Preliminary x-ray crystallographic study of the principal subunit of the lens structural protein, bovine beta-crystallin

Close packing of an oligomeric eye lens beta-crystallin induces loss of symmetry and ordering of sequence extensions

beta-Crystallins are oligomeric eye lens proteins that are related to monomeric gamma-crystallins. The main sequence difference between the two families is the presence of sequence extensions in the beta-crystallins. A major question concerns the role that these extensions play in mediating interactions at the high protein concentrations found in the lens. The predominant beta-crystallin polypeptide, beta B2, can be crystallized in two different space groups, I222 and C222. The I222 crystal structure revealed that the protein packed as a tetramer with perfect 222 symmetry but that the extensions were disordered. The X-ray structure of the C222 lattice of beta B2 has now been refined at 3.3 A, the structure analysed and compared with the I222 lattice. The protein is also a tetramer with 222 symmetry in the C222 lattice but differs in that parts of the N-terminal extensions have been visualized. In the asymmetric unit of the C222 lattice there are four subunits, each comprising a single polypeptide chain, in which certain flexible loops in the N-terminal domains and the N-terminal extensions have various conformations. The tetramers in the C222 lattice are more tightly packed than in the I222 form. Analysis of the tetramer contacts shows that the sites of interaction break the 222 symmetry of the tetramers. The N-terminal extensions play a major role in directing interactions between tetramers. One of the N-terminal extensions interacts with a hydrophobic patch on the N-terminal domain of another tetramer. These crystallographic observations obtained over a physiological concentration range indicate how, in beta-crystallin oligomers, the N-terminal extensions of beta B2 can switch from interacting with water to interacting with protein depending on their relative concentrations. This could be useful in maintaining a gradient of refractive index.

Estimation of the secondary structure and conformation of bovine lens crystallins by infrared spectroscopy: quantitative analysis and resolution by Fourier self-deconvolution and curve fit

The secondary structure of six bovine lens protein fractions (two alpha, three beta and one gamma-crystallin) are examined in solution and in solid forms for the first time using FTIR spectroscopy. Films of the nuclear and cortical regions of the bovine lens are also examined. The structure is quantitatively estimated from the vibrational analysis of the resolution-enhanced amide-I profile achieved by Fourier self-deconvolution and linear least-squares curve-fit algorithm. All the protein fractions fold predominantly in a beta-pleated sheet structure with little or no alpha-helical domains in solution or in lyophilized solid form. These proteins also retain their predominant beta-sheet conformation in the cellular phospholipid environment of the lens, in conformity with the structure obtained for all the mammalian species examined to date. Despite structural homology, vibrational data indicate subtle structural differences within each class of the crystallins probably due to presence of several minor substructures/subconformations. Substantial high amounts of turns (approx. 40%) observed in the beta-fractions may have a fundamental implication in stabilizing the tertiary structure of the uniquely folded-proteins vital for the transparency of the lens. These proteins in solid KBr-matrix undergo a major structural change, induced primarily by ionic interactions which refold them in a helical conformation. IR spectroscopy together with band-narrowing procedures has proven to be an effective tool to obtain structural information of proteins in solution, as solid substrates or in a complex biological tissue, such as ocular lens.

Alteration of dynamic quaternary structure and calcium-binding ability of beta-crystallin by light

beta-crystallin, one of the three main constituent proteins of the eye lens, exists as an equilibrium population of oligomeric (beta H), trimeric (beta L1) and dimeric (beta L2) species. This equilibrium is dependent on various factors such as the protein concentration, ionic strength and pH of the medium. We have studied the effect of ultraviolet B radiation on the aggregational patterns of beta-crystallin, using size-exclusion chromatography. Irradiation of a solution of beta H-crystallin at 295 nm for about 30 min causes the deaggregation of the hexameric population into dimers. Irradiation for a longer time, however, produces cross-linked high molecular weight products. Irradiation of a beta L2 solution for 30 min does not perturb the elution profile, while irradiation for a longer time increases the content of beta L1 (trimeric) crystallin. Irradiation also causes a decrease in the calcium-binding affinity of the beta-crystallins.

