Structure and stability of gamma-crystallins. I. Spectroscopic evaluation of secondary and tertiary structure in solution

Development of a selective and sensitive analytical method to detect isomerized aspartic acid residues in crystallin using a combination of derivatization and liquid chromatography mass spectrometry

The isomerization of amino acids in peptides and proteins induces structural changes and aggregation. The isomerization rate of aspartic acid (Asp) is high and causes various serious diseases including Alzheimer's disease and cataract. Herein, a method for the comprehensive separation and sensitive detection of isomerized crystallin containing Asp (l-α-Asp, l-β-Asp, d-α-Asp, and d-β-Asp) was developed using chiral derivatization and reversed-phase UHPLC separation. Of three candidate derivatization reagents tested for the separation of peptides containing isomerized aspartic acid, 2,5-dioxopyrrolidin-1-yl-1-(4,6-dimethoxy-1,3,5-triazine-2-yl) pyrrolidine-2-carboxylate (DMT-(R)-Pro-OSu) was the most suitable reagent for separating isomerized peptides and improved the sensitivity of mass spectrometry by 50-fold. This method was applied to analyze heat-denatured crystallin. Asp58 and Asp151 residues in αA-crystallin (AAC) exhibited the highest isomerization rate in heated crystallin. Furthermore, the analysis of α-crystallin extracted from bovine eye lens identified isomerized Asp residues (Asp24/35, Asp58, and Asp151 in AAC and Asp140 in αB-crystallin (ABC)). These results indicate that the newly developed method using chiral derivatization provides selective and sensitive analysis of isomerized Asp sites in α-crystallin protein. This novel method will allow for the identification and quantification of isomerized amino acids in crystallin proteins.

A rapid and sensitive detection of D-Aspartic acid in Crystallin by chiral derivatized liquid chromatography mass spectrometry

A method for the determination of D-Aspartic acid (D-Asp) and its D/L ratio in peptides and proteins has been developed. This method was carried out with good separation of the D/L chiral peptide pairs by combination of a chiral derivatization and an ADME column separation. Furthermore, a cationic derivatization reagent, DBD-Py-NCS, increased the sensitivity of the ESI-MS/MS detection. To confirm the comprehensive peptide analysis, synthesized α-Crystallin tryptic peptides, which included D-Asp residues, were analyzed. The 5 pairs of D/L-Asp that included peptide diastereomers were well separated. Their peak resolutions were more than 1.5 and the results were reproducible (RSD<0.05, n=5). As an application of this method, we analyzed the α-Crystallin standard and UV irradiated α-Crystallin. After trypsin digestion and DBD-Py-NCS derivatization, the tryptic peptide derivatives were applied to LC-MS/MS. Based on the results of peptide sequence identification, almost all the tryptic peptides of the αA- and αB-Crystallin homologous subunits of α-Crystallin were detected as DBD-Py NCS derivatives. However, there was no D-Asp residue in the standard proteins. In the case of the UV irradiated α-Crystallin, Asp⁷⁶ and Asp⁸⁴ in the αA-Crystallin and Asp⁹⁶ in αB-Crystallin were racemized to D-Asp. These results show that this proposed chiral peptide LC-MS/MS method using chiral derivatization provides a rapid and sensitive analysis for post translational Asp racemization sites in aging proteins.

Cataract-associated mutant E107A of human gammaD-crystallin shows increased attraction to alpha-crystallin and enhanced light scattering

