alpha m-Crystallin: the native form of the protein?

α‐crystallin: a review of its structure and function

alpha-crystallin, the major protein of the mammalian lens in most species, is an aggregate assembled from two polypeptides, each with a molecular weight around 20,000 Da. It is polydisperse and can be isolated in a variety of forms, including spherical particles with molecular weights ranging upwards from about 200 kDa. Sequence comparisons reveal that it is a member of the small heat shock protein (shsp) family. These proteins are aggregates assembled from polypeptides of 10 to 25 kDa that share a common central domain of about 90 residues (the 'alpha-crystallin domain') with variable N- and C-terminal extensions. alpha-crystallin has been intensively studied for more than 50 years but its three-dimensional structure remains unknown because it has not been possible to obtain crystals for X-ray studies and it is too large for NMR measurements. Structural information has been derived from a variety of solution studies. Because of the protein's polydispersity, interpretation of data has been difficult. This led to different viewpoints and vigorous debate on its structure and properties. Recently, the crystal structures of two closely-related small heat shock proteins have been determined. These have provided some insight into the structure of a-crystallin and explanations of previous observations. Like many other heat shock proteins, alpha-crystallin exhibits chaperone-like properties, including the ability to prevent the precipitation of denatured proteins and to increase cellular tolerance to stress. It has been suggested that these functions are important for the maintenance of lens transparency and the prevention of cataract.

Hard-sphere-like dynamics in highly concentrated alpha-crystallin suspensions

The dynamics of concentrated suspensions of the eye-lens protein alpha crystallin have been measured using x-ray photon correlation spectroscopy. Measurements were made at wave vectors corresponding to the first peak in the hard-sphere structure factor and volume fractions close to the critical volume fraction for the glass transition. Langevin dynamics simulations were also performed in parallel to the experiments. The intermediate scattering function f(q,τ) could be fit using a stretched exponential decay for both experiments and numerical simulations. The measured relaxation times show good agreement with simulations for polydisperse hard-sphere colloids.

NEW INSIGHTS INTO STRUCTURAL CHANGES OF LENS PROTEINS

X-ray scattering techniques were used to study the effects of heating on whole eye lens and α-crystallin gels. The temperature range used was from 20 to 70°C. The position of single X-ray reflection seen in whole lens was unchanged in the temperature range 20 to 45°C, with a continuous spacing of 152 Å. However, at 50°C the spacing increased from 152 Å to 165 Å. An interpretation of these results is that in eye lens, α-crystallin is protecting other lens proteins from super-aggregation up to 50°C. In α-crystallin gels a moderate increase in both the spacing and intensity of the reflection was observed from 20 to 45°C, followed by a dramatic increase from 45 to 70°C. Over the whole temperature range the spacing changed from 138 Å at 70°C to 195 Å at 70°. After eleven hours of cooling, this effect was found to be irreversible.

Mechanism of suppression of protein aggregation by α-crystallin

This review summarizes experimental data illuminating the mechanism of suppression of heat-induced protein aggregation by alpha-crystallin, one of the small heat shock proteins. The dynamic light scattering data show that the initial stage of thermal aggregation of proteins is the formation of the initial aggregates involving hundreds of molecules of the denatured protein. Further sticking of the starting aggregates proceeds in a regime of diffusion-limited cluster-cluster aggregation. The protective effect of alpha-crystallin is due to transition of the aggregation process to the regime of reaction-limited cluster-cluster aggregation, wherein the sticking probability for the colliding particles becomes lower than unity.

Liquid-liquid phase separation and static light scattering of concentrated ternary mixtures of bovine alpha and gammaB crystallins

We have used light scattering, turbidimetry, and thermodynamic analysis to study the phase diagram of concentrated aqueous mixtures of the bovine lens proteins, gammaB crystallin, and alpha crystallin. We find that dilute alpha crystallin raises the phase separation temperature of concentrated gammaB crystallin, while more concentrated alpha crystallin suppresses phase separation. Very concentrated alpha/gammaB mixtures can reversibly cloud above 37 degrees C, even though gammaB alone phase separates only below temperatures near 0 degrees C, and alpha does not phase separate. At the scattering vector magnitude used, high-concentration alpha/gammaB mixtures scatter less light than the weighted average of their component alpha and gammaB solutions, while low-concentration alpha/gammaB mixtures scatter more than such a weighted average. We use a mean-field thermodynamic analysis of such ternary mixtures to show that the observed light scattering and phase boundaries of alpha and gammaB crystallin mixtures give evidence for prominent local fluctuations of relative protein composition. In the single phase, these fluctuations scatter comparatively little light, but are associated with enhanced thermodynamic instability. By applying this analysis to the experimental tie lines we estimate the magnitude of the saddlelike component of the free energy near the aqueous-gammaB critical point.

Trehalose effects on α-crystallin aggregates

alpha-Crystallin in its native state is a large, heterogeneous, low-molecular weight (LMW) aggregate that under certain conditions may progressively became part of insoluble high-molecular weight (HMW) systems. These systems are supposed to play a relevant role in eye lens opacification and vision impairment. In this paper, we report the effects of trehalose on alpha-crystallin aggregates. The role of trehalose in alpha-crystallin stress tolerance, chaperone activity and thermal stability is studied. The results show that trehalose stabilizes the alpha-crystallin native structure, inhibits alpha-crystallin aggregation, and disaggregates preformed LMW systems not affecting its chaperone activity.

The necessity of functional proteomics: protein species and molecular function elucidation exemplified by in vivo alpha A crystallin N-terminal truncation

Ten years after the establishment of the term proteome, the science surrounding it has yet to fulfill its potential. While a host of technologies have generated lists of protein names, there are only a few reported studies that have examined the individual proteins at the covalent chemical level defined as protein species in 1997 and their function. In the current study, we demonstrate that this is possible with two-dimensional gel electrophoresis (2-DE) and mass spectrometry by presenting clear evidence of in vivo N-terminal alpha A crystallin truncation and relating this newly detected protein species to alpha crystallin activity regulation by protease cleavage in the healthy young murine lens. We assess the present state of technology and suggest a shift in resources and paradigm for the routine attainment of the protein species level in proteomics.

