Stability, homodimerization, and calcium-binding properties of a single, variant betagamma-crystallin domain of the protein absent in melanoma 1 (AIM1)

Purification and Functional Characterization of the C-Terminal Domain of the β-Actin-Binding Protein AIM1 In Vitro

The protein absent in melanoma 1 (AIM1) is a member of the βγ-crystal lens superfamily that is associated with the development of multiple cancers. The binding of AIM1 to β-actin affects the migration and invasion of prostate cancer epithelial cells. The C-terminus of AIM1 is required for the β-actin interaction. However, the characteristics of AIM1 in vitro and the interaction mode between AIM1 and β-actin remain unknown. We describe novel methods to prepare pure recombinant AIM1 and identify possible binding modes between AIM1 and β-actin; we also obtain the crystal of the first two βγ-crystallin domains of AIM1 (g1g2) for future structural biology research. We first express and purify AIM1 after cloning the sequence into a modified pET-28a_psp expression vector. Next, we define the minimum unit formed by the βγ-crystallin domain repeats that bound to β-actin and perform its physiological function. Finally, we made the structural model of the AIM1 g1g2 that can be used to guide future biomedical investigations and prostate cancer research.

Exploring the sequence-structure-function relationship for the intrinsically disordered βγ-crystallin Hahellin

βγ-Crystallins are a superfamily of proteins containing crystallin-type Greek key motifs. Some βγ-crystallin domains have been shown to bind Ca²⁺. Hahellin is a newly identified intrinsically disordered βγ-crystallin domain from Hahella chejuensis. It folds into a typical βγ-crystallin structure upon Ca²⁺ binding and acts as a Ca²⁺-regulated conformational switch. Besides Hahellin, another two putative βγ-crystallins from Caulobacter crescentus and Yersinia pestis are shown to be partially disordered in their apo-form and undergo large conformational changes upon Ca²⁺ binding, although whether they acquire a βγ-crystallin fold is not known. The extent of conformational disorder/order of a protein is determined by its amino acid sequence. To date how this sequence-structure relationship is reflected in the βγ-crystallin superfamily has not been investigated. In this work, we comparatively studied the sequence and structure of Hahellin with those of Protein S, an ordered βγ-crystallin, via various computational biophysical techniques. We found that several factors, including presence of a C-terminal disorder prone region, high content of energetic frustrations, and low contact density, may promote the formation of the disordered state of apo-Hahellin. We also analyzed the disorder propensities for other putative disordered βγ-crystallin domains. This study provides new clues for further understanding the sequence-structure-function relationship of βγ-crystallins.

Microbial βγ-crystallins

βγ-Crystallins have emerged as a superfamily of structurally homologous proteins with representatives across the domains of life. A major portion of this superfamily is constituted by members from microorganisms. This superfamily has also been recognized as a novel group of Ca(2+)-binding proteins with huge diversity. The βγ domain shows variable properties in Ca(2+) binding, stability and association with other domains. The various members present a series of evolutionary adaptations culminating in great diversity in properties and functions. Most of the predicted βγ-crystallins are yet to be characterized experimentally. In this review, we outline the distinctive features of microbial βγ-crystallins and their position in the βγ-crystallin superfamily.

Ca2+-binding motif of βγ-crystallins

βγ-Crystallin-type double clamp (N/D)(N/D)XX(S/T)S motif is an established but sparsely investigated motif for Ca(2+) binding. A βγ-crystallin domain is formed of two Greek key motifs, accommodating two Ca(2+)-binding sites. βγ-Crystallins make a separate class of Ca(2+)-binding proteins (CaBP), apparently a major group of CaBP in bacteria. Paralleling the diversity in βγ-crystallin domains, these motifs also show great diversity, both in structure and in function. Although the expression of some of them has been associated with stress, virulence, and adhesion, the functional implications of Ca(2+) binding to βγ-crystallins in mediating biological processes are yet to be elucidated.