Several point mutations in human γD-crystallin (HGD) are now known to be associated with cataract. So far, the in vitro studies of individual mutants of HGD alone have been sufficient in providing plausible molecular mechanisms for the associated cataract in vivo. Nearly all the mutant proteins in solution showed compromised solubility and enhanced light scattering due to altered homologous γ-γ crystallin interactions. In sharp contrast, here we present an intriguing case of a human nuclear cataract-associated mutant of HGD--namely Glu107 to Ala (E107A), which is nearly identical to the wild type in structure, stability, and solubility properties, with one exception: Its pI is higher by nearly one pH unit. This increase dramatically alters its interaction with α-crystallin. There is a striking difference in the liquid-liquid phase separation behavior of E107A-α-crystallin mixtures compared to HGD-α-crystallin mixtures, and the light-scattering intensities are significantly higher for the former. The data show that the two coexisting phases in the E107A-α mixtures differ much more in protein density than those that occur in HGD-α mixtures, as the proportion of α-crystallin approaches that in the lens nucleus. Thus in HGD-α mixtures, the demixing of phases occurs primarily by protein type while in E107A-α mixtures it is increasingly governed by protein density. Analysis of these results suggests that the cataract due to the E107A mutation could result from the instability caused by the altered attractive interactions between dissimilar proteins--i.e., heterologous γ-α crystallin interactions--primarily due to the change in surface electrostatic potential in the mutant protein.

Transtactin: a universal transmembrane delivery system for Strep-tag II-fused cargos

The delivery of molecules into cells poses a critical problem that has to be solved for the development of diagnostic tools and therapeutic agents acting on intracellular targets. Cargos which by themselves cannot penetrate cellular membranes due to their biophysical properties can achieve cell membrane permeability by fusion to protein transduction domains (PTDs). Here, we engineered a universal delivery system based on PTD-fused Strep-Tactin, which we named Transtactin. Biochemical characterization of Transtactin variants bearing different PTDs indicated high thermal stabilities and robust secondary structures. Internalization studies demonstrated that Transtactins facilitated simple and safe transport of Strep-tag II-linked small molecules, peptides and multicomponent complexes, or biotinylated proteins into cultured human cells. Transtactin-introduced cargos were functionally active, as shown for horseradish peroxidase serving as a model protein. Our results demonstrate that Transtactin provides a universal and efficient delivery system for Strep-tag II-fused cargos.

Mechanism of the Highly Efficient Quenching of Tryptophan Fluorescence in Human γD-Crystallin†

Quenching of the fluorescence of buried tryptophans (Trps) is an important reporter of protein conformation. Human gammaD-crystallin (HgammaD-Crys) is a very stable eye lens protein that must remain soluble and folded throughout the human lifetime. Aggregation of non-native or covalently damaged HgammaD-Crys is associated with the prevalent eye disease mature-onset cataract. HgammaD-Crys has two homologous beta-sheet domains, each containing a pair of highly conserved buried tryptophans. The overall fluorescence of the Trps is quenched in the native state despite the absence of the metal ligands or cofactors. We report the results of detailed quantitative measurements of the fluorescence emission spectra and the quantum yields of numerous site-directed mutants of HgammaD-Crys. From fluorescence of triple Trp to Phe mutants, the homologous pair Trp68 and Trp156 were found to be extremely quenched, with quantum yields close to 0.01. The homologous pair Trp42 and Trp130 were moderately fluorescent, with quantum yields of 0.13 and 0.17, respectively. In an attempt to identify quenching and/or electrostatically perturbing residues, a set of 17 candidate amino acids around Trp68 and Trp156 were substituted with neutral or hydrophobic residues. None of these mutants showed significant changes in the fluorescence intensity compared to their own background. Hybrid quantum mechanical-molecular mechanical (QM-MM) simulations with the four different excited Trps as electron donors strongly indicate that electron transfer rates to the amide backbone of Trp68 and Trp156 are extremely fast relative to those for Trp42 and Trp130. This is in agreement with the quantum yields measured experimentally and consistent with the absence of a quenching side chain. Efficient electron transfer to the backbone is possible for Trp68 and Trp156 because of the net favorable location of several charged residues and the orientation of nearby waters, which collectively stabilize electron transfer electrostatically. The fluorescence emission spectra of single and double Trp to Phe mutants provide strong evidence for energy transfer from Trp42 to Trp68 in the N-terminal domain and from Trp130 to Trp156 in the C-terminal domain. The backbone conformation of tryptophans in HgammaD-Crys may have evolved in part to enable the lens to become a very effective UV filter, while the efficient quenching provides an in situ mechanism to protect the tryptophans of the crystallins from photochemical degradation.