Age-related changes of alpha-crystallin aggregate in human lens

Lens alpha-crystallin, composed of two subunits alpha A- and alpha B-crystallin, forms large aggregates in the lens of the eye. The present study investigated the aggregate of human lens alpha-crystallin from elderly and young donors. Recombinant alpha A- and alpha B-crystallins in molar ratios of alpha A to alpha B at 1:1, corresponding to the aged sample, were also studied in detail. We found by ultra-centrifugation analysis that the alpha-crystallin aggregate from elderly donors was large and heterogeneous with an average sedimentation coefficient of 30 S and a range of 20-60 S at 37 degrees C. This was higher compared to the young samples that had an average sedimentation coefficient of 17 S. The sedimentation coefficients of recombinant alpha A- and alpha B-crystallins were approximately 12 S and 15 S, respectively. Even when recombinant alpha-crystallins were mixed in molar ratios equivalent to those found in vivo, similar S values as the native aged alpha-crystallin aggregates were not obtained. Changes in the self-association of alpha-crystallin aggregate were correlated to changes in chaperone activity. Alpha-crystallin from young donors, and recombinant alpha A- and alpha B-crystallin and their mixtures showed chaperone activity, which was markedly lost in samples from the aged alpha-crystallin aggregates.

Structural Changes in α-Crystallin and Whole Eye Lens During Heating, Observed by Low-angle X-ray Diffraction

Whole eye lens and alpha-crystallin gels and solutions were investigated using X-ray scattering techniques at temperatures ranging from 20 degrees C to 70 degrees C. In whole lens isolated in phosphate-buffered saline, the spacing of the dominant X-ray reflection seen with low-angle scattering was constant from 20 degrees C to 45 degrees C but increased at 50 degrees C from 15.2 nm to 16.5 nm. At room temperature, the small-angle X-ray diffraction pattern of the intact lens was very similar to the pattern of alpha-crystallin gels at near-physiological concentration (approximately 300 mg/ml), so it is reasonable to assume that the alpha-crystallin pattern dominates the pattern of the intact lens. Our results therefore indicate that in whole lens alpha-crystallin is capable of maintaining its structural properties over a wide range of temperature. This property would be useful in providing protection for other lens proteins super-aggregating. In the alpha-crystallin gels, a moderate increase in both the spacing and intensity of the reflection was observed from 20 degrees C to 45 degrees C, followed by an accelerated increase from 45 degrees C to 70 degrees C. Upon cooling, this effect was found to be irreversible over 11 hours. Qualitatively similar results were observed for alpha-crystallin solutions at a variety of lower concentrations.

Viscosity of alpha-crystallin solutions

Accommodation in the mammalian lens requires flexure of lens fibres and some redistribution of their contents involving limited viscous flow. Shear-dependent viscosity of bovine alpha-crystallin solutions was determined with the Contraves Low-Shear Rheometer between 4.4 and 347 mg ml(-1), and at 15.5, 25, 30, and 37 degrees C. All solutions showed significant shear thinning, with markedly higher viscosity at physiological levels of approximately 300 mg ml(-1). Viscosity-concentration graphs were similar at low (1.0 s(-1)) and high (94.5 s(-1)) shear rates, indicating low molecular interaction in solution. Arrhenius plots which might have indicated the size of the energy barrier to displacement of molecules or aggregates were inconclusive.

Effect of trifluoroethanol on the structural and functional properties of alpha-crystallin

Alpha crystallin is an eye lens protein with a molecular weight of approximately 800 kDa. It belongs to the class of small heat shock proteins. Besides its structural role, it is known to prevent the aggregation of beta- and gamma-crystallins and several other proteins under denaturing conditions and is thus believed to play an important role in maintaining lens transparency. In this communication, we have investigated the effect of 2,2,2-trifluoroethanol (TFE) on the structural and functional features of the native alpha-crystallin and its two constituent subunits. A conformational change occurs from the characteristic beta-sheet to the alpha-helix structure in both native alpha-crystallin and its subunits with the increase in TFE levels. Among the two subunits, alphaA-crystallin is relatively stable and upon preincubation prevents the characteristic aggregation of alphaB-crystallin at 20% and 30% (v/v) TFE. The hydrophobicity and chaperone-like activity of the crystallin subunits decrease on TFE treatment. The ability of alphaA-crystallin to bind and prevent the aggregation of alphaB-crystallin, despite a conformational change, could be important in protecting the lens from external stress. The loss in chaperone activity of alphaA-crystallin exposed to TFE and the inability of peptide chaperone--the functional site of alphaA-crystallin--to stabilize alphaB-crystallin at 20-30% TFE suggest that the site(s) involved in subunit interaction and chaperone-like function are quite distinct.

Studies of the denaturation patterns of bovine alpha-crystallin using an ionic denaturant, guanidine hydrochloride and a non-ionic denaturant, urea

The effects of non-ionic and ionic denaturation and denaturation/renaturation on the native structure of alpha-crystallin at room temperature were examined. Native alpha-crystallin, at concentrations above and below the previously reported critical micelle concentration (CMC) range, was denatured by varying concentrations of urea and guanidine hydrochloride. The resulting denatured samples were examined by gel filtration fast performance liquid chromatography (FPLC), circular dichroism spectropolarimetry (CD), and transmission electron microscopy. Elution peak samples from gel filtration chromatography with sufficiently high concentrations were examined for subunit composition by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The studies presented herein demonstrate that the denaturation and renaturation of alpha-crystallin via non-ionic urea denaturation results in different renaturation species, depending upon the initial concentration of alpha-crystallin which is denatured and the concentration of urea, including certain species which, by gel filtration FPLC, have an apparent molecular weight greater than the native 800 kD aggregate. Transmission electron microscopy has also demonstrated the existence of a high molecular weight aggregate form for denatured samples. Ionic dissociation, in contrast, proceeds much in the same manner above and below the CMC range, the major difference occurring at 2 M guanidine hydrochloride. alpha B-crystallin is preferentially removed from the native alpha-crystallin aggregate upon treatment with 2 M guanidine hydrochloride indicating, once again, differences between the two subunits. Above and below the CMC range, dissociation with guanidine hydrochloride appears to plateau after 4 M guanidine hydrochloride as indicated by the presence of two apparent homotetrameric species and no further dissociation of these species with increasing guanidine hydrochloride concentrations. CD demonstrates that some secondary structure, which is lost with lower concentrations of alpha-crystallin, is still present when concentrations of alpha-crystallin, well above the critical micelle concentration range, are treated with high concentrations of urea at room temperature. In contrast, concentrations both above and below the CMC range demonstrate a significant loss of secondary structure upon treatment with 2 M guanidine hydrochloride. Finally, ionic denaturation and subsequent renaturation results in the formation of a species which is functionally incapable of protecting gamma-crystallin from heat-induced aggregation.