Disability for function: loss of Ca(2+)-binding is obligatory for fitness of mammalian βγ-crystallins

Vertebrate βγ-crystallins belonging to the βγ-crystallin superfamily lack functional Ca(2+)-binding sites, while their microbial homologues do not; for example, three out of four sites in lens γ-crystallins are disabled. Such loss of Ca(2+)-binding function in non-lens βγ-crystallins from mammals (e.g., AIM1 and Crybg3) raises the possibility of a trade-off in the evolutionary extinction of Ca(2+)-binding. We test this hypothesis by reconstructing ancestral Ca(2+)-binding motifs (transforming disabled motifs into the canonical ones) in the lens γB-crystallin by introducing minimal sets of mutations. Upon incorporation of serine at the fifth position in the N/D-N/D-X-X-S/T(5)-S motif, which endowed a domain with microbial characteristics, a decreased domain stability was observed. Ca(2+) further destabilized the N-terminal domain (NTD) and its serine mutants profoundly, while the incorporation of a C-terminal domain (CTD) nullified this destabilization. On the other hand, Ca(2+)-induced destabilization of the CTD was not rescued by the introduction of an NTD. Of note, only one out of four sites is functional in the NTD of γB-crystallins responsible for weak Ca(2+) binding, but the deleterious effects of Ca(2+) are overcome by introduction of a CTD. The rationale for the onset of cataracts by certain mutations, such as R77S, which have not been clarified by structural means, could be explained by this work. The findings presented here shed light on the evolutionary innovations in terms of the functional loss of Ca(2+)-binding and acquisition of a bilobed domain, besides imparting additional advantages (e.g., protection from light) required for specialized functions.

Association properties and unfolding of a βγ-crystallin domain of a Vibrio-specific protein

The βγ-crystallin superfamily possesses a large number of versatile members, of which only a few members other than lens βγ-crystallins have been studied. Understanding the non-crystallin functions as well as origin of crystallin-like properties of such proteins is possible by exploring novel members from diverse sources. We describe a novel βγ-crystallin domain with S-type (Spherulin 3a type) Greek key motifs in protein vibrillin from a pathogenic bacterium Vibrio cholerae. This domain is a part of a large Vibrio-specific protein prevalent in Vibrio species (found in at least fourteen different strains sequenced so far). The domain contains two canonical N/D-N/D-X-X-S/T-S Ca²⁺-binding motifs, and bind Ca²⁺. Unlike spherulin 3a and other microbial homologues studied so far, βγ-crystallin domain of vibrillin self-associates forming oligomers of various sizes including dimers. The fractionated dimers readily form octamers in concentration-dependent manner, suggesting an association between these two major forms. The domain associates/dissociates forming dimers at the cost of monomeric populations in the presence of Ca²⁺. No such effect of Ca²⁺ has been observed in oligomeric species. The equilibrium unfolding of both forms follows a similar pattern, with the formation of an unfolding intermediate at sub-molar concentrations of denaturant. These properties exhibited by this βγ-crystallin domain are not shown by any other domain studied so far, demonstrating the diversity in domain properties.

Aggregation-prone near-native intermediate formation during unfolding of a structurally similar nonlenticular βγ-crystallin domain

The folding and unfolding of structurally similar proteins belonging to a family have long been a focus of investigation of the structure-(un)folding relationship. Such studies are yet to reach a consensus about whether structurally similar domains follow common or different unfolding pathways. Members of the βγ-crystallin superfamily, which consists of structurally similar proteins with limited sequence similarity from diverse life forms spanning microbes to mammals, form an appropriate model system for exploring this relationship further. We selected a new member, Crybg3_D3, the third βγ-crystallin domain of non-lens vertebrate protein Crybg3 from mouse brain. The crystal structure determined at 1.86 Å demonstrates that the βγ-crystallin domain of Crybg3 resembles more closely the lens βγ-crystallins than the microbial crystallins do. However, interestingly, this structural cousin follows a quite distinct (un)folding pathway via formation of an intermediate state. The intermediate species is in a nativelike conformation with variation in flexibility and tends to form insoluble aggregates. The individual domains of lens βγ-crystallins (and microbial homologues) do not follow such an unfolding pattern. Thus, even the closest members of a subfamily within a superfamily do not necessarily follow similar unfolding paths, suggesting the divergence acquired by these domains, which could be observed only by unfolding. Additionally, this study provides insights into the modifications that this domain has undergone during its recruitment into the non-lens tissues in vertebrates.