Crystal cataracts: human genetic cataract caused by protein crystallization

Several human genetic cataracts have been linked recently to point mutations in the gammaD crystallin gene. Here we provide a molecular basis for lens opacity in two genetic cataracts and suggest that the opacity occurs because of the spontaneous crystallization of the mutant proteins. Such crystallization of endogenous proteins leading to pathology is an unusual event. Measurements of the solubility curves of crystals of the Arg-58 to His and Arg-36 to Ser mutants of gammaD crystallin show that the mutations dramatically lower the solubility of the protein. Furthermore, the crystal nucleation rate of the mutants is enhanced considerably relative to that of the wild-type protein. It should be noted that, although there is a marked difference in phase behavior, there is no significant difference in protein conformation among the three proteins.

Solution structure, backbone dynamics, and stability of a double mutant single-chain monellin. structural origin of sweetness

Single-chain monellin (SCM), which is an engineered 94-residue polypeptide, has been characterized as being as sweet as native two-chain monellin. Data from gel-filtration high performance liquid chromatography and NMR has proven that SCM exists as a monomer in aqueous solution. In order to determine the structural origin of the taste of sweetness, we engineered several mutant SCM proteins by mutating Glu(2), Asp(7), and Arg(39) residues, which are responsible for sweetness. In this study, we present the solution structure, backbone dynamics, and stability of mutant SCM proteins using circular dichroism, fluorescence, and NMR spectroscopy. Based on the NMR data, a stable alpha-helix and five-stranded antiparallel beta-sheet were identified for double mutant SCM. Strands beta1 and beta2 are connected by a small bulge, and the disruption of the first beta-strand were observed with SCM(DR) comprising residues of Ile(38)-Cys(41). The dynamical and folding characteristics from circular dichroism, fluorescence, and backbone dynamics studies revealed that both wild type and mutant proteins showed distinct dynamical as well as stability differences, suggesting the important role of mutated residues in the sweet taste of SCM. Our results will provide an insight into the structural origin of sweet taste as well as the mutational effect in the stability of the engineered sweet protein SCM.

Molecular basis of a progressive juvenile-onset hereditary cataract

In a recent paper, patients with a progressive juvenile-onset hereditary cataract have been reported to have a point mutation in the human gammaD crystallin gene (Stephan, D. A., Gillanders, E., Vanderveen, D., Freas-Lutz, D., Wistow, G., Baxevanis, A. D., Robbins, C. M., VanAuken, A., Quesenberry, M. I., Bailey-Wilson, J., et al. (1999) Proc. Natl. Acad. Sci. USA 96, 1008-1012). This mutation results in the substitution of Arg-14 in the native protein by a Cys residue. It is not understood how this mutation leads to cataract. We have expressed recombinant wild-type human gammaD crystallin (HGD) and its Arg-14 to Cys mutant (R14C) in Escherichia coli and show that R14C forms disulfide-linked oligomers, which markedly raise the phase separation temperature of the protein solution. Eventually, R14C precipitates. In contrast, HGD slowly forms only disulfide-linked dimers and no oligomers. These data strongly suggest that the observed cataract is triggered by the thiol-mediated aggregation of R14C. The aggregation profiles of HGD and R14C are consistent with our homology modeling studies that reveal that R14C contains two exposed cysteine residues, whereas HGD has only one. Our CD, fluorescence, and differential scanning calorimetric studies show that HGD and R14C have nearly identical secondary and tertiary structures and stabilities. Thus, contrary to current views, unfolding or destabilization of the protein is not necessary for cataractogenesis.

