramers, and higher-order complexes to maintain lens transparency and refractivity. A previous study has shown that dimerization of betaB2- and betaA3-crystallins is energetically highly favored and entropically driven. While heterodimers further associate into higher-order complexes in vivo, a significant level of reversibly associated tetrameric crystallin has not been previously observed in vitro. To enhance our understanding of the interactions between beta-crystallins, we characterized the association of betaB1-crystallin, a major component of large beta-crystallin complexes (beta-high), with itself and with betaA3-crystallin. Mouse betaB1-crystallin and human betaA3-crystallin were expressed in Escherichia coli and purified chromatographically. Their association was then characterized using size-exclusion chromatography, native gel electrophoresis, isoelectric focusing, and analytical sedimentation equilibrium centrifugation. When present alone, each beta-crystallin associates into homodimers; however, no tetramer formation is seen. Once mixing has taken place, formation of a heterocomplex between betaB1- and betaA3-crystallins is observed using size-exclusion chromatography, native gel electrophoresis, isoelectric focusing, and sedimentation equilibrium. In contrast to results previously obtained after betaB2- and betaA3-crystallins had been mixed, mixed betaB1- and betaA3-crystallins show a dimer-tetramer equilibrium with a K d of 1.1 muM, indicating that these two beta-crystallins associate predominantly into heterotetramers in vitro. Thus, while each purified beta-crystallin associates only into homodimers and under the conditions studied mixed betaB2- and betaA3-crystallins form a mixture of homo- and heterodimers, mixed betaB1- and betaA3-crystallins associate predominantly into heterotetramers in equilibrium with heterodimers. These findings suggest a unique role for betaB1-crystallin in promoting higher-order crystallin association in the lens.

Mechanism of the efficient tryptophan fluorescence quenching in human gammaD-crystallin studied by time-resolved fluorescence

Human γD-crystallin (HγD-Crys) is a two-domain, β-sheet eye lens protein found in the lens nucleus. Its long-term solubility and stability are important to maintain lens transparency throughout life. HγD-Crys has four highly conserved buried tryptophans (Trps), with two in each of the homologous β-sheet domains. In situ, these Trps will be absorbing ambient UV radiation that reaches the lens. The dispersal of the excited-state energy to avoid covalent damage is likely to be physiologically relevant for the lens crystallins. Trp fluorescence is efficiently quenched in native HγD-Crys. Previous steady-state fluorescence measurements provide strong evidence for energy transfer from Trp42 to Trp68 in the N-terminal domain and from Trp130 to Trp156 in the C-terminal domain [Chen, J., et al. (2006) Biochemistry 45, 11552−11563]. Hybrid quantum mechanical−molecular mechanical (QM-MM) simulations indicated that the fluorescence of Trp68 and Trp156 is quenched by fast electron transfer to the amide backbone. Here we report additional information obtained using time-resolved fluorescence spectroscopy. In the single-Trp-containing proteins (Trp42-only, Trp68-only, Trp130-only, and Trp156-only), the highly quenched Trp68 and Trp156 have very short lifetimes, τ ∼0.1 ns, whereas the moderately fluorescent Trp42 and Trp130 have longer lifetimes, τ ∼3 ns. In the presence of the energy acceptor (Trp68 or Trp156), the lifetime of the energy donor (Trp42 or Trp130) decreased from ∼3 to ∼1 ns. The intradomain energy transfer efficiency is 56% in the N-terminal domain and is 71% in the C-terminal domain. The experimental values of energy transfer efficiency are in good agreement with those calculated theoretically. The absence of a time-dependent red shift in the time-resolved emission spectra of Trp130 proves that its local environment is very rigid. Time-resolved fluorescence anisotropy measurements with the single-Trp-containing proteins, Trp42-only and Trp130-only, indicate that the protein rotates as a rigid body and no segmental motion is detected. A combination of energy transfer with electron transfer results in short excited-state lifetimes of all Trps, which, together with the high rigidity of the protein matrix around Trps, could protect HγD-Crys from excited-state reactions causing permanent covalent damage.