High resolution structure of an oligomeric eye lens beta-crystallin. Loops, arches, linkers and interfaces in beta B2 dimer compared to a monomeric gamma-crystallin

beta-Crystallins are polydisperse, oligomeric structural proteins that have a major role in forming the high refractive index of the eye lens. Using single crystal X-ray crystallography with molecular replacement, the structure of beta B2 dimer has been solved at 2.1 A resolution. Each subunit comprises an N and C-terminal domain that are very similar and each domain is formed from two similar "Greek key" motifs related by a local dyad. Sequence differences in the internally quadruplicated molecules, analysed in terms of their beta-sheets, hairpins and arches, give rise to structural differences in the motifs. Whereas the related family of gamma-crystallins are monomers, beta-crystallins are always oligomers. In the beta B2 subunit, the domains, each comprising two motifs, are separated by an extended linking peptide. A crystallographic 2-fold axis relates the two subunits of the dimer so that the N-terminal domain of one subunit of beta B2 and the C-terminal domain of the symmetry-related subunit are topologically equivalent to the two covalently connected domains of gamma B-crystallin. The intersubunit domain interface is very similar to the intradomain interface of gamma B, although many sequence differences have resulted in an increase in polar interactions between domains in beta B2. Comparison of the structures of beta B2 and gamma B-crystallins shows that the two families differ largely in the conformation of their connecting peptides. A further extensive lattice contact indicates a tetramer with 222 symmetry. The ways in which insertions and extensions in the beta-crystallin effect oligomer interactions are described. The two kinds of crystallin are analysed for structural features that account for their different stabilities. These studies are a basis for understanding formation of higher aggregates in the lens.

Near-infrared Fourier transform Raman and conventional Raman studies of calf gamma-crystallins in the lyophilized state and in solution

We present in this report a detailed structural study of calf gamma-crystallins both in the solid state and in solution by the newly developed technique of near-infrared (IR) Fourier transform (FT)-Raman spectroscopy as well as by the conventional Raman method. In comparison with conventional laser Raman spectroscopy, the near-IR FT-Raman approach exhibits several attractive features such as fluorescence rejection capability, frequency accuracy, and the FT's multiplex and throughput advantages. These distinct characteristics combined form the basis for the particular suitability of FT-Raman in crystallin structural analysis and elucidation. We have thus obtained evidence in support of the view that native calf gamma-II crystallin does not contain a disulfide bond either in the lyophilized state or in solution. In addition, conventional Raman spectra are examined for all four gamma-crystallin fractions. gamma-S, gamma-II, gamma-III, and gamma-IV, and the results indicate a high degree of structural similarities among them. It is also found that the sulfhydryl groups in all four gamma-crystallins are highly resistant to air oxidation and are capable of maintaining their reduced state during isolation in the absence of added reductants or such chelating agents as EDTA.

Rapid separation of bovine beta-crystallin subunits beta B1, beta B2, beta B3, beta A3 and beta A4

Bovine beta-crystallin aggregates, beta H-, beta L1- and beta L2-crystallins, prepared by rapid gel filtration, are each subjected to anion-exchange chromatography in deaggregating media using a Pharmacia Fast Protein Liquid Chromatography System. beta B1, beta B2 and beta A4 subunits are rapidly isolated using a one step Mono Q column from beta H-, beta L2- and beta L1-crystallin, respectively. beta B3 and beta A3 are separated from each other using a second Mono Q column starting from beta L2- and beta L1-crystallin respectively. Whereas beta B2, beta B3 and beta A4 are common to all sizes of aggregate, beta B1 is restricted to beta H-crystallin and beta A3 is absent from beta L2-crystallin.

Crystallization of a new form of the eye lens protein beta B2-crystallin

A new crystal form of the bovine oligomeric lens protein beta B2 has been grown in the presence of calcium acetate. The crystals are orthorhombic, I222 or I2(1)2(1)2(1), with cell dimensions a = 77.8 A, b = 83.6 A, c = 109.2 A. This new crystal form, which diffracts to at least 2.5 A, has a and b cell dimensions that are half those of the original crystal form, although there is no simple relationship between the c cell dimensions. The new crystal form reported here contains only one subunit per asymmetric unit, indicating that the dimer lies on a crystallographic 2-fold axis, and is a suitable candidate for molecular replacement studies.