Thermodynamic stability of human lens recombinant alphaA- and alphaB-crystallins

Lens alpha-crystallin is a 600-800-kDa heterogeneous oligomer protein consisting of two subunits, alphaA and alphaB. The homogeneous oligomers (alphaA- and alphaB-crystallins) have been prepared by recombinant DNA technology and shown to differ in the following biophysical/biochemical properties: hydrophobicity, chaperone-like activity, subunit exchange rate, and thermal stability. In this study, we studied their thermodynamic stability by unfolding in guanidine hydrochloride. The unfolding was probed by three spectroscopic parameters: absorbance at 235 nm, Trp fluorescence intensity at 320 nm, and far-UV circular dichroism at 223 nm. Global analysis indicated that a three-state model better describes the unfolding behavior than a two-state model, an indication that there are stable intermediates for both alphaA- and alphaB-crystallins. In terms of standard free energy (DeltaG(NU)(H(2)(O))), alphaA-crystallin is slightly more stable than alphaB-crystallin. The significance of the intermediates may be related to the functioning of alpha-crystallins as chaperone-like molecules.

Thermodynamic and kinetic characterization of calf lens gammaF-crystallin

Gamma-crystallin is reported to be conformationally stable because of its internal structural symmetry, and gammaF (gammaIVa) is the most stable among the various gamma-crystallin gene products. However, there is no detailed report on its thermodynamic and kinetic stability. In the present study, detailed unfolding of gammaF-crystallin was investigated by equilibrium and kinetics methods with fluorescence and far-UV CD spectroscopic measurements. The GdnHCl-induced unfolding curves probed by Trp emission maximum and intensity showed a sharp single-step transition. Upon widening the unfolding transition with the use of urea in 1.5 M GdnHCl, a more proper fit for thermodynamic analysis was obtained. GammaF-Crystallin underwent a straightforward two-state process (N <==> U) without showing any measurable amount of intermediate. The conformational stability, as measured by deltaG(D)H2O (approximately 9 kcal/mol), indicates that gammaF-crystallin is a very stable protein. The high activation energy deltaG++H2O (approximately 24 kcal/mol), calculated from unfolding kinetics monitored by far-UV CD at 218 nm, also indicates that the native and unfolded states are separated by a high activation energy barrier.

Mutational analysis of hydrophobic domain interactions in gamma B-crystallin from bovine eye lens

gamma B-crystallin is a monomeric member of the beta gamma-superfamily of vertebrate eye lens proteins. It consists of two similar domains with all-beta Greek key topology associating about an approximate two-fold axis. At pH 2, with urea as the denaturant, the domains show independent equilibrium unfolding transitions, suggesting different intrinsic stabilities. Denaturation experiments using recombinant one- or two-domain proteins showed that the N-terminal domain on its own exhibits unaltered intrinsic stability but contributes significantly to the stability of its C-terminal partner. It has been suggested that docking of the domains is determined by a hydrophobic interface that includes phenylalanine at position 56 of the N-terminal domain. In order to test this hypothesis, F56 was substituted by site-directed mutagenesis in both complete gamma B-crystallin and its isolated N-terminal domain. All mutations destabilize the N-terminal domain to about the same extent but affect the C-terminal domain in a different way. Replacement by the small alanine side chain or the charged aspartic acid residue results in a significant destabilization of the C-terminal domain, whereas the more bulky tryptophan residue causes only a moderate decrease in stability. In the mutants F56A and F56D, equilibrium unfolding transitions obtained by circular dichroism and intrinsic fluorescence differ, suggesting a more complex denaturation behavior than the one observed for gamma B wild type. These results confirm how mutations in one crystallin domain can affect the stability of another when they occur at the interface. The results strongly suggest that size, hydrophobicity, and optimal packing of amino acids involved in these interactions are critical for the stability of gamma B-crystallin.