Decoding the molecular design principles underlying Ca(2+) binding to βγ-crystallin motifs

Numerous proteins belonging to the recently expanded βγ-crystallin superfamily bind Ca(2+) at the double-clamp N/D-N/D-X(1)-X(2)-S/T-S motif. However, there have been no attempts to understand the intricacies involving Ca(2+) binding, such as the determinants of Ca(2+)-binding affinity and their contributions to gain in stability. This work is an in-depth analysis of understanding the modes and determinants of Ca(2+) binding to βγ-crystallin motifs. We have performed extensive naturally occurring substitutions from related proteins on the βγ-crystallin domains of flavollin, a low-affinity Ca(2+)-binding protein, and clostrillin, a moderate-affinity protein. We monitored the consequences of these modifications on Ca(2)(+) binding by isothermal titration calorimetry, thermal stability and conformational and crystal structure analyses. We demonstrate that Ca(2)(+) binding to the two sites of a βγ-domain is interdependent and that the presence of Arg at the fifth position disables a site. A change from Thr to Ser, or vice versa, influences Ca(2+)-binding affinity, highlighting the basis of diversity found in these domains. A subtle change in the first site has a greater influence on Ca(2)(+) binding than a similar alteration in the second site. Thus, the second site is more variable in nature. Replacing an acidic or hydrophobic residue in a binding site alters the Ca(2+)-binding properties drastically. While it appears from their binding site sequence that these domains have evolved randomly, our examination illustrates the subtlety in the design of these modules. Decoding such design schemes would aid in our understanding of the functional themes underlying differential Ca(2)(+) binding and in predicting these in emerging sequence information.

Characterization of the βγ-crystallin domains of βγ-CAT, a non-lens βγ-crystallin and trefoil factor complex, from the skin of the toad Bombina maxima

βγ-CAT is a naturally existing 72-kDa complex of a non-lens βγ-crystallin (α-subunit, CAT-α) and a trefoil factor (β-subunit, CAT-β) that contains a non-covalently linked form of αβ(2) and was isolated from the skin secretions of the toad Bombina maxima. The N-terminal region of CAT-α (CAT-αN, residues 1-170) contains two βγ-crystallin domains while the C-terminal region (CAT-αC) has sequence homology to the membrane insertion domain of the Clostridium perfringens epsilon toxin. To examine the biochemical characteristics of the βγ-crystallin domains of βγ-CAT, CAT-αN, CAT-αC and CAT-β were expressed in Escherichia coli. Co-immunoprecipitation of the naturally assembled βγ-CAT confirmed that the CAT-α and CAT-β complex always exists. Furthermore, recombinant CAT-β bound recombinant CAT-αN. Ca(2+)-binding motifs were identified in CAT-αN, and recombinant CAT-αN was able to bind the calcium probe terbium. However, the conformation of CAT-αN was not significantly altered upon Ca(2+) binding. βγ-CAT possesses strong hemolytic activity toward human erythrocytes, and treatment of erythrocytes with βγ-CAT resulted in a rapid Ca(2+) influx, eventually leading to hemolysis. However, in the absence of extracellular Ca(2+), no significant hemolysis was detected, even though the binding and oligomerization of βγ-CAT in the erythrocyte membrane was observed. Our data demonstrate the binding of CAT-β (a trefoil factor) to CAT-αN (βγ-crystallin domains) and provide a basis for the formation of a βγ-crystallin and trefoil factor complex in vivo. Furthermore, the βγ-crystallin domains of βγ-CAT are able to bind Ca(2+), and βγ-CAT-induced hemolysis is Ca(2+) dependent.

