Crystal structure of R120G disease mutant of human αB-crystallin domain dimer shows closure of a groove.

Small heat shock proteins form large cytosolic assemblies from an "α-crystallin domain" (ACD) flanked by sequence extensions. Mutation of a conserved arginine in the ACD of several human small heat shock protein family members causes many common inherited diseases of the lens and neuromuscular system. The mutation R120G in αB-crystallin causes myopathy, cardiomyopathy and cataract. We have solved the X-ray structure of the excised ACD dimer of human αB R120G close to physiological pH and compared it with several recently determined wild-type vertebrate ACD dimer structures. Wild-type excised ACD dimers have a deep groove at the interface floored by a flat extended "bottom sheet." Solid-state NMR studies of large assemblies of full-length αB-crystallin have shown that the groove is blocked in the ACD dimer by curvature of the bottom sheet. The crystal structure of R120G ACD dimer also reveals a closed groove, but here the bottom sheet is flat. Loss of Arg120 results in rearrangement of an extensive array of charged interactions across this interface. His83 and Asp80 on movable arches on either side of the interface close the groove by forming two new salt bridges. The residues involved in this extended set of ionic interactions are conserved in Hsp27, Hsp20, αA- and αB-crystallin sequences. They are not conserved in Hsp22, where mutation of the equivalent of Arg120 causes neuropathy. We speculate that the αB R120G mutation disturbs oligomer dynamics, causing the growth of large soluble oligomers that are toxic to cells by blocking essential processes.

Crystal structures of alpha-crystallin domain dimers of alphaB-crystallin and Hsp20.

Small heat shock proteins (sHsps) are a family of large and dynamic oligomers highly expressed in long-lived cells of muscle, lens and brain. Several family members are upregulated during stress, and some are strongly cytoprotective. Their polydispersity has hindered high-resolution structure analyses, particularly for vertebrate sHsps. Here, crystal structures of excised alpha-crystallin domain from rat Hsp20 and that from human alphaB-crystallin show that they form homodimers with a shared groove at the interface by extending a beta sheet. However, the two dimers differ in the register of their interfaces. The dimers have empty pockets that in large assemblies will likely be filled by hydrophobic sequence motifs from partner chains. In the Hsp20 dimer, the shared groove is partially filled by peptide in polyproline II conformation. Structural homology with other sHsp crystal structures indicates that in full-length chains the groove is likely filled by an N-terminal extension. Inside the groove is a symmetry-related functionally important arginine that is mutated, or its equivalent, in family members in a range of neuromuscular diseases and cataract. Analyses of residues within the groove of the alphaB-crystallin interface show that it has a high density of positive charges. The disease mutant R120G alpha-crystallin domain dimer was found to be more stable at acidic pH, suggesting that the mutation affects the normal dynamics of sHsp assembly. The structures provide a starting point for modelling higher assembly by defining the spatial locations of grooves and pockets in a basic dimeric assembly unit. The structures provide a high-resolution view of a candidate functional state of an sHsp that could bind non-native client proteins or specific components from cytoprotective pathways. The empty pockets and groove provide a starting model for designing drugs to inhibit those sHsps that have a negative effect on cancer treatment.

A reference dataset for circular dichroism spectroscopy tailored for the betagamma-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.

The P23T cataract mutation causes loss of solubility of folded gammaD-crystallin.

Mutations in the human gammaD-crystallin gene have been linked to several types of congenital cataracts. In particular, the Pro23 to Thr (P23T) mutation of human gammaD crystallin has been linked to cerulean, lamellar, coralliform, and fasciculiform congenital cataracts. We have expressed and purified wild-type human gammaD, P23T, and the Pro23 to Ser23 (P23S) mutant. Our measurements show that P23T is significantly less soluble than wild-type human gammaD, with P23S having an intermediate solubility. Using synchrotron radiation circular dichroism spectroscopy, we have determined that the P23T mutant has a slightly increased content of beta-sheet, which may be attributed to the extension of an edge beta-strand due to the substitution of Pro23 with a residue able to form hydrogen bonds. Neither of the point mutations appears to have reduced the thermal stability of the protein significantly, nor its resistance to guanidine hydrochloride-induced unfolding. These results suggest that insolubility, rather than loss of stability, is the primary basis for P23T congenital cataracts.

Characterization of the G91del CRYBA1/3-crystallin protein: a cause of human inherited cataract.