Contributions of tryptophan side chains to the far-ultraviolet circular dichroism of proteins

It has often been assumed that the role of aromatic side chains in the far-ultraviolet region of protein circular dichroism (CD) is negligible. However, some proteins have positive CD bands in the 220-230 nm region which are almost certainly due to aromatic side chains. The contributions to the CD of interactions between tryptophan side chains and the nearest neighbor peptide groups have been studied, focusing on the indole Bb transition which occurs near 220 nm. Calculations on idealized peptide conformations show that the CD depends strongly on both backbone and side-chain conformation. Because of the low symmetry of indole, rotation about the C beta C gamma bond (dihedral angle chi 2) by 180 degrees generally leads to large changes in the CD, often causing the Bb band to reverse sign. When side-chain conformational preferences are taken into account, there is no strong bias for either positive or negative Bb rotational strengths. The observation that simple tryptophan derivatives such as N-acetyl-L-tryptophan methylamide have positive CD near 220 nm implies either that these derivatives prefer the alpha R region over the beta region, or that there is little preference for chi 2 < 180 degrees over chi 2 > 180 degrees. Nearest-neighbor-only calculations on individual tryptophans in 15 globular proteins also reveal a small bias toward positive Bb bands. Rotational strengths of the Bb transition for some conformations can be as large as approximately 1.0 Debye-Bohr magnetons in magnitude, corresponding to maximum molar ellipticities greater than 10(5) deg cm2/dmol. Although a substantial amount of cancellation occurs in most of the examples considered here, such CD contributions could be significant, especially in proteins of low helix content.

Picosecond fluorescence decay of lens protein gamma-II crystallin

The fluorescence decay of tryptophan residues in the bovine lens protein gamma-II crystallin has been measured in aqueous buffer solutions. Results were obtained as a function of emission wavelength, temperature, dissolved oxygen, and denaturing solvent. The protein displayed complex fluorescence decay which fit a biexponential model with a long component (ns) and a short component (few hundred ps). Measured fluorescence quantum yields data for gamma-II crystallin allowed calculation of radiative and non-radiative rate constants. The radiative rate constant was consistent with that observed in other indole derivatives, while the non-radiative rate constant was quite large and accounted for the short lifetime in gamma-II. The temperature dependence of the non-radiative decay in gamma-II crystallin yielded a small activation energy of only 1-2 kcal/mol, compared to 4 kcal/mol for the reference compound NATA whose barrier is known to derive from the rotamer model.

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.

Deconvolution of the circular dichroism spectra of proteins: the circular dichroism spectra of the antiparallel beta-sheet in proteins

A recently developed algorithm, called Convex Constraint Analysis (CCA), was successfully applied to determine the circular dichroism (CD) spectra of the pure beta-pleated sheet in globular proteins. On the basis of X-ray diffraction determined secondary structures, the original data set used (Perczel, A., Hollosi, M., Tusnady, G. Fasman, G.D. Convex constraint analysis: A natural deconvolution of circular dichroism curves of proteins, Prot. Eng., 4:669-679, 1991), was improved by the addition of proteins with high beta-pleated sheet content. The analysis yielded CD curves of the pure components of the main secondary structural elements (alpha-helix, antiparallel beta-pleated sheet, beta-turns, and unordered conformation), as well as a curve attributed to the "aromatic contribution" in the wavelength range of 195-240 nm. Upon deconvolution the curves obtained were assigned to various secondary structures. The calculated weights (percentages determining the contributions of each pure component curve in the measured CD spectra of a given protein) were correlated with the X-ray diffraction determined percentages in an assignment procedure and were evaluated. The Pearson product correlation coefficients (R) are significant for all five components. The new pure component curves, which were obtained through deconvolution of the protein CD spectra alone, are promising candidates for determining the percentages of the secondary structural components in globular proteins without the necessity of adopting an X-ray database. The CD spectrum of the CheY protein was interesting because it has the characteristic shape associated with the alpha-helical structure, but upon analysis yielded a considerable amount of beta-sheet in agreement with the X-ray structure.

The interaction of gamma-crystallins with model surfaces

Three biophysical techniques were employed to study the structure and thermal stability of a series of homologous bovine lens gamma-crystallins upon binding to three model surfaces. The surfaces in order of increasing hydrophobicity were silica, methyl silica, and diphenyl silica. Secondary structure was analyzed by deconvolution Fourier transform infrared spectroscopy, while tertiary structure alterations were probed by front surface fluorescence spectroscopy. The effect of surface binding on protein thermal stability was analyzed by fluorescence and differential scanning calorimetry. The comparison of free and surface-bound protein with variations in the electrostatic and hydrophobic character of both the protein and the adsorbent surface with these techniques demonstrated that: (i) destabilization on hydrophobic surfaces is greater than on a more hydrophilic interface, (ii) detectable conformational changes tend to increase as the hydrophobicity of the surface increases, and (iii) subtle structural differences among proteins can play an important role in determining differences in protein stability and structure upon surface adsorption.