Biophysical characterization of the olfactomedin domain of myocilin, an extracellular matrix protein implicated in inherited forms of glaucoma

Myocilin is an eye protein found in the trabecular extracellular matrix (TEM), within the anatomic region that controls fluid flow. Variants of myocilin, localized to its olfactomedin (OLF) domain, have been linked to inherited forms of glaucoma, a disease associated with elevated intraocular pressure. OLF domains have also been implicated in psychiatric diseases and cancers by their involvement in signaling, neuronal growth, and development. However, molecular characterization of OLFs has been hampered by challenges in recombinant expression, a hurdle we have recently overcome for the myocilin OLF domain (myoc-OLF). Here, we report the first detailed solution biophysical characterization of myoc-OLF to gain insight into its structure and function. Myoc-OLF is stable in the presence of glycosaminoglycans, as well as in a wide pH range in buffers with functional groups reminiscent of such glycosaminoglycans. Circular dichroism (CD) reveals significant β-sheet and β-turn secondary structure. Unexpectedly, the CD signature is reminiscent of α-chymotrypsin as well as another ocular protein family, the βγ-crystallins. At neutral pH, intrinsic tryptophan fluorescence and CD melts indicate a highly cooperative transition with a melting temperature of ∼55°C. Limited proteolysis combined with mass spectrometry reveals that the compact core structural domain of OLF consists of approximately residues 238-461, which retains the single disulfide bond and is as stable as the full myoc-OLF construct. The data presented here inform new testable hypotheses for interactions with specific TEM components, and will assist in design of therapeutic agents for myocilin glaucoma.

The betagamma-crystallin superfamily contains a universal motif for binding calcium

The betagamma-crystallin superfamily consists of evolutionarily related proteins with domain topology similar to lens beta- and gamma-crystallins, formed from duplicated Greek key motifs. Ca(2+) binding was found in a few betagamma-crystallin members earlier, although its prevalence and diversity as inherent molecular properties among members of the superfamily are not well studied. To increase our understanding of Ca(2+) binding in various betagamma-crystallins, we undertook comprehensive structural and Ca(2+)-binding studies of seven members of the superfamily from bacteria, archaea, and vertebrates, including determination of high-resolution crystal structures of three proteins. Our structural observations show that the determinants of Ca(2+) coordination remain conserved in the form of an N/D-N/D-#-I-S/T-S motif in all domains. However, binding of Ca(2+) elicits varied physicochemical responses, ranging from passive sequestration to active stabilization. The motif in this superfamily is modified in some members like lens crystallins where Ca(2+)-binding abilities are partly or completely compromised. We show that reduction or loss of Ca(2+) binding in members of the superfamily, particularly in vertebrates, is due to the selective presence of unfavorable amino acids (largely Arg) at key Ca(2+)-ligation positions and that engineering of the canonical Ca(2+)-binding residues can confer binding activity on an otherwise inactive domain. Through this work, we demonstrate that betagamma-crystallins with the N/D-N/D-#-I-S/T-S motif form an extensive set of Ca(2+)-binding proteins prevalent in all of the three kingdoms of life.

Biomarkers for predicting the sensitivity of cancer cells to TRAIL-R1 agonistic monoclonal antibody

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and an agonistic monoclonal antibody to TRAIL-R1 (TRAIL-R1 mAb) induce apoptosis and show anti-proliferative activity in vitro and in vivo. However, some TRAIL-R1-expressing cell lines are not sensitive to either TRAIL-R1 mAb or TRAIL. We have identified four genes (STK17B, SP140L, CASP8, and AIM1) whose expression levels differ significantly between TRAIL-R1 mAb-sensitive and resistant cell lines. Using the expression levels of these genes, we predicted TRAIL-R1 mAb and TRAIL sensitivity in our test cell lines with 75% (9/12) and 84% (21/25) accuracy, respectively. Knockdown of STK17B in TRAIL-R1 mAb-sensitive cells augmented Bcl-2 expression and suppressed TRAIL-R1 mAb-induced apoptosis. Our results may be useful for predicting the response of cancers to TRAIL-agonistic drugs in the clinic.