Congenital cataract is a leading cause of visual disability in children. Inherited isolated (non-syndromic) cataract represents a significant proportion of cases and the identification of genes responsible for inherited cataract will lead to a better understanding of the mechanism of cataract formation at the molecular level both in congenital and age-related cataract. Crystallins are abundantly expressed in the developing human lens and represent excellent candidate genes for inherited cataract. A genome-wide search of a five-generation family with autosomal dominant lamellar cataract demonstrated linkage to the 17p12-q11 region. Screening of the CRYBA1/3 gene showed a 3 bp deletion, which resulted in a G91del mutation within the tyrosine corner, that co-segregated with disease and was not found in 96 normal controls. In order to understand the molecular basis of cataract formation, the mutant protein was expressed in vitro and its unfolding and refolding characteristics assessed using far-UV circular dichroism spectroscopy. Defective folding and a reduction in solubility were found. As the wild-type protein did not refold into the native conformation following unfolding, a corresponding CRYBB2 mutant was genetically engineered and its refolding characteristics analysed and compared with wild-type CRYBB2. Its biophysical properties support the hypothesis that removal of the glycine residue from the tyrosine corner impairs the folding and solubility of beta-crystallin proteins. This study represents the first comprehensive description of the biophysical consequences of a mutant beta-crystallin protein that is associated with human inherited cataract.

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

Crystallins are bulk structural proteins of the eye lens that have to last a life time. They gradually become modified with age, denature and form light scattering centres. High thermodynamic and kinetic stability of the crystallins enables them to resist unfolding and delay cataract. Here we have made recombinant human betaA1-, betaA3-, and betaA4-crystallins. The betaA3-crystallin formed higher oligomers that lead to precipitation at ambient temperature. Heat-induced precipitation of betaA3-crystallin was compared with human and calf betaB2-crystallins, showing that the human proteins start to precipitate above 50 degrees C while the calf betaB2-crystallin stays in solution even when unfolded. The stabilities of these human acidic beta-crystallin homo-oligomers have been estimated by measuring their unfolding in urea at neutral pH. BetaA3/1/betaB1 and betaA4/betaB1-crystallin hetero-oligomers have been prepared from homo-oligomers by subunit exchange. The resolution of the methodology used was insufficient to detect a stabilization of the betaA4-crystallin subunit in the hetero-oligomer, the betaA1-crystallin subunit was clearly stabilized by its interaction with betaB1-crystallin. Circular dichroism and fluorescence spectroscopies show that homo-dimer surface tryptophans become buried in the betaA3/1/betaB1-crystallin hetero-dimer concomitant with changes in polypeptide chain conformation.

Gamma-D crystallin gene (CRYGD) mutation causes autosomal dominant congenital cerulean cataracts.

Congenital cataracts are a major cause of bilateral visual impairment in childhood. We mapped the gene responsible for autosomal congenital cerulean cataracts to chromosome 2q33-35 in a four generation family of Moroccan descent. The maximum lod score (7.19 at recombination fraction theta=0) was obtained for marker D2S2208 near the gamma-crystallin gene (CRYG) cluster. Sequencing of the coding regions of the CRYGA, B, C, and D genes showed the presence of a heterozygous C>A transversion in exon 2 of CRYGD that is associated with cataracts in this family. This mutation resulted in a proline to threonine substitution at amino acid 23 of the protein in the first of the four Greek key motifs that characterise this protein. We show that although the x ray crystallography modelling does not indicate any change of the backbone conformation, the mutation affects a region of the Greek key motif that is important for determining the topology of this protein fold. Our data suggest strongly that the proline to threonine substitution may alter the protein folding or decrease the thermodynamic stability or solubility of the protein. Furthermore, this is the first report of a mutation in this gene resulting in autosomal dominant congenital cerulean cataracts.

Crystal structure of eta-crystallin: adaptation of a class 1 aldehyde dehydrogenase for a new role in the eye lens.

Eta-crystallin is a retinal dehydrogenase that has acquired a role as a structural protein in the eye lens of elephant shrews, members of an ancient order of mammals. While it retains some activity toward retinal, which is oxidized to retinoic acid, the protein has acquired a number of specific sequence changes that have presumably been selected to enhance the lens role. The crystal structure of eta-crystallin, in common with class 1 and 2 ALDHs, is a dimer of dimers. It has a better-defined NAD binding site than those of related mammalian ALDH1 enzymes with the cofactor bound in the "hydride transfer" position in all four monomers with small differences about the dimer dyads. Although the active site is well conserved, the substrate-binding site is larger in eta-crystallin, and there are some mutations to the substrate access tunnel that might affect binding or release of substrate and product. It is possible that eta-crystallin has lost flexibility to improve its role in the lens. Enhanced binding of cofactor could enable it to act as a UV/blue light filter in the lens, improving visual acuity. The structure not only gives a view of a "natural mutant" of ALDH1 illustrating the adaptive conflict that can arise in multifunctional proteins, but also provides a well-ordered NAD binding site structure for this class of enzymes with important roles in development and health.