alpha-Crystallin polymers and polymerization: the view from down under

Several models have been proposed for the quaternary structure of alpha-crystallin. Some suggest the subunits are arranged in concentric shells. Others propose that the subunits are in a micelle-like arrangement. However, none is able to satisfactorily account for all observations on the protein and the quaternary structure of alpha-crystallin remains to be established. In this review, factors contributing to the assembly and polymerization are examined in order to evaluate the different models. Consideration of the variations in particle size and molecular weight under different conditions leads to the conclusion that alpha-crystallin cannot be a micelle or a layered structure. Instead, it is suggested that the protein may be assembled from a 'monomeric' unit comprising eight subunits arranged in two tetramers with cyclic symmetry. The octameric unit is proposed to be disc-like particle with a diameter of 9.5 nm and a height of 3 nm. The larger particles, chains and sheet-like structures commonly observed are assembled from the octamers. Structural predictions indicate that the polypeptide may be folded into three independent domains which have different roles in the structural organization and functions of the protein. It is suggested that the tetramers are stabilized through interactions involving the second domain (residues 64-104) while assembly into the octamers and higher polymers requires hydrophobic interactions involving the N-terminal domain. Deletion of parts of this domain by site directed mutagenesis revealed that residues 46-63 play a critical role in the assembly. Current research aims to identify the specific amino acids involved.

Environmental factors influencing the chaperone-like activity of alpha-crystallin

The effects of mild environmental changes (e.g. the addition of divalent cations or EDTA, as well as variations of buffer pH) on the heat stability and chaperone-like activity of native alpha-crystallin, and denatured-renatured alpha-crystallin in the native molar isoform ratio, have been investigated using circular dichroism (CD) spectropolarimetry and functional assays. The presence or absence of divalent cations has little or no effect on the secondary structure of renatured samples, although chaperone-like activity levels can vary widely; the only relevant spectral difference observed is a loss of some alpha-helical content in all the renatured samples relative to the native protein, but this change has no impact on function. The range of concentration over which the inhibitory Mg2+ effect is observed is 10-fold higher for dialyzed fresh protein than for protein renatured into buffers containing Mg2+, but for both sets of samples, the full effect is established below physiological Mg2+ concentrations. Renaturing into various pH buffers, in contrast, affects both heat stability and chaperone-like activity below pH 7.0, with essentially no functionality observed at pH 6.0. CD spectra of these samples indicate that acidic conditions lead to some degree of unfolding, and that this unfolding correlates directly with functionality. Similar results are obtained for fresh protein dialyzed against these pH levels. Overall, these results suggest that heat stability is a function of the protein's secondary structure and folding state, while chaperone-like activity is primarily a function of factors at the tertiary and quaternary levels of organization.

Quaternary structure of bovine alpha-crystallin: influence of temperature

The tertiary and quaternary structure of alpha-crystallin is still a matter of controversy. We have characterized the native alpha-crystallin quaternary structure by isolating it at the in vivo temperature and solvent conditions. It can be represented by a distribution of expanded particles with a weight average molar mass of 550,000 g/mol. On decreasing (to 4 degrees C) or increasing (up to 50 degrees C) the temperature, the size distribution increases to larger particles. Only at lower temperatures (4 degrees C), a stable population of particles is obtained with weight average molar mass of 700,000 g/mol. In all conditions, alpha-crystallin behaves as a very expanded particle with a maximum hydrodynamic volume of 3.15 ml/g. The transitions in quaternary structure are rather slow: it takes several hours to evolve from a population of aggregates, characteristic for given solvent conditions, to another distribution in size and quaternary structure on changing the environment. The quaternary structure of alpha-crystallin is an uncharacteristic parameter of the particle: a broad distribution of values can be obtained on changing the environment. Any realistic model should include this property. Our studies favor an open loose structure, where peptides can be added or removed without drastic changes of secondary and tertiary structure of the peptides.

Structural perturbation of alpha-crystallin and its chaperone-like activity

alpha-Crystallin is a multimeric lenticular protein that has recently been shown to be expressed in several non-lenticular tissues as well. It is shown to prevent aggregation of non-native proteins as a molecular chaperone. By using a non-thermal aggregation model, we could show that this process is temperature-dependent. We investigated the chaperone-like activity of alpha-crystallin towards photo-induced aggregation of gamma-crystallin, aggregation of insulin and on the refolding induced aggregation of beta- and gamma-crystallins. We observed that alpha-crystallin could prevent photo-aggregation of gamma-crystallin and this chaperone-like activity of alpha-crystallin is enhanced several fold at temperatures above 30 degrees C. This enhancement parallels the exposure of its hydrophobic surfaces as a function of temperature, probed using hydrophobic fluorescent probes such as pyrene and 8-anilinonaphthalene-1-sulfonate. We, therefore, concluded that alpha-crystallin prevents the aggregation of other proteins by providing appropriately placed hydrophobic surfaces; a structural transition above 30 degrees C involving enhanced or re-organized hydrophobic surfaces of alpha-crystallin is important for its chaperone-like activity. We also addressed the issue of conformational aspects of target proteins and found that their aggregation prone molten globule states bind to alpha-crystallin. We trace these developments and discuss some new lines that suggest the role of tertiary structural aspects in the chaperone process.

alpha-Crystallin quaternary structure and interactive properties control eye lens transparency

The eye lens is the foremost biological system where function is directly under control of the physico-chemical properties of the cytoplasmic macromolecular solution. Indeed, lens transparency and opacity, lens refractive index gradient and viscosity, are the result of the structural and interactive properties of the crystallins, of their stability, of the fine tuning of their interaction potentials and associations at different levels of organization. Among the different crystallin classes, alpha-crystallins have represented a major challenge for a long time. The alpha-crystallin secondary, tertiary and quaternary structures are still unknown. On the functional side, however, it is established that alpha-crystallin quaternary structure and repulsive interactions determine lens transparency, whereas the alpha-crystallin chaperone effect most probably plays a role in the aging process. In the present paper, we recall the physico-chemical properties and the quaternary structure features of alpha-crystallins that were demonstrated to control light scattering and transparency. The interest of a crystallin mixture for lens function is discussed. Then, a formal approach is proposed to design models for the alpha-crystallin quaternary structure, including the question of whether alpha-crystallins assemble with symmetry. An hypothesis relevant to the fold of the alpha-crystallin C-terminal domain is presented in another paper in this issue.