Three-dimensional domain swapping in nitrollin, a single-domain betagamma-crystallin from Nitrosospira multiformis, controls protein conformation and stability but not dimerization

The betagamma-crystallin superfamily has a well-characterized protein fold, with several members found in both prokaryotic and eukaryotic worlds. A majority of them contain two betagamma-crystallin domains. A few examples, such as ciona crystallin and spherulin 3a exist that represent the eukaryotic single-domain proteins of this superfamily. This study reports the high-resolution crystal structure of a single-domain betagamma-crystallin protein, nitrollin, from the ammonium-oxidizing soil bacterium Nitrosospira multiformis. The structure retains the characteristic betagamma-crystallin fold despite a very low sequence identity. The protein exhibits a unique case of homodimerization in betagamma-crystallins by employing its N-terminal extension to undergo three-dimensional (3D) domain swapping with its partner. Removal of the swapped strand results in partial loss of structure and stability but not dimerization per se as determined using gel filtration and equilibrium unfolding studies. Overall, nitrollin represents a distinct single-domain prokaryotic member that has evolved a specialized mode of dimerization hitherto unknown in the realm of betagamma-crystallins.

Calcium binds to leptospiral immunoglobulin-like protein, LigB, and modulates fibronectin binding

Pathogenic Leptospira spp. express immunoglobulin-like proteins, LigA and LigB, which serve as adhesins to bind to extracellular matrices and mediate their attachment on host cells. However, nothing is known about the mechanism by which these proteins are involved in pathogenesis. We demonstrate that LigBCen2 binds Ca(2+), as evidenced by inductively coupled plasma optical emission spectrometry, energy dispersive spectrometry, (45)Ca overlay, and mass spectrometry, although there is no known motif for Ca(2+) binding. LigBCen2 binds four Ca(2+) as determined by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The dissociation constant, K(D), for Ca(2+) binding is 7 mum, as measured by isothermal titration calorimetry and calcium competition experiments. The nature of the Ca(2+)-binding site in LigB is possibly similar to that seen in the betagamma-crystallin superfamily, since structurally, both families of proteins possess the Greek key type fold. The conformation of LigBCen2 was significantly influenced by Ca(2+) binding as shown by far- and near-UV CD and by fluorescence spectroscopy. In the apo form, the protein appears to be partially unfolded, as seen in the far-UV CD spectrum, and upon Ca(2+) binding, the protein acquires significant beta-sheet conformation. Ca(2+) binding stabilizes the protein as monitored by thermal unfolding by CD (50.7-54.8 degrees C) and by differential scanning calorimetry (50.0-55.7 degrees C). Ca(2+) significantly assists the binding of LigBCen2 to the N-terminal domain of fibronectin and perturbs the secondary structure, suggesting the involvement of Ca(2+) in adhesion. We demonstrate that LigB is a novel bacterial Ca(2+)-binding protein and suggest that Ca(2+) binding plays a pivotal role in the pathogenesis of leptospirosis.

Exploring the limits of sequence and structure in a variant betagamma-crystallin domain of the protein absent in melanoma-1 (AIM1)

Betagamma-crystallins belong to a superfamily of proteins in prokaryotes and eukaryotes that are based on duplications of a characteristic, highly conserved Greek key motif. Most members of the superfamily in vertebrates are structural proteins of the eye lens that contain four motifs arranged as two structural domains. Absent in melanoma 1 (AIM1), an unusual member of the superfamily whose expression is associated with suppression of malignancy in melanoma, contains 12 betagamma-crystallin motifs in six domains. Some of these motifs diverge considerably from the canonical motif sequence. AIM1g1, the first betagamma-crystallin domain of AIM1, is the most variant of betagamma-crystallin domains currently known. In order to understand the limits of sequence variation on the structure, we report the crystal structure of AIM1g1 at 1.9 A resolution. Despite having changes in key residues, the domain retains the overall betagamma-crystallin fold. The domain also contains an unusual extended surface loop that significantly alters the shape of the domain and its charge profile. This structure illustrates the resilience of the betagamma fold to considerable sequence changes and its remarkable ability to adapt for novel functions.