Crystal structure and assembly of a eukaryotic small heat shock protein.

The 2.7 A structure of wheat HSP16.9, a member of the small heat shock proteins (sHSPs), indicates how its alpha-crystallin domain and flanking extensions assemble into a dodecameric double disk. The folding of the monomer and assembly of the oligomer are mutually interdependent, involving strand exchange, helix swapping, loose knots and hinged extensions. In support of the chaperone mechanism, the substrate-bound dimers, in temperature-dependent equilibrium with higher assembly forms, have unfolded N-terminal arms and exposed conserved hydrophobic binding sites on the alpha-crystallin domain. The structure also provides a model by which members of the sHSP protein family bind unfolded substrates, which are involved in a variety of neurodegenerative diseases and cataract formation.

Lens crystallins and their microbial homologs: structure, stability, and function.

abg-Crystallins are the major protein components in the vertebrate eye lens--a as a molecular chaperone and b and g as structural proteins. Surprisingly, the latter two share some structural characteristics with a number of microbial stress proteins. The common denominator is not only the Greek key topology of their polypeptide chains but also their high intrinsic stability, which, in certain microbial crystallin homologs, is further enhanced by high-affinity Ca2+-binding. Recent studies of natural and mutant vertebrate bg-crystallins as well as spherulin 3a from Physarum polycephalum and Protein S from Myxococcus xanthus allowed the correlation of structure and stability of crystallins to be elucidated in some detail. From the thermodynamic point of view, stability increments come from (1) local interactions involved in the close packing of the cooperative units, (2) the all-b secondary structure of the Greek-key motif, (3) intramolecular interactions between domains, (4) intermolecular domain interactions, including 3D domain swapping and (v) excluded volume effects due to "molecular crowding" at the high cellular protein concentrations. Apart from these contributions to the Gibbs free energy of stability, significant kinetic stabilization originates from the high activation energy barrier determining the rate of unfolding from the native to the unfolded state. From the functional point of view, the high stability is responsible for the long-term transparency of the eye lens, on the one hand, and the stress resistance of the microorganisms in their dormant state on the other. Local structural perturbations due to chemical modification, wrong protein interactions, or other irreversible processes may lead to protein aggregation. A leading cataract hypothesis is that only after a-crystallin, a member of the small heat-shock protein family, is titrated out does pathological opacity occur. Understanding the structural basis of protein stability in the healthy eye lens is the route to solve the enormous medical and economical problem of cataract.

Association behaviour of human betaB1-crystallin and its truncated forms.

betaB1-crystallin plays an important role in the assembly of betaH-crystallin yet is known to be subject to N-terminal sequence truncations during human lens development and ageing. Here we have over-expressed human betaB1-crystallin, and various truncated forms in Escherichia coli and used mass spectrometry to monitor the monomer molecular weight. Gel permeation chromatography and laser light scattering have been used to estimate the assembly size of the various polypeptides as a function of protein concentration. The full-length betaB1-crystallin behaves as a dimer, like recombinant human betaB2-crystallin, but undergoes further self-association at high protein concentrations, unlike the betaB2-crystallin. Major truncations from the N-terminal extension lead to anomalous behaviour on gel permeation chromatography indicative of altered interactions with the column matrix, whereas light scattering indicated dimers at low protein concentration that self-associate as a function of protein concentration. Loss of 41 residues from the N-terminus, equivalent to an in vivo truncation site, resulted in temperature-dependent phase separation behaviour of the shortened betaB1-crystallin. Good crystals have been grown of a truncated version of human betaB1-crystallin using an in vitro cleavage protocol.

Crystal structure of the calcium-loaded spherulin 3a dimer sheds light on the evolution of the eye lens betagamma-crystallin domain fold.