Chaperone-like activity and temperature-induced structural changes of alpha-crystallin

alpha-Crystallin is known to exhibit chaperone-like activity. We have studied its chaperone-like activity toward the aggregation of betaL-crystallin upon refolding of this protein from its unfolded state in guanidinium chloride. The chaperone-like activity of alpha-crystallin is less pronounced below 30 degrees C and is enhanced above this temperature. The plot of percentage protection as a function of temperature shows two transitions; one at 30 degrees C and another at around 55 degrees C. We have performed steady state fluorescence, fluorescence polarization, fluorescence quenching, circular dichroism, sedimentation analysis, and gel filtration chromatography to probe the temperature-induced structural changes of alpha-crystallin. Our results show that at above 50 degrees C, alpha-crystallin undergoes a transition to a multimeric molten globule-like state. Above 30 degrees C, a minor but detectable perturbation in its tertiary structure occurs that might lead to the observed exposure of its hydrophobic surfaces. These results support our earlier hypothesis that alpha-crystallin prevents the aggregation of other proteins by providing appropriately placed hydrophobic surfaces; a structural transition above 30 degrees C involving enhanced or reorganized hydrophobic surfaces of alpha-crystallin is important for its chaperone-like activity. It is possible that a structural alteration induced by temperature forms a part of the general mechanism of chaperone function, because they are required to function more effectively at nonpermissible temperatures.

Analysis of the factors involved in the loss and restoration of the chaperone-like function of alpha-crystallin

alpha-crystallin, the major protein component of the crystallin lens of mammalian eyes, is found in vivo as two separate gene products. Both isoforms are expressed in different major tissues of the body, with the lens the only location where both are found together. Both sequences can be phosphorylated, though at different locations. Both exhibit a high sequence homology to the small heat shock proteins, and it has been shown that alpha-crystallin also resists heat-induced denaturation. Horwitz [J. Horowitz (1992) Proc. Natl. Acad. Sci. USA 89, 10449-10453] demonstrated that alpha-crystallin can exhibit chaperone-like protection against heat-induced turbidity increases, and it has been suggested that this may be an in vivo function as well. However, neither isoform, when purified, shows the same overall level of chaperone-like activity as the native species, except for one phosphorylated species [M. A. M. van Boekel, S. E. A. Hoogakker, J. J. Harding, and W. W. de Jong (1996) Ophthalmic Res. 28(Suppl. 1), 32-38]. Experiments designed to determine the factors leading to loss of chaperone-like activity indicate that strong ionic conditions, such as those used in isoform separation and/or the presence of divalent cations reduce the efficiency of this function and that the presence of EDTA fully restores it irrespective of prior treatment or buffer conditions. Heat stability is essentially preserved under all conditions. These results suggest that alpha-crystallin may serve primarily as a heat shock protein in vivo and that the chaperone-like function may be inhibited under physiological conditions.

Rapid refolding studies on the chaperone-like alpha-crystallin. Effect of alpha-crystallin on refolding of beta- and gamma-crystallins

alpha-Crystallin, a multimeric protein present in the eye lens, is shown to have chaperone-like activity in preventing thermally induced aggregation of enzymes and other crystallins. We have studied the rapid refolding of alpha-crystallin, and compared it with other calf eye lens proteins, namely beta- and gamma-crystallins. alpha-Crystallin forms a clear solution upon rapid refolding from 8 M urea. The refolded alpha-crystallin has native-like secondary, tertiary, and quaternary structures as revealed by circular dichroism and fluorescence characteristics as well as gel filtration and sedimentation velocity measurements. On rapid refolding, beta- and gamma-crystallins aggregate and form turbid solutions. The presence of alpha-crystallin in the refolding buffer marginally increases the recovery of beta- and gamma-crystallins in the soluble form. However, unfolding of these crystallins together with alpha-crystallin using 8 M urea and subsequent refolding significantly increases the recovery of these proteins in the soluble form. These results indicate that an intermediate of alpha-crystallin formed during refolding is more effective in preventing the aggregation of beta- and gamma-crystallins. This supports our earlier hypothesis (Raman, B., and Rao, C. M. (1994) J. Biol. Chem. 269, 27264-27268) that the chaperone-like activity of alpha-crystallin is more pronounced in its structurally perturbed state.

Interaction of ATP and lens alpha crystallin characterized by equilibrium binding studies and intrinsic tryptophan fluorescence spectroscopy

alpha-Crystallin, the most prevalent protein in vertebrate lenses, is a high molecular weight aggregate composed of alpha A and alpha B subunits. Evidence is presented that ATP, a major phosphorus metabolite of the lens binds to alpha-crystallin extracted from calf lenses. The following parameters were obtained from equilibrium binding studies conducted at 37 degrees C: binding sites per 400 kDa aggregate = 10 and Ka = 8.1 x 10(3) M-1; and an essentially identical Ka of 7.84 x 10(3) M-1 and 22 binding sites were determined for a 850 kDa aggregate. The cooperativity parameter, alpha H, approximates unity which denotes that the binding of ligand is at independent sites. Binding was not significant at 22 degrees C and was absent at 4 degrees C. The specificity of the binding site for ATP was established by intrinsic tryptophan fluorescence spectroscopy. In the presence of increasing concentrations of ATP (0.05-0.3 mM), tryptophan fluorescence decreases in a concentration dependent manner to a minimum of 0.2 mM above which there is a non-linear response. Quenching of fluorescence was not evident with P(i), AMP or ADP. GTP elicited a minimal quenching of fluorescence only at the highest concentration (0.30 mM). Modulation of both supramolecular organization and lens metabolism is predicted as a consequence of ATP/alpha-crystallin binding.

Structure and modifications of the junior chaperone alpha-crystallin. From lens transparency to molecular pathology

alpha-Crystallin is a high-molecular-mass protein that for many decades was thought to be one of the rare real organ-specific proteins. This protein exists as an aggregate of about 800 kDa, but its composition is simple. Only two closely related subunits termed alpha A- and alpha B-crystallin, with molecular masses of approximately 20 kDa, form the building blocks of the aggregate. The idea of organ-specificity had to be abandoned when it was discovered that alpha-crystallin occurs in a great variety of nonlenticular tissues, notably heart, kidney, striated muscle and several tumors. Moreover alpha B-crystallin is a major component of ubiquinated inclusion bodies in human degenerative diseases. An earlier excitement arose when it was found that alpha B-crystallin, due to its very similar structural and functional properties, belongs to the heat-shock protein family. Eventually the chaperone nature of alpha-crystallin could be demonstrated unequivocally. All these unexpected findings make alpha-crystallin a subject of great interest far beyond the lens research field. A survey of structural data about alpha-crystallin is presented here. Since alpha-crystallin has resisted crystallization, only theoretical models of its three-dimensional structure are available. Due to its long life in the eye lens, alpha-crystallin is one of the best studied proteins with respect to post-translational modifications, including age-induced alterations. Because of its similarities with the small heat-shock proteins, the findings about alpha-crystallin are illuminative for the latter proteins as well. This review deals with: structural aspects, post-translational modifications (including deamidation, racemization, phosphorylation, acetylation, glycation, age-dependent truncation), the occurrence outside of the eye lens, the heat-shock relation and the chaperone activity of alpha-crystallin.