Deamidation destabilizes and triggers aggregation of a lens protein, betaA3-crystallin

Protein aggregation is a hallmark of several neurodegenerative diseases and also of cataracts. The major proteins in the lens of the eye are crystallins, which accumulate throughout life and are extensively modified. Deamidation is the major modification in the lens during aging and cataracts. Among the crystallins, the betaA3-subunit has been found to have multiple sites of deamidation associated with the insoluble proteins in vivo. Several sites were predicted to be exposed on the surface of betaA3 and were investigated in this study. Deamidation was mimicked by site-directed mutagenesis at Q42 and N54 on the N-terminal domain, N133 and N155 on the C-terminal domain, and N120 in the peptide connecting the domains. Deamidation altered the tertiary structure without disrupting the secondary structure or the dimer formation of betaA3. Deamidations in the C-terminal domain and in the connecting peptide decreased stability to a greater extent than deamidations in the N-terminal domain. Deamidation at N54 and N155 also disrupted the association with the betaB1-subunit. Sedimentation velocity experiments integrated with high-resolution analysis detected soluble aggregates at 15%-20% in all deamidated proteins, but not in wild-type betaA3. These aggregates had elevated frictional ratios, suggesting that they were elongated. The detection of aggregates in vitro strongly suggests that deamidation may contribute to protein aggregation in the lens. A potential mechanism may include decreased stability and/or altered interactions with other beta-subunits. Understanding the role of deamidation in the long-lived crystallins has important implications in other aggregation diseases.

Multi-crystallin complexes exist in the water-soluble high molecular weight protein fractions of aging normal and cataractous human lenses

The purpose of the study was to identify non-covalently held complexes that exist in the water-soluble high molecular weight (WS-HMW) protein fractions of normal human lenses of 20-year-old and 60- to 70-year-old, and in the age-matched 60- to 70-year-old cataractous lenses. The WS protein fractions were prepared from five pooled normal lenses of 20-year-old donors or five pooled lenses of 60- to 70-year-old donors or four pooled cataractous lenses (with nuclear opacity) of 60- to 70-year-old donors. Each WS protein fraction was subjected to size-exclusion chromatography using an Agarose A 5m column to recover the void volume WS-HMW protein fraction. A method known as blue-native polyacrylamide gel electrophoresis (BN-PAGE), which allows the isolation of large multi-protein complexes (MPCs) in their native state for further characterization, was used to separate such complexes from individual WS-HMW protein fractions. The protein species that existed as a complex were excised from a gel and trypsin-digested, and the amino acid sequences of the tryptic fragments analyzed by electrospray tandem mass spectrometry (ES-MS/MS). After the second-dimensional sodium dodecyl sulfate-PAGE during BN-PAGE, protein complexes containing a total of 16, 12, and 24 species with M(r) between 10 and 90 kDa were identified in the HMW protein fractions of normal lenses of 20-year-old, 60- to 70-year-old and cataractous lenses of 60- to 70-year-old donors, respectively. Based on the amino acid sequences of tryptic peptides of individual protein species in the complexes by the ES-MS/MS method, the presence of alpha-, beta-, and gamma-crystallin species along with beaded filament proteins (filensin and phakinin) was observed in the 20-year-old normal lenses. The 60- to 70-year-old normal lenses contained filensin and aldehyde dehydrogenase in addition to the above crystallins. Similarly, the age-matched cataractous lenses also contained the above crystallins and aldehyde dehydrogenase but lacked beaded filament proteins. Protein complexes, held mostly via non-covalent bonding, were seen in the WS-HMW proteins of 20-year-old normal, 60- to 70-year-old normal, and 60- to 70-year-old cataractous lenses. The complexes in the normal lenses were made of alpha-, beta-, and gamma-crystallin species, beaded filament proteins (filensin and/or phakinin), and aldehyde dehydrogenase. The complexes in the age-matched cataractous lenses also contained these crystallins, and aldehyde dehydrogenase, but not the beaded filament proteins. Further, the crystallin fragments were greater in number in the cataractous lenses compared to the age-matched normal lenses. During multi-angle light scattering (MALS), the HMW proteins from cataractous lenses exhibited species with lower molecular weight range than age-matched normal lenses. The HMW protein preparations from both normal and cataractous lenses showed spherical structures on electron microscopic analysis.