Physicochemical characterization of beta-crystallins from bovine lenses: hydrodynamic and aggregation properties

A detailed investigation of the hydrodynamic and aggregation behaviors has been made on the beta-crystallins of bovine lens. Results from this study indicated that beta H (high-molecular-weight beta-crystallin) and beta L (low-molecular-weight beta-crystallin) exhibited considerable heterogeneity in their native structures and subunit polypeptides. Low-speed sedimentation equilibrium showed a heterogeneous paucidisperse system in each beta-crystallin fraction. Viscosity and circular dichroism studies pointed to a compact and globular shape and the presence of beta-sheet and beta-turns in these crystallins. Dissociation of beta H by urea and guanidinium HCl followed by reassociation during gel-filtration chromatography produced an elution pattern with two fractions corresponding to beta L crystallin and high-molecular-weight aggregates without the formation of native beta H. By contrast, under similar treatment, about 60% beta L reassociated into the correct native structure and the rest into high-molecular-weight fractions. Amino acid analyses of beta H and beta L and their corresponding subunit polypeptides demonstrated the close similarity of these crystallins. Trace element analyses indicated that both Ca and Mg are present in beta H and beta L crystallins and may be involved in maintaining the native quarternary structures of these proteins.

Heat-induced changes in the conformation of alpha- and beta-crystallins: unique thermal stability of alpha-crystallin

Of the crystallin proteins of the lens, the principal subunit of the beta-crystallin, beta B2 (beta Bp), has been considered to be the only heat-stable protein because it does not precipitate upon heating. In our recent investigations, however, we have found that the alpha-crystallin from bovine lenses is not only heat stable but also does not denature at temperatures up to 100 degrees C. Using circular dichroism and fluorescence to monitor the conformational changes of alpha- and beta B2-crystallins upon heating, we found that alpha-crystallin maintains a high degree of structure, whereas the beta B2-crystallin shows a reversible sigmoidal order-disorder transition at about 58 degrees C.

Evolutionary and functional relationships between the basic and acidic beta-crystallins

beta-Crystallins are complex oligomers composed of many related subunits. In order to understand their interactions we have built molecular models of several bovine beta-crystallins, based on their sequence similarity to the well-defined gamma-II crystallin structure, using interactive computer graphics techniques. Their common origin with gamma-crystallin is displayed in both the retention of four-fold sequence repeats of critical residues involved with stabilizing a folded beta-hairpin and the conservation of core-filling hydrophobic side-chains. The beta-crystallins have been built as bilobal molecules with each domain composed of two 'Greek key' motifs which associate about an approximate two-fold axis to form beta-sheets. The beta-crystallin sequences have previously been shown to comprise two families, the basic and acidic subunits, which have extensions of sequence. The three-dimensional models show how the two families appear to stabilize the folded beta-hairpin in the N- and C-terminal domains in ways which suggest that they have diverged from a common ancestor in different ways. Acidic beta-crystallins, like gamma-crystallins, have a regular array of charges on their N-terminal domain which has been interrupted in basic beta-crystallins by hydrophobic residues which may be related to the presence of a C-terminal extension. beta-Crystallins are more highly charged than gamma-crystallins although their charge density is higher in certain regions of the N-terminal domain, particularly in beta B1-crystallin. beta-crystallins also differ from gamma-crystallins in the virtual absence of core-filling sulphydryl groups whereas they have numerous sulphur-containing side-chains together with tryptophan and histidine rings protruding from the globular domains, particularly in the acidic subunits. The burial of these residues in subunit contacts is consistent with their spectroscopic and electrostatic properties. Protein subunit aggregation commonly occurs through hydrophobic interaction or beta-sheet extension. Analysis of the subunit surfaces has identified an N-terminal hydrophobic region common to beta B1 and beta B2 whereas a C-terminal hydrophobic loop region is common to beta B1 and beta A1 and may be correlated with their association properties. It is suggested that the polar C-terminal domain of beta B2 contributes towards the solubility of higher aggregates by interactions involving beta-sheet structure.