A comparison of the inhibition of porcine pancreatic elastase and human neutrophil elastase by alpha-crystallin

Bovine lens alpha-crystallin inhibited both porcine pancreatic elastase (PPE) and human neutrophil elastase (HNE), but not in the same manner. PPE was immediately inhibited with a stoichiometry of 10 moles of PPE inhibited per mole of alpha-crystallin. The inhibition was markedly decreased by the addition of even low levels of salts. The inhibition was transient, as PPE activity returned to normal with a t1/2 of 30 min even in low salt. HNE required a short preincubation to show maximum inhibition with a stoichiometry of approximately one mole of HNE inhibited per mole of alpha-crystallin. The inhibition of HNE was only slightly decreased by the addition of 0.1 M salt, and HNE activity returned slowly exhibiting a t1/2 of 30 hrs under these conditions. The inhibition of each enzyme by alpha-crystallin was evaluated by Dixon plots giving Ki values of 1.5 nM for PPE and 0.25 nM for HNE. DFP-trypsin was able to compete with PPE for binding to alpha-crystallin and cause the release of PPE already bound to alpha-crystallin. The inhibition of HNE, however, was unaffected by the addition of DFP-trypsin. A mixture of HNE and alpha-crystallin in 0.1 M NaCl was incubated at 25 degrees C for 6 hours. Aliquots showed a slow, continuous cleavage of the alpha-crystallin subunits by SDS-PAGE, but little or no increase in HNE activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Probing the microenvironments of tryptophan residues in the monomeric crystallins of the bovine lens

Tryptophan microenvironments have been examined in bovine beta s-, gamma II-, gamma IIIa-, gamma IIIb-, gamma IVa- and gamma IVb-crystallins by fluorescence methods. The proteins could be divided into two groups on the basis of the accessibilities of their tryptophan residues. The first group, comprising beta s, gamma II and gamma IIIb, appeared to have a compact structure with none of the tryptophans accessible to KI and only moderately so to acrylamide. By contrast in gamma IIIa, gamma IVa and gamma Vb, all tryptophans were readily accessible to acrylamide and 70% of the fluorescence could be quenched with KI. Spectral analysis, before and after quenching, time-resolved spectroscopy and simulations of the quenching curves suggested that gamma IIIa, gamma IVa and gamma IVb contain two classes of tryptophan residues. One class (tau 0 = 0.52 ns, fa = 0.3, lambda max = 324 nm) which was completely inaccessible to KI and relatively inaccessible to acrylamide (Ksv = 0.25 M-1), was assigned to the topologically equivalent residues in positions 42 and 131. The other class (tau 0 = 2.1-3.4 ns, fa = 0.7, lambda max = 330 nm) was accessible to both quenchers (Ksv = 5.00-5.15 M-1 and 2.47-2.60 M-1, for acrylamide and KI, respectively) and corresponded to the tryptophan residues in positions 68 and 157. The same classes may be present in the other low molecular weight proteins (tau 0 = 0.47-0.55 and 1.55-1.74) but the lower emission and low accessibilities to quenchers prevented their distinction and suggested that these proteins had more compact structures.

Further characterization of structural and electric properties of non-spherical alpha-crystallin

Recently we have shown that the population of native alpha-crystallin, isolated using size-exclusion chromatography from eye lenses of calves, is multimodal. Most of the protein probably possesses an almost spherical appearance, but at least one of the other modes represents more extended, ellipsoidally and/or cylindrically shaped molecules [Van Haeringen, B., Eden, D., Van den Bogaerde, M.R., Van Grondelle, R. & Bloemendal, M. (1992) Eur. J. Biochem. 210, 211-216]. In the present study, we characterize various subpools of a single alpha-crystallin size-exclusion chromatography elution peak by means of transient-electric-birefringence measurements, ultraviolet linear-dichroism spectroscopy and analytical fast protein liquid chromatography. It is concluded that the fractions have a well-defined stable mass and are not in reversible equilibrium with each other. All pools appear to be composed of at least two types of differently shaped molecules. The hydrodynamic dimensions and electric properties of the different alpha-crystallin species are characterized. The non-spherical alpha-crystallin is found to be optically and electrically more anisotropic, and to contain a larger permanent electric dipole moment than the spherical form. A model for the composition of the alpha-crystallin pool is presented.

Acid-induced dissociation of alpha A- and alpha B-crystallin homopolymers

Homopolymers were constructed from the alpha A and alpha B polypeptides isolated from the lens protein alpha-crystallin. As the pH is lowered from 7.0 to 3.4, these homopolymers dissociate to smaller species with molecular masses ranging from 80 to 250 kDa for the alpha A and around 140 kDa for the alpha B dissociation products. The pKa for this dissociation was 3.8 +/- 0.2 for alpha A and 4.1 +/- 0.1 for alpha B homopolymers. Further decreases in pH, to 2.5, resulted in the presence of only denatured alpha B polypeptides, whereas the alpha A dissociation products remained intact. Fractionation of the acid dissociation products from the alpha A homopolymer at pH 2.5 yielded stable species with molecular masses of 220 +/- 30, 160 +/- 20, and 90 +/- 10 kDa. The majority of the population at acid pH consisted of the 160 kDa species. Conformational analysis of these species revealed that most of the secondary structure of the original alpha A homopolymer was retained but that the tertiary structure was perturbed. Fluorescence quenching and energy transfer measurements suggested that the molecule had undergone acid expansion, with the greatest perturbation observed in the smallest particles. The results from this work suggest that alpha A homopolymers are heterogeneous populations of aggregates of a "monomeric" molecule with a molecular mass of 160 kDa. This "monomeric" molecule may be formed from the association of two tetrameric units.