BACKGROUND: The betagamma-crystallins belong to a superfamily of two-domain proteins found in vertebrate eye lenses, with distant relatives occurring in microorganisms. It has been considered that an eukaryotic stress protein, spherulin 3a, from the slime mold Physarum polycephalum shares a common one-domain ancestor with crystallins, similar to the one-domain 3-D structure determined by NMR. RESULTS: The X-ray structure of spherulin 3a shows it to be a tight homodimer, which is consistent with ultracentrifugation studies. The (two-motif) domain fold contains a pair of calcium binding sites very similar to those found in a two-domain prokaryotic betagamma-crystallin fold family member, Protein S. Domain pairing in the spherulin 3a dimer is two-fold symmetric, but quite different in character from the pseudo-two-fold pairing of domains in betagamma-crystallins. There is no evidence that the spherulin 3a single domain can fold independently of its partner domain, a feature that may be related to the absence of a tyrosine corner. CONCLUSION: Although it is accepted that the vertebrate two-domain betagamma-crystallins evolved from a common one-domain ancestor, the mycetezoan single-domain spherulin 3a, with its unique mode of domain pairing, is likely to be an evolutionary offshoot, perhaps from as far back as the one-motif ancestral stage. The spherulin 3a protomer stability appears to be dependent on domain pairing. Spherulin-like domain sequences that are found within bacterial proteins associated with virulence are likely to bind calcium.

Structural studies of duck delta 1 and delta 2 crystallin suggest conformational changes occur during catalysis.

Duck delta1 and delta2 crystallin are 94% identical in amino acid sequence, and while delta2 crystallin is the duck orthologue of argininosuccinate lyase (ASL) and catalyzes the reversible breakdown of argininosuccinate to arginine and fumarate, the delta1 isoform is enzymatically inactive. The crystal structures of wild type duck delta1 and delta2 crystallin have been solved at 2.2 and 2.3 A resolution, respectively, and the refinement of the turkey delta1 crystallin has been completed. These structures have been compared with two mutant duck delta2 crystallin structures. Conformational changes were observed in two regions of the N-terminal domain with intraspecies differences between the active and inactive isoforms localized to residues 23-32 and both intra- and interspecies differences localized to the loop of residues 74-89. As the residues implicated in the catalytic mechanism of delta2/ASL are all conserved in delta1, the amino acid substitutions in these two regions are hypothesized to be critical for substrate binding. A sulfate anion was found in the active site of duck delta1 crystallin. This anion, which appears to mimic the fumarate moiety of the argininosuccinate substrate, induces a rigid body movement in domain 3 and a conformational change in the loop of residues 280-290, which together would sequester the substrate from the solvent. The duck delta1 crystallin structure suggests that Ser 281, a residue strictly conserved in all members of the superfamily, could be the catalytic acid in the delta2 crystallin/ASL enzymatic mechanism.

A temperature-sensitive mutation of Crygs in the murine Opj cataract.

In Opj, an inherited cataract in mice, opacity is associated with a mutation in Crygs, the gene for gammaS-crystallin, the first mutation to be associated with this gene. A single base change causes replacement of Phe-9, a key hydrophobic residue in the core of the N-terminal domain, by serine. Despite this highly non-conservative change, mutant protein folds normally at low temperature. However, it exhibits a marked, concentration-dependent decrease in solubility, associated with loss of secondary structure, at close to physiological temperatures. This is reminiscent of processes thought to occur in human senile cataracts in which normal proteins become altered and aggregate. The Opj cataract is progressive and more severe in Opj/Opj than in Opj/+. Lens histology shows that whereas fiber cell morphology in Opj/+ mice is essentially normal, in Opj/Opj, cortical fiber cell morphology and the loss of maturing fiber cell nuclei are both severely disrupted from early stages. This may indicate a loss of function of gammaS-crystallin which would be consistent with ideas that members of the betagamma-crystallin superfamily may have roles associated with maintenance of cytoarchitecture.

The N-terminal domain of betaB2-crystallin resembles the putative ancestral homodimer.

betagamma-crystallins from the eye lens are proteins consisting of two similar domains joined by a short linker. All three-dimensional structures of native proteins solved so far reveal similar pseudo-2-fold pairing of the domains reflecting their presumed ancient origin from a single-domain homodimer. However, studies of engineered single domains of members of the betagamma-crystallin superfamily have not revealed a prototype ancestral solution homodimer. Here we report the 2.35 A X-ray structure of the homodimer of the N-terminal domain of rat betaB2-crystallin (betaB2-N). The two identical domains pair in a symmetrical manner very similar to that observed in native betagamma-crystallins, where N and C-terminal domains (which share approximately 35% sequence identity) are related by a pseudo-2-fold axis. betaB2-N thus resembles the ancestral prototype of the betagamma-crystallin superfamily as it self-associates in solution to form a dimer with an essentially identical domain interface as that between the N and C domains in betagamma-crystallins, but without the benefit of a covalent linker. The structure provides further evidence for the role of two-domain pairing in stabilising the protomer fold. These results support the view that the betagamma-crystallin superfamily has evolved by a series of gene duplication and fusion events from a single-domain ancestor capable of forming homodimers.