Enhancing the thermostability of a novel beta-agarase AgaB through directed evolution

To increase the thermostability of beta-agarase AgaB by directed evolution, the mutant gene libraries were generated by error-prone polymerase chain reaction (PCR) and deoxyribonucleic acid (DNA) shuffling. Mutants with high thermostability were screened by a simple method based on agarase-degrading agar to generate a clear zone on the agar plate. A mutant S2 was obtained through two rounds of error-prone PCR and a single round of DNA shuffling and selection. It has higher thermostability and slightly increased catalytic activity than wild-type AgaB. Melting temperature (T(m)) of S2, as determined by circular dichroism, is 4.6 degrees C higher than that of wild-type AgaB, and the half-life of S2 is 350 min at 40 degrees C, which is 18.4-fold longer than that of the wild-type enzyme. Saturation mutagenesis and hydrophobic cluster analysis indicated that hydrophobic interaction might be the key factor that enhances the enzyme stability.

Molecular diversity and genomic organisation of the alpha, beta and gamma eye lens crystallins from the Antarctic toothfish Dissostichus mawsoni

The eye lens of the Antarctic toothfish living in the -2 degrees C Southern Ocean is cold-stable. To investigate the molecular basis of this cold stability, we isolated, cloned and sequenced 22 full length crystallin cDNAs. We found two alpha crystallins (alphaA, alphaB), six beta crystallins (betaA1, betaA2, betaA4, betaB1, betaB2, betaB3) and 14 gamma crystallins (gammaN, gammaS1, gammaS2, gammaM1, gammaM3, gammaM4, gammaM5, gammaM7, gammaM8a, gammaM8b, gammaM8c, gammaM8d, gammaM8e, and gammaM9). Alignments of alpha, beta and gamma with other known crystallin sequences indicate that toothfish alpha and beta crystallins are relatively conserved orthologues of their vertebrate counterparts, but the toothfish and other fish gammaM crystallins form a distinct group that are not orthologous to mammalian gamma crystallins. A preliminary Fingerprinted Contig analysis of clones containing crystallin genes screened from a toothfish BAC library indicated alpha crystallin genes occurred in a single genomic region of ~266 kbp, beta crystallin genes in ~273 kbp, while the gamma crystallin gene family occurred in two separate regions of ~180 and ~296 kbp. In phylogenetic analysis, the gammaM isoforms of the ectothermic toothfish displayed a diversity not seen with endothermic mammalian gamma crystallins. Similar to other fishes, several toothfish gamma crystallins are methionine-rich (gammaM isoforms) which may have predisposed the toothfish lens to biochemically attenuate gamma crystallin hydrophobicity allowing for cold adaptation. In addition to high methionine content, conservation of alphabeta crystallins both in sequence and abundance suggests greater functional constraints relative to gamma crystallins. Conversely, reduced constraints upon gamma crystallins could have allowed for greater evolutionary plasticity resulting in increased polydispersity of gamma crystallins contributing to the cold-stability of the Antarctic toothfish lens.

Association of partially folded lens betaB2-crystallins with the alpha-crystallin molecular chaperone

Age-related cataract is a result of crystallins, the predominant lens proteins, forming light-scattering aggregates. In the low protein turnover environment of the eye lens, the crystallins are susceptible to modifications that can reduce stability, increasing the probability of unfolding and aggregation events occurring. It is hypothesized that the alpha-crystallin molecular chaperone system recognizes and binds these proteins before they can form the light-scattering centres that result in cataract, thus maintaining the long-term transparency of the lens. In the present study, we investigated the unfolding and aggregation of (wild-type) human and calf betaB2-crystallins and the formation of a complex between alpha-crystallin and betaB2-crystallins under destabilizing conditions. Human and calf betaB2-crystallin unfold through a structurally similar pathway, but the increased stability of the C-terminal domain of human betaB2-crystallin relative to calf betaB2-crystallin results in the increased population of a partially folded intermediate during unfolding. This intermediate is aggregation-prone and prevents constructive refolding of human betaB2-crystallin, while calf betaB2-crystallin can refold with high efficiency. alpha-Crystallin can effectively chaperone both human and calf betaB2-crystallins from thermal aggregation, although chaperone-bound betaB2-crystallins are unable to refold once returned to native conditions. Ordered secondary structure is seen to increase in alpha-crystallin with elevated temperatures up to 60 degrees C; structure is rapidly lost at temperatures of 70 degrees C and above. Our experimental results combined with previously reported observations of alpha-crystallin quaternary structure have led us to propose a structural model of how activated alpha-crystallin chaperones unfolded betaB2-crystallin.