Studies on the location of aromatic amino acids in alpha-crystallin

The locations of tryptophan residues in alpha-crystallin and homopolymers constructed from the alpha A- and alpha B-chains were examined by comparing their fluorescence emission properties and their accessibilities to quenchers. Two classes of tryptophan could be distinguished on the basis of differences in their spectral characteristics, fluorescence decay lifetimes, quenching with acrylamide and exposure by increasing concentrations of urea. Polarization measurements indicated that the tryptophan residues were associated with flexible segments of the polypeptide chains. The two classes could be assigned, one to Trp-9 (in both A- and B-chains) which is in an hydrophobic environment, and one to Trp-60 (B-chain) which appeared to be nearer the surface of the aggregate. No evidence was found for residues inaccessible to the quenchers. An apparent partition coefficient of 40 was obtained for the association of acrylamide with the protein. From temperature-dependence studies, it was concluded that there was a significant energy barrier to the penetration of acrylamide into the protein matrix (Ea = 5.8 kcal/mol) and that entry of the quencher was through channels produced by temporary disruption of the matrix (delta s = 1.5 eu). The phenolic side-chains of tyrosine residues in several different alpha-crystallins were found to ionize with pK values above pH 11, indicative of residues highly shielded from the solvent. Comparisons of polypeptide sequences, together with tyrosine fluorescence emission data and the pK values, permitted a tentative assignment of residue locations. All of the data are consistent with a possible micelle-like structure for alpha-crystallin but not with a layered structure.

Alpha-crystallin exists in a non-spherical form. A study on the rotational properties of native and reconstituted alpha-crystallin

Native alpha-crystallin, obtained from the cortex of calf lenses with FPLC (Pharmacia) was characterized by means of transient-electric-birefringence measurements and ultraviolet linear-dichroism spectroscopy. These techniques were also performed on 6-M-urea-dissociated and reconstituted alpha-crystallin. Transient-electric-birefringence measurements offer the possibility to characterize the often observed, but usually neglected, non-spherical occurrences of alpha-crystallin in more detail. Although not distinguishable with size-exclusion chromatography, we could identify at least two different classes of both native and reconstituted alpha-crystallin, from which at least one consists of non-spherical molecules. The results are compared with those obtained with electron microscopy using different staining methods. From the three independent techniques used we find evidence that a fraction of the alpha-crystallin exists in a more extended quaternary structure. The results are difficult to explain with a concentric three-layer model for alpha-crystallin as proposed by Tardieu et al. [Tardieu, A., Laporte, D., Licinio, P., Krop, B. & Delaye, M. (1986) J. Mol. Biol. 192, 711-724].

Identification by 1H NMR spectroscopy of flexible C-terminal extensions in bovine lens alpha-crystallin

Two-dimensional 1H NMR spectroscopy of bovine eye lens alpha-crystallin and its isolated alpha A and alpha B subunits reveals that these aggregates have short and very flexible C-terminal extensions of eight (alpha A) and ten (alpha B) amino acids which adopt little preferred conformation in solution. Total alpha-crystallin forms a tighter aggregate than the isolated alpha A and alpha B subunit aggregates. Our results are consistent with a micelle model for alpha-crystallin quaternary structure. The presence of terminal extensions is a general feature of those crystallins, alpha and beta, which form aggregates.

Biophysical characterization of alpha-crystallin aggregates: validation of the micelle hypothesis

The size of alpha-crystallin aggregates, as well as the structural organization of each particle's subunits, is currently unknown, although a number of different laboratories have suggested both structures and average molecular weights (Thomson, J.A. and Augusteyn, R.C. (1984) Proc. Int. Soc. Eye Res. 3, 152). One hypothesis, compatible with literature reports and consistent with what is known of subunit primary structure and physiological function, is that alpha-crystallin exists in vivo as a naturally occurring protein micelle (Sen, A.C. and Chakrabarti, B. (1991) Biophysical J. 59, 108a.) To test this hypothesis, experiments were performed on this protein to determine its behavior under increased hydrostatic pressure and the effect of its concentration on aqueous surface tension. With increasing hydrostatic pressure, the turbidity of an alpha-crystallin solution increases exponentially to a plateau at about 6000-8000 psi; upon release of pressure, the samples slowly return to their original turbidity level. Other naturally aggregating proteins, such as skeletal muscle myosin, demonstrate a decrease in turbidity under the same conditions. The surface tension of alpha-crystallin in aqueous solution decreases to a plateau with increasing protein subunit concentration, with an inflection point over the range 0.18-0.25 mM; cholate and other amphiphiles exhibit similar behavior. In contrast, plots of surface tension over the equivalent concentration range for other protein aggregates in the same buffer more closely approximate the types of curve obtained with short chain aliphatic acids. These results indicate that alpha-crystallin behaves like the protein version of a micelle.

Expression and aggregation of recombinant alpha A-crystallin and its two domains

The 20 kDa alpha A and alpha B subunits of alpha-crystallin from mammalian eye lenses form large aggregates with an average molecular weight of 800,000. To get insight into the interactions responsible for aggregate formation, we expressed in Escherichia coli the putative N- and C-terminal domains of alpha A-crystallin, as well as the intact alpha A-crystallin chain. The proteins are expressed in a stable form and in relatively high amounts (20-60% of total protein). Recombinant alpha A-crystallin and the C-terminal domain are expressed in a water-soluble form. Recombinant alpha A-crystallin forms aggregates comparable with alpha-crystallin aggregates from calf lenses, whereas the C-terminal domain forms dimers or tetramers. The N-terminal domain is expressed in an initially water-insoluble form. After solubilization, denaturation and reaggregation the N-terminal domain exists in a high molecular weight multimeric form. These observations suggest that the interactions leading to aggregation of alpha A-crystallin subunits are mainly located in the N-terminal half of the chain.