Cloning, modeling, and chromosomal localization for a small leucine-rich repeat proteoglycan (SLRP) family member expressed in human eye.

PURPOSE: To examine a highly abundant novel transcript from human iris. METHODS: Expressed sequence tag (EST) analysis of an adult human iris cDNA library revealed an abundant (>0.7%) transcript for a novel member of the small leucine-rich proteoglycan (SLRP) family. Other 3' ESTs from retina were also detected in dbEST. The structure of the leucine-rich repeat (LRR) domain was investigated by molecular modeling. Antisera were raised against a specific peptide and used in western blots of human and rat eye tissues. RESULTS: From its prevalence in the eye and its superfamily relationships, this SLRP protein has been given the names oculoglycan or opticin (Optc). Sequence analysis suggests that Optc has a signal peptide and two structural domains, the larger of which is the LRR domain. Modeling of the LRR domain reveals structural variability in the repeat motifs, forming potential interaction sites for binding partners. Antiserum to a specific peptide detected a protein of approximately 48 kDa, in human iris, ciliary body and retina while the major protein detected in rat ocular tissues was 37 kDa in size. This may reflect a species difference in post-translational modification. Radiation hybrid mapping shows that the gene for OPTC is located on chromosome 1q31, close to the inherited eye diseases ARMD1 and AXPC1. CONCLUSIONS: Optc is a newly identified SLRP family member, which appears to have eye-preferred expression. Molecular modeling reveals local deviations from the familiar LRR structure, which are candidates for specific interaction sites. Western blotting with a specific peptide antibody detects Optc in iris, ciliary body and retina in the human eye and suggests that the protein is post-translationally modified. In rat, the antibody detects Optc in several eye tissues and in brain but the protein appears to have undergone much less modification, suggesting that this is not essential for all aspects of function. Considering its eye-preferred expression, the OPTC gene has the potential for involvement in inherited eye disease. Indeed, it maps close to at least two disease loci for which no gene has so far been identified.

Structure of the crystallins.

The lens is formed from two protein superfamilies, the alpha- and beta gamma-crystallins. Representative three-dimensional structures show they both have a basic 2-beta-sheet domain fold, with the beta gamma-domain being made from two intercalating Greek keys. X-ray structures of monomeric gamma-crystallins and simple oligomeric beta-crystallins show how multiple gene duplications can give rise to highly symmetrical assemblies based on paired domains. These protein folds have been engineered by directed mutagenesis to investigate the roles of the critical region in domain pairing and assembly. Inherited human cataracts have been described that are associated with representatives of each of the crystallin protein families. Mutations to certain beta- and gamma-crystallin genes cause expression of truncated polypeptides that would not be expected to fold properly; instead they would randomly aggregate causing light scattering. As crystallin proteins are not renewed, age-related cataract is a gradual accumulation of small changes to pre-existing normal proteins. The precise sites of post-translational modifications are now being mapped to the various crystallins.

Vitamin C induced oxidation of eye lens gamma crystallins.

Crystallins are long-lived proteins of the eye lens that have specific structures that maintain lens transparency. Lens crystallins are known to undergo changes with age that include oxidation. Oxidation may contribute to cataract development. In this study the effect of metal-catalysed oxidation of vitamin C (ascorbate) on gamma-crystallins was investigated based on polyacrylamide gel electrophoresis and electrospray mass spectrometry. Cross-linking, aggregation and denaturation occurred when two members of the gamma-crystalline family, gamma B and gamma S, were challenged with copper (II) and ascorbate. These proteins form a dimer, with copper alone or with the addition of ascorbate, which may be an early marker of oxidation. It was found that alpha-ketoglutarate and pyruvate were very effective in the inhibition of oxidation.

The C-terminal domains of gammaS-crystallin pair about a distorted twofold axis.

The 2-domain gammaS-crystallin, a highly conserved early evolutionary off-shoot of the gamma-crystallin family, is located in the water-rich region of eye lenses. The expressed C-terminal domain, gammaS-C, has been crystallized and the 2.56 A X-ray structure determined. There are two domains in the asymmetric unit which pair about a distorted twofold axis. One of the domains has an altered conformation in a highly conserved region of the protein, the tyrosine corner. The distorted gammaS-C dimer of domains is compared with the highly symmetrical, equivalent recombinant dimer of C-terminal domains from gammaB-crystallin. Sequence changes close to the interface, that distinguish gammaS from the other gamma-crystallins, are examined in order to evaluate their role in symmetrical domain pairing.