Folding and stability of the isolated Greek key domains of the long-lived human lens proteins gammaD-crystallin and gammaS-crystallin

The transparency of the eye lens depends on the high solubility and stability of the lens crystallin proteins. The monomeric gamma-crystallins and oligomeric beta-crystallins have paired homologous double Greek key domains, presumably evolved through gene duplication and fusion. Prior investigation of the refolding of human gammaD-crystallin revealed that the C-terminal domain folds first and nucleates the folding of the N-terminal domain. This result suggested that the human N-terminal domain might not be able to fold on its own. We constructed and expressed polypeptide chains corresponding to the isolated N- and C-terminal domains of human gammaD-crystallin, as well as the isolated domains of human gammaS-crystallin. Both circular dichroism and fluorescence spectroscopy indicated that the isolated domains purified from Escherichia coli were folded into native-like monomers. After denaturation, the isolated domains refolded efficiently at pH 7 and 37 degrees C into native-like structures. The in vitro refolding of all four domains revealed two kinetic phases, identifying partially folded intermediates for the Greek key motifs. When subjected to thermal denaturation, the isolated N-terminal domains were less stable than the full-length proteins and less stable than the C-terminal domains, and this was confirmed in equilibrium unfolding/refolding experiments. The decrease in stability of the N-terminal domain of human gammaD-crystallin with respect to the complete protein indicated that the interdomain interface contributes of 4.2 kcal/mol to the overall stability of this very long-lived protein.

Biophysical properties of gammaC-crystallin in human and mouse eye lens: the role of molecular dipoles

The eye lens is packed with soluble crystallin proteins, providing a lifetime of transparency and light refraction. gamma-Crystallins are major components of the dense, high refractive index central regions of the lens and generally have high solubility, high stability and high levels of cysteine residues. Human gammaC belongs to a group of gamma-crystallins with a pair of cysteine residues at positions 78 and 79. Unlike other gamma-crystallins it has relatively low solubility, whereas mouse gammaC, which has the exposed C79 replaced with arginine, and a novel mouse splice variant, gammaCins, are both highly soluble. Furthermore, human gammaC is extremely stable, while the mouse orthologs are less stable. Evolutionary pressure may have favoured stability over solubility for human gammaC and the reverse for the orthologs in the mouse. Mutation of C79 to R79, in human gammaC, greatly increased solubility, however, neither form produced crystals. Remarkably, when the human gammaD R36S crystallization cataract mutation was mimicked in human gammaC-crystallin, the solubility of gammaC was dramatically increased, although it still did not crystallize. The highly soluble mouse gammaC-crystallin did crystallize. Its X-ray structure was solved and used in homology modelling of human gammaC, and its mutants C79R and R36S. The human gammaD R36S mutant was also modelled from human gammaD coordinates. Molecular dynamics simulation of the six molecules in the solution state showed that the human gammaCs differed from gammaDs in domain pairing, behaviour that correlates with interface sequence changes. When the fluctuations of the calculated molecular dipoles, for the six structures, over time were analysed, characteristic patterns for soluble gammaC and gammaD proteins were observed. Individual sequence changes that increase or decrease solubility correlated well with changes in the magnitude and direction of these dipoles. It is suggested that changes in surface residues have allowed adaptation for the differing needs of human and mouse lenses.

Protein-protein interactions among human lens acidic and basic beta-crystallins

Human lens beta-crystallin contains four acidic (betaA1-->betaA4) and three basic (betaB1-->betaB3) subunits. They oligomerize in the lens, but it is uncertain which subunits are involved in the oligomerization. We used a two-hybrid system to detect protein-protein interactions systematically. Proteins were also expressed for some physicochemical studies. The results indicate that all acidic-basic pairs (betaA-betaB) except betaA4-betaBs pairs show strong hetero-molecular interactions. For acidic or basic pairs, only two pairs (betaA1-betaA1 and betaA3-betaA3) show strong self-association. betaA2 and betaA4 show very weak self-association, which arises from their low solubility. Confocal fluorescence microscopy shows enormous protein aggregates in betaA2- or betaA4-crystallin transfected cells. However, coexpression with betaB2-crystallin decreased both the number and size of aggregates. Circular dichroism indicates subtle differences in conformation among beta-crystallins that may have contributed to the differences in interactions.