Calcium-binding to lens βB2- and βA3-crystallins suggests that all β-crystallins are calcium-binding proteins

Crystallins are the major proteins of a mammalian eye lens. The topologically similar eye lens proteins, beta- and gamma-crystallins, are the prototype and founding members of the betagamma-crystallin superfamily. Betagamma-crystallins have until recently been regarded as structural proteins. However, the calcium-binding properties of a few members and the potential role of betagamma-crystallins in fertility are being investigated. Because the calcium-binding elements of other member proteins, such as spherulin 3a, are not present in betaB2-crystallin and other betagamma-crystallins from fish and mammalian genomes, it was argued that lens betagamma-crystallins should not bind calcium. In order to probe whether beta-crystallins can bind calcium, we selected one basic (betaB2) and one acidic (betaA3) beta-crystallin for calcium-binding studies. Using calcium-binding assays such as 45Ca overlay, terbium binding, Stains-All and isothermal titration calorimetry, we established that both betaB2- and betaA3-crystallin bind calcium with moderate affinity. There was no significant change in their conformation upon binding calcium as monitored by fluorescence and circular dichroism spectroscopy. However, 15N-1H heteronuclear single quantum correlation NMR spectroscopy revealed that amide environment of several residues underwent changes indicating calcium ligation. With the corroboration of calcium-binding to betaB2- and betaA3-crystallins, we suggest that all beta-crystallins bind calcium. Our results have important implications for understanding the calcium-related cataractogenesis and maintenance of ionic homeostasis in the lens.

A reference dataset for circular dichroism spectroscopy tailored for the βγ-crystallin lens proteins

Circular dichroism (CD) spectroscopy is a powerful solution technique for the study of protein secondary structure. As hierarchical euclidean clustering analyses of high quality crystallin synchrotron radiation circular dichroism (SRCD) spectral data can be separated into structural groups based solely on spectral information, the technique can potentially be improved to more accurately determine secondary structures and monitor conformational changes in crystallins. Secondary structure estimates can be determined through use of reference datasets of circular dichroism spectra from proteins with determined crystal structures. As with any empirical method, the accuracies of the analyses are dependent upon how closely the reference dataset characteristics match those of the protein to be studied. To date, crystallin proteins have not been well analysed by CD because existing reference datasets do not contain good representations of their structural characteristics. This work describes a betagamma-crystallin specific reference dataset, CRYST175, which was created solely for the study of betagamma-crystallin secondary structures. Prediction accuracy was assessed for the new dataset using several deconvolution algorithms and it was found to substantially outperform existing more general reference datasets.

Age-related changes in cochlear gene expression in normal and shaker 2 mice

The vertebrate cochlea is a complex organ optimized for sound transduction. Auditory hair cells, with their precisely arranged stereocilia bundles, transduce sound waves to electrical signals that are transmitted to the brain. Mutations in the unconventional myosin XV cause deafness in both human DFNB3 families and in shaker 2 (sh2) mice as a result of defects in stereocilia. In these mutant mice, hair cells have relatively normal spatial organization of stereocilia bundles but lack the graded, stair-step organization. We used sh2 mice as an experimental model to investigate the molecular consequences of the sh2 mutation in the Myo15 gene. Gene expression profiling with Affymetrix GeneChips in deaf homozygous (sh2/sh2) mice at 3 weeks and 3 months of age, and in age-matched, normal-hearing heterozygotes (+/sh2) identified only a few genes whose expression was affected by genotype, but a large number with age-associated changes in expression in both normal mice and sh2/sh2 homozygotes. Microarray data analyzed using Robust Multiarray Average identified Aim1, Dbi, and Tm4sf3 as genes with increased expression in sh2/sh2 homozygotes. These increases were confirmed by quantitative reverse transcription-polymerase chain reaction. Genes exhibiting altered expression with age encoded collagens and proteins involved in collagen maturation, extracellular matrix, and bone mineralization. These results identified potential cellular pathways associated with myosin XV defects, and age-associated molecular events that are likely to be involved in maturation of the cochlea and auditory function.