Partial sequence homology of human myc oncogene protein to beta and gamma crystallins

The human cellular myc gene is one of about 20 cellular oncogenes which code for a variety of proteins including protein kinases and growth factors. The human gene is related to the avian myelocytomatosis leukaemia virus MC29 and produces a binding protein which may be involved in regulation of gene expression and cellular differentiation and proliferation. The crystallins are proteins in the eye lens synthesised at different stages of cell differentiation and proliferation, and whose short range order is necessary for lens transparency. Computer-based sequence comparisons show that beta Bp and gamma II crystallins, which show partial sequence homology and conservation of 'Greek Key' motives are also partially homologous to two regions on the human myc protein, though this protein probably does not conserve the 'Greek Key' structural motives.

Aggregation behavior of the bovine beta-crystallin Bp chain studied by limited proteolysis

The bovine beta-crystallin Bp chain is organized into two very similar domains, with short extensions at both N- and C-termini, and two alternative models for the beta Bp dimer have been proposed (Wistow, G., Slingsby, C., Blundell, T., Driessen, H.P.C., De Jong, W.W. and Bloemendal, H. (1981) FEBS Lett. 133, 9-16). By limited proteolysis the C-terminal arms can be cleaved off rapidly from the beta Bp dimer, while the N-terminal arms are more difficult to remove. Trypsin divides the beta Bp chain into two fragments which approximately correspond to the two structural domains. Dissociation and reassociation of the different products of limited proteolysis indicated that: the C-terminal arm extends freely from the surface and is not involved in subunit-contact; at least one N-terminal arm seems required for dimer formation; the N-terminal domains have a greater tendency to associate than the C-terminal domains and, when mixed, the purified domains reassociate partially to a Mr 50 000 structure like native beta Bp. These findings support the more extended dimer model of beta Bp.

Structural homology of lens crystallins. III. Secondary structure estimation from circular dichroism and prediction from amino acid sequences

Circular dichroism spectra (196-240 nm) of calf alpha-, beta H-, beta L- and gamma-crystallins were measured and analyzed over the entire wavelength range with five curve-fitting procedures for estimating protein secondary structure. For gamma-crystallin the estimates are in good agreement with the X-ray structure. For all four crystallins the estimates are very similar: 0-9% alpha-helix and 51-68% beta-sheet. This is in accordance with the three-dimensional homology of beta Bp- and gamma 2-crystallin polypeptide chains as postulated from their 30% sequence homology, and suggests that alpha A- and alpha B-crystallin chains may also have a corresponding structure. Secondary structure elements in the four amino acid sequences were predicted using two different comprehensive prediction methods. For gamma 2-crystallin the predictions of beta-sheet are in good agreement with the X-ray structure and with circular dichroism estimates. For beta Bp-crystallin only the C-terminal domain secondary structure predictions are considered satisfactory, which possibly relates to the proposed role of the N-terminal domain in subunit interactions. The combined predictions for alpha A- and alpha B-chains (3% helix, 49% sheet) are in excellent agreement with circular dichroism. Moreover, the good alignment of predicted beta-sheet segments in alpha-crystallin chains with known beta-sheet strands in gamma 2- (and presumably beta Bp-) crystallin strongly supports a similar 4-motif folding pattern in all four calf crystallin chains.