The effect of urea on the aggregate state and elastase inhibitor activity of the water-insoluble fraction from bovine and human lens

Preparations of alpha-crystallin from bovine and human lens exhibited elastase inhibitor activity with a specific activity of 100-250 U mg-1 protein. A washed water-insoluble fraction from bovine, human and cataractous lens nuclei, when solubilized by sonication, gave specific activities of 910, 950 and 1270 U mg-1, respectively. Disaggregation of these water-insoluble fractions in 8.0 M urea, with subsequent reaggregation by urea removal, resulted in a decrease in inhibitor activity. Agarose A-5m gel filtration chromatography after the urea treatment resolved a residual high molecular weight (HMW) fraction and a peak which eluted at the position of water soluble alpha-crystallin. Assays showed that the urea-induced 'alpha-crystallin' peaks from all three preparations had specific activities, equivalent to native alpha-crystallin, whereas the HMW fractions retained their original high specific activity. We conclude that the increased elastase inhibitor activity of the water-insoluble fraction is a property of the aggregate state of the component alpha-crystallin molecules, which is lost upon reaggregation to an 800-kDa alpha-crystallin. Amino acid analysis of the bovine water-insoluble fraction suggested a content of 85-90% alpha-crystallin and 10-15% beta H-crystallin, which was confirmed by SDS-PAGE.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of isolation buffers on the properties of alpha-crystallin

This work was undertaken in order to resolve some of the controversy in the literature concerning the properties of alpha-crystallins isolated in different laboratories. Bovine lens proteins were extracted and isolated by gel chromatography using 'Hoenders buffer' (0.02 M Tris-HCl, 1 mM EDTA, 80 mM NaCl, pH 7.3), 'Tardieu buffer' (0.04 M phosphate, 1 mM EDTA, 0.2 mM DTT, 0.06 M KCl, pH 6.8) and 'Thomson/Augusteyn' buffer (0.05 M Tris-HCl, 2 mM EDTA, 0.2 mM DTT, pH 8.0). The alpha-crystallin peaks were then divided into 12-16 pools and subjected to detailed physicochemical characterization. Fractionation by HPLC-GPC and quasi-elastic light scattering indicated that the size of the proteins decreased with increasing elution volume and that they were stable for at least 9 months at 20 degrees C. Molecular masses were found to range from over 2 mDa at the front of the peaks to around 600 kDa at the end. The size distributions, for the three buffers, were indistinguishable. No differences could be detected in the polypeptide distributions by SDS-PAGE. The proteins were also identical in their near- and far-UV circular dichroism spectra, accessibility of their sulphydryl groups to DTNB, tryptophan accessibility to quenching by acrylamide and iodide, and immunoreactivity with two monoclonal antibodies with different specificities. It is concluded that identical alpha-crystallins are isolated with the three different buffers and that variations in pH (6.9-8.0), ionic strength (60-150 mM) and cation (K, Na, Tris) during the isolation do not affect the properties of the protein. Claims that differing observations on the properties of alpha-crystallin may be attributed to the buffers used, are untenable.

Purification and properties of the low-molecular-weight alpha-crystallin from normal goat lens: comparison with bovine lens

Low-molecular-weight alpha-crystallin (alpha L-crystallin) isolated from decapsulated lens of goat (Capra hiscus) has been purified to an apparently homogeneous population. Goat alpha L-crystallin closely resembles its bovine counterpart in size, shape, exposition of sulfhydryl groups, subunit composition and the nature of its UV-absorption profile. Like bovine alpha L-crystallin, dissociated subunits of goat alpha L-crystallin assemble upon reassociation into a particle of almost half the size of the native one. However, subunits of goat alpha L-crystallin are found to contain more aromatic amino acid residues than those of bovine subunits leading to a higher value of extinction coefficient (E1cm1%) at 280 nm for the goat protein.

Structural properties of polydisperse biopolymer solutions: a light scattering study of bovine alpha-crystallin

We have measured mean value of RHz, mean value of R2G1/2z, and mean value of Mw for individual fractions of the protein alpha-crystallin obtained by gel filtration of bovine lens nuclear extracts. A strong and monotonic decrease of mean value of RHz and mean value of Mw with increasing elution volume could be observed, indicating a broad size distribution. The experimental results are quantitatively consistent with a polymerization of monomeric units into linear chains, which may have a certain degree of flexibility. Using theoretical expressions for mean value of R2G and mean value of RH originally derived for semiflexible polymers in solution, we can self-consistently analyse the data from static and dynamic light scattering, and from electron microscopy experiments. We thus obtain detailed information on the molecular weight distribution and the quaternary structure of alpha-crystallin in these solutions.

The effects of sonication on alpha-crystallin

Sonication of bovine alpha-crystallin increases its molecular mass from around 770 kDa to in excess of 2,300 kDa. Exposure to 2M urea or 0.1 M glycine pH 7, did not affect the size of the sonicated protein, indicating that it did not consist of dimers and higher polymers of the original molecule. Sonication of a mixture of alpha-crystallins labelled on the A chain sulphydryl group with either an aminonaphthalene or a fluorescein chromophore, generated a product exhibiting substantial energy transfer. The average distance between the probes was calculated to be 5 nm. These observations suggest that sonication has generated a new quaternary structure, incorporating subunits from two or more different alpha-crystallin molecules. No significant differences were observed in the microenvironments of tryptophan residues although those in the sonicated protein could be more easily exposed by controlled denaturation with urea. A small decrease was observed in the quenchability of a fluorescent probe attached to the sulphydryl group and a small increase in the uptake of an hydrophobic probe. These data suggest that sonication may have altered the conformation of the subunits at, or near the surface of the protein.

A dynamic quaternary structure of bovine alpha-crystallin as indicated from intermolecular exchange of subunits

The structural bovine eye lens protein alpha-crystallin was dissociated in 7 M urea and its four subunits, A1, A2, B1, and B2, were separated by means of ion-exchange chromatography. Homopolymeric reaggregates of these subunits were prepared by removal of the denaturant via dialysis. It was found that subunits were exchanged upon incubation of mixtures of two homopolymers under native conditions. New hybrid species were formed within 24 h as demonstrated by isoelectric focusing. Moreover, native alpha-crystallin molecules also exchanged subunits when incubated with homopolymeric aggregates of B2 subunits. Subunit exchange between native alpha-crystallin molecules is postulated, and a "dynamic quaternary structure" is presented that allows the polydisperse protein to adapt to changes in cytoplasmic conditions upon aging of the lens tissue.

Excimer fluorescence in lens proteins labelled with N-(1-pyrene)maleimide

Lens proteins labelled with the sulphydryl reagent, N-(1-pyrene)maleimide exhibit pyrene excimer fluorescence around 470 nm. This was found to be associated with both the beta- and gamma-crystallins but not the alpha-crystallins. Disappearance of the 470 nm peak on exposure to denaturants indicates it arose from an intramolecular excimer. This excimer fluorescence provides a new and sensitive tool for monitoring conformational changes in the beta- and gamma-crystallins.