Identification of crystallin family proteins in vitreous body in rat endotoxin-induced uveitis: Involvement of crystallin truncation in uveitis pathogenesis

Endotoxin-induced uveitis (EIU) is an animal model of acute ocular inflammation. To characterize the mechanism of EIU, we analyzed the infiltration of proteins in the vitreous bodies of rats with EIU and normal rats using 2-DE and micro LC/LC-MS/MS. Twenty spots were identified in vitreous bodies of rats. Eighteen of these spots were members of the crystallin family. The truncated form of beta A4- and beta B2-crystallin were predominant in normal vitreous bodies, but there were intact form of crystallins in lipopolysaccharide-injected rats with EIU. These results suggest that crystallin family proteins are the major group of proteins involved in uveitic vitreous and that C-terminal truncation of beta-crystallins may play a role in EIU-related disease progression.

Crystallization and preliminary X-ray crystallographic investigations on a betagamma-crystallin domain of absent in melanoma 1 (AIM1), a protein from Homo sapiens

AIM1g1 is a single betagamma-crystallin domain from the protein absent in melanoma 1 (AIM1), which appears to play a role in the suppression of melanomas. This domain is known to bind calcium and its structure would help in identifying calcium-coordinating sites in vertebrate crystallins, which have hitherto been believed to have lost this ability during evolution. Crystallization of this domain was performed by the hanging-drop vapour-diffusion method. Crystals diffracted to a maximum resolution of 1.86 A and were found to belong to space group P6(1) or P6(5), with unit-cell parameters a = b = 54.98, c = 59.73 A. Solvent-content analysis indicated the presence of one monomer per asymmetric unit.

Transparency and non-refractive functions of crystallins--a proposal

Based on the premise that all crystallins have cellular and metabolically relevant catalytic activities, we propose that aberrant changes in non-crystallin (non-refractive) functions presage the appearance of cataractous pathologies in an otherwise highly stable edifice of transparency. This proposal is based on accumulating evidence from developmental, molecular and genetic studies that have established that crystallins are more than inanimate building blocks of the transparent lens fiber mass. The published work does not support the perceived dichotomy in the relevance of crystallin function (as essential) and non-crystallin function (as either of secondary importance or not essential at all), to the emergence and maintenance of the phenotype of transparency. A number of crystallin mutations have stage-specific phenotypes at developmental times when their concentrations have not reached 'crystallin' (high) proportions. There is heterogeneity in the cataract phenotypes associated with similar or identical mutations in different populations; the cataracts have disparate phenotypes even when the mutations are in the same gene. These data suggest that non-crystallin function is not merely a non-lens activity of a crystallin but an essential requirement within the lens itself.

Calcium-binding Crystallins from Yersinia pestis

Betagamma-crystallin is a superfamily with diverse members from vertebrate lens to microbes. However, not many members have been identified and studied. Here, we report the identification of a putative exported protein from Yersinia pestis as a member of the betagamma-crystallin superfamily. Even though calcium has been known to play an important role in the physiology and virulence of the Yersinia genus, calcium-binding proteins have not yet been identified. We have studied the calcium-binding properties of two of the three crystallin domains present in this putative exported protein designated "Yersinia crystallin." These two domains (D1 and D2) have unique AA and BB types of arrangement of their Greek key motifs unlike the domains of other members of the betagamma-crystallin superfamily, which are either AB or BA types. These domains bind two calcium ions with low and high affinity-binding sites. We showed their calcium-binding properties using various probes for calcium and the effect of calcium on their secondary and tertiary structures. Although both domains bind calcium, D1 underwent drastic changes in secondary and tertiary structure and hydrodynamic volume upon calcium binding. Domain D1, which is intrinsically unstructured in the apo form, requires calcium for the typical betagamma-crystallin fold. Calcium exerted an extrinsic stabilization effect on domain D1 but not on D2, which is also largely unstructured. We suggest that this protein might be involved in calcium-dependent processes, such as stress response or physiology in the Yersinia genus, similar to its microbial relatives and mammalian lens crystallins.