X-ray analysis of the eye lens protein gamma-II crystallin at 1.9 A resolution

We report the X-ray structure analysis and refinement at 1.9 A resolution of calf gamma-II crystallin, a lens-specific protein. The sequence of Croft (1972) has been modified to give a polypeptide chain of 174 residues (cf. 165). The protein has a symmetrical, hierarchical structure of two globular domains each comprising two similar "Greek key" motifs, consecutive along the polypeptide chain, and related by a pseudo 2-fold axis. The two domains pack together with a single connection and are related by a further pseudo 2-fold axis which bisects the angle between the intra-domain dyads. Forty-two pairs of C alpha positions for the two most similar motifs have root-mean-square separation at best fit of 0.69 A. The N and C-terminal domains gave root-mean-square separation of 0.89 A for 82 pairs of C alpha atoms at best fit. In each domain the two Greek key motifs form a pair of four-stranded antiparallel beta-pleated sheets, each sheet composed of three stands from one motif and one from the other. The sheets pack together in a wedge shape, closed at the top by the loops connecting the third and fourth strands of each motif. The first two strands of each motif form an extended beta-hairpin which is folded on to the beta-sheet. The packing of each motif into the globular domains involves a staggered bilayer of side-chains between each pair of beta-sheets which does not preserve the pseudo 2-fold axes observed in the C alpha position topology. In the core of each domain there are interactions between polarizable aromatic groups and sulphur-containing residues which may contribute to stability and may also serve to protect aromatic side-chains from ultraviolet light damage in the lens. At the surface of the molecule over half the ionic side-chains are closely paired, which probably stabilizes the tertiary fold and may reduce the water bound. Crystal lattice interactions are described which may be similar to those occurring in vivo in the lens between crystallins. Seven cysteine residues have been identified in the structure and these may have a role in the thermodynamic stability of the molecule, its intermolecular interactions under the normal reducing conditions of the lens, and also in the aggregation and cross-linking which occur in some forms of cataract. Three of these residues, Cys18, Cys23 and Cys74, form a cluster in the N-terminal domain.(ABSTRACT TRUNCATED AT 400 WORDS)

beta-Crystallin: endogenous substrate of lens transglutaminase. Characterization of the acyl-donor site in the beta Bp chain

Incubation of calf lens cortex homogenate with [14C]putrescine or dansylcadaverine, followed by two-dimensional gel electrophoresis and fluorography, enabled the identification of three different beta-crystallin chains as the endogenous substrates of Ca2+-dependent lens transglutaminase (R-glutaminyl-peptide:amine-gamma-glutamylyltransferase, EC 2.3.2.13). One of these is beta Bp, the predominant subunit of beta-crystallin, of which the amino acid sequence is known. The site of amine-labeling in beta Bp could be located, by limited proteolysis, in the N-terminal domain of this chain. Tryptic digestion of the N-terminal domain and subdigestion with elastase of the N-terminal tryptic peptide identified glutamine-7 as the single residue to which the amines are bound. This is the first example of an endogenous substrate of intracellular transglutaminase in which the site of the acyl-donor glutamine residue has been established. Tryptic digestion of the putrescine-labeled beta-crystallin aggregate, followed by high-voltage paper electrophoresis, provided a preliminary characterization of the labeled peptides originating from the other two labeled beta subunits.

Primary gene products of bovine beta-crystallin and reassociation behavior of its aggregates

beta-Crystallin from calf lens cortex was fractionated in three different aggregates of increasing size: beta L2- beta L1 and beta H. of which the subunit composition was revealed by 2-dimensional gel electrophoresis. While beta L2 mainly consists of beta Bp (the major polypeptide chain in all three aggregates). beta L1 is characterized by the addition of a neutral and two acidic chains, and beta H contains moreover two basic chains. Translation of calf lens polyribosomes in a reticulocyte cell-free system allowed the identification of six beta-crystallin subunits as primary gene products. The distribution of these newly synthesized polypeptides over the three aggregates was established after gel filtration in the presence of carrier lens proteins. The aggregation behavior of the beta-crystallin chains was studied by dissociation reassociation experiments. The three separate aggregates could be reversibly dissociated. Reassociation of basic, neutral and acidic polypeptides, isolated by ion-exchange chromatography of beta-crystallin, produced a beta H-like aggregate. The neutral and acidic polypeptides reassociated into a beta L1-like aggregate, while the neutral polypeptides gave dimers like beta L2. A beta H-like aggregate could also be obtained by reaggregation of beta L2 with the acidic and basic chains of beta H. On the basis of these results a preliminary model for the formation of beta-crystallin aggregates is discussed.