Folding-unfolding and aggregation-dissociation of bovine alpha-crystallin subunits; evidence for unfolding intermediates of the alpha A subunits

The aggregation and dissociation behavior of bovine alpha-crystallin as well as the folding and unfolding of its subunits were investigated by equilibrium studies using tryptophan fluorescence measurements and two isoelectric focusing techniques, viz. isoelectric focusing across a urea gradient and isoelectric focusing in two dimensions with different concentrations of urea. It was found that the alpha B chains lose their ability to aggregate and start unfolding at a lower concentration of urea than the alpha A chains. Equilibrium intermediates were found upon unfolding or refolding of alpha A subunits, which can be explained by a two-domain organization of these molecules.

On the structure of alpha-crystallin: construction of hybrid molecules and homopolymers

The alpha A2 and alpha B2 subunits of bovine alpha-crystallin were purified by chromatofocussing in urea and assembled into homopolymers. Light-scattering measurements indicated their molecular masses were 360 and 420 kDa. The alpha A2 and alpha B2 polypeptides were also used to construct a series of hybrid molecules with alpha A/alpha B ratios ranging from 7:1 to 1:7. Sedimentation velocity analyses, isoelectric focussing under non-deaggregating conditions, circular dichroism spectroscopy and immunochemical analysis indicated that all of the subunits had copolymerized to alpha-crystallin-like aggregates with complete regeneration of the native structure. The polymers could be distinguished on the basis of their differing affinities for the antiserum. This was directly related to the proportion of alpha A2 subunits in each polymer. It was concluded that the alpha A2 and alpha B2 subunits are structurally equivalent and occupy equivalent site in the alpha-crystallin aggregates. It was also concluded that a micellar-like quaternary structure was consistent with most previous observations on the protein.

Evaluation of equilibrium constants from precipitin curves: interaction of alpha-crystallin with an elicited monoclonal antibody

A simple procedure, based on the precipitin curve and the antibody:antigen ratios of the precipitates, is described for evaluation of the intrinsic association constant (k) governing the interaction between a multivalent antigen and a bivalent antibody. Its application is illustrated with a study of the interaction between alpha-crystallin and an elicited monoclonal antibody, which is shown to exhibit essentially identical affinities (k = 9 X 10(4) M-1) for the alpha m and alpha c forms of the antigen.

Specific dissociation of alpha B subunits from alpha-crystallin

Exposure of bovine alpha-crystallin to 0.1 M glycine at pH 7 decreases the average molar mass of the protein from 700 to 420 kDa. When the pH is lowered to 2.5, in the same buffer, the alpha B chains specifically dissociate from the aggregates, leaving a particle of 290 kDa containing only alpha A chains. The decrease in the molar mass corresponds to the mass of the alpha B chains in the original aggregate. The pH-dependent dissociation is fully reversible. Similar changes were observed with rat and kangaroo alpha-crystallins but the dogfish protein was not affected. Sedimentation velocity analyses and fluorescence spectroscopy yielded a pK, for the dissociation, of 3.7 for alpha-crystallin and 4.0 for a homopolymer constructed from purified alpha B2 polypeptides. An alpha A2 homopolymer was virtually unaffected by the lowering of pH. The products from the dissociation were isolated and their properties studied by sedimentation analysis and acrylamide quenching of tryptophan fluorescence. The alpha B chains were found to be completely denatured, whereas the structure of the alpha A chains, in the 290 kDa, particle, were only slightly altered. Comparisons of the sequences of the various proteins examined suggested that decreased ionization of aspartic acid 127 in the alpha B chain was responsible for the specific dissociation of this polypeptide.

On the structure of alpha-crystallin: the minimum molecular weight

alpha-crystallin can be isolated in two forms depending on the temperature at which the lens is extracted. At 4 degrees C, alpha c-crystallin is obtained while at 37 degrees C, a smaller molecule, alpha m-crystallin can be isolated. The apparent molecular weight of bovine foetal alpha m- and alpha c-crystallins were determined in 5 different ways using sedimentation velocity, sedimentation equilibrium and intensity fluctuation spectroscopy analyses and the experimentally determined diffusion coefficients, intrinsic viscosity and partial specific volume. Values ranged from 291,000 to 369,000 for alpha m and from 604,000 to 760,000 for alpha c due to the differential effects of the protein's polydispersity on the different methods. Subfractionation of the protein by gel filtration yielded much less polydisperse minimum species populations with molecular weights of 280,000 and 529,000 for alpha m and alpha c respectively. It was concluded that alpha-crystallin is probably synthesized as a symmetrical dodecamer and that the polydispersity of most preparations can be attributed to age-related modification in vivo as well as in vitro supra-aggregation due to variations in experimental conditions.

Isolation of alpha-crystallin and its subunits by affinity chromatography on immobilized monoclonal antibodies

Use has been made of the specific interactions between monoclonal antibodies and the alpha A or alpha B subunits of alpha-crystallin to devise methods for the purification of the intact protein or its subunits. alpha A and alpha B subunits were separated by affinity chromatography on an immobilized monoclonal antibody specific for alpha A chains, using a pH gradient. Use of an antibody which binds both subunits has enabled the isolation of intact alpha-crystallin aggregates. Gel electrophoresis, conformational probing and size analysis showed that the affinity purified proteins were purer but otherwise indistinguishable from the alpha-crystallins isolated by other methods.

Quenching of tryptophan fluorescence in bovine lens proteins by acrylamide and iodide

The microenvironments of tryptophan residues in bovine alpha-, beta H-, beta L and gamma-crystallins have been examined using acrylamide and KI quenching of fluorescence. From a consideration of the differential effects of the two quenchers, the quenching efficiencies and spectral changes, it was possible to distinguish tryptophans in different environments and to assign these to specific residues in the sequence. Two classes of tryptophan were identified in gamma-crystallin, one buried and one moderately accessible. The buried class contained tryptophans 42A and 125 which lie in the angles of the wedge-shaped domains of the protein. These residues, which had emission maxima at 326 nm, were not accessible to quenching by iodide. The more accessible residues, emitting at 334 nm, corresponded to tryptophans 64 and 148 which are in the widest part of the wedge-shaped subunit and close to the surface of the protein. The two beta-crystallins were virtually indistinguishable. They contained two buried tryptophans, probably residues 58 and 150, and three close to the surface, residues 81, 84 and 166. The quenching efficiencies for these two classes were lower than those observed with gamma-crystallin. Since the three-dimensional structures of the beta- and gamma-crystallins are probably very similar, this suggests that the polymeric nature of the beta-crystallins is responsible for the decreased accessibility of the tryptophans to the quenchers. alpha-crystallin demonstrated unusually high static quenching which made it difficult to distinguish different classes of tryptophan.(ABSTRACT TRUNCATED AT 250 WORDS)