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.

Average orientation of aromatic residues in proteins determined from linear dichroism spectroscopy. A comparison of results on bovine gamma-crystallins with X-ray data

The structures of the two very closely related proteins, bovine gamma II- and gamma IVa-crystallin have been studied by means of near-ultra-violet linear dichroism spectroscopy on squeezed polyacrylamide gel systems. The crystallin spectra are discussed in terms of the spectra of the aromatic chromophores present in these proteins and provide detailed information on the average orientation of these residues in the proteins. A comparison of our results with information based on crystallographic X-ray experiments shows excellent agreement, reflecting even some of the minor differences between the two proteins studied. Since linear dichroism measurements as performed here take a few days only, and can be done on most aqueous protein solutions, linear dichroism spectroscopy may give a valuable contribution to structural studies on proteins.

The reaction of singlet oxygen with proteins, with special reference to crystallins

Photosensitized oxidation of the eye lens proteins, the crystallins, is thought to lead to protein crosslinks and high molecular weight aggregates. Such protein modifications may be important factors in the formation of lens opacities or cataracts. We focus attention here on type 2 photo-oxidation involving the reaction of singlet oxygen (1O2) with crystallins and some "control" proteins. We find that: (1) trp residues are oxidized to N-formyl kynurenine and related products, but this in itself does not lead to the production of high molecular weight protein aggregates of the protein; (2) tyr residues react with 1O2 but we do not detect dihydroxyphenylalanine or bityrosine nor are protein crosslinks formed as a result; (3) oxidation of his residues appears necessary for high molecular weight protein covalent aggregates to form. Proteins devoid of his, e.g. melittin or bovine pancreatic trypsin inhibitor, do not form high molecular weight products upon reaction with 1O2. Prior reaction and blocking of his inhibits the crosslinking reactions. (4) The oxidized protein is seen to be more acidic than the parent and has an altered tertiary structure. (5) Among the crystallins, reactivity towards 1O2 varies in the order gamma greater than beta greater than alpha and also gamma A/E greater than gamma D greater than gamma B crystallin.

The conformational status of a protein influences the aerobic photolysis of its tryptophan residues: melittin, beta-lactoglobulin and the crystallins

We have studied the aerobic photolysis of the tryptophan residues of the proteins melittin and beta-lactoglobulin when the proteins are in ordered conformations and when they are in randomly coiled states. The results suggest that the conformational status of the protein is a factor that influences the photolysis of the constituent tryptophan residues. This point appears to be of relevance to the photo-oxidation of the tryptophan residues of the eye lens proteins crystallins.

Packing interactions in the eye-lens. Structural analysis, internal symmetry and lattice interactions of bovine gamma IVa-crystallin

gamma-Crystallins are a family of low molecular weight proteins found in high concentration in the densely packed regions of high refractive index in vertebrate lenses. Certain members have the characteristic property of a high critical temperature (tc) for phase separation. We report the three-dimensional structure determination of such a protein, bovine lens gamma IVa-crystallin, which has been refined to give an X-ray R-factor of 0.143. Its high tc contrasts with the low tc gamma II-crystallin, whose structure we have already published. The root mean square difference between the alpha-carbon atoms of these two proteins is 0.70 A and gamma IVa has an internal symmetry even higher than that of gamma II. The presence of a protein that exhibits the phenomenon of phase separation at body temperature renders the lens very susceptible to a transformation from transparent to an opaque state due to irregularities in the refractive index. Protein interactions of gamma IVa-crystallin have implications for the mechanism of cataract formation. Modes of self-association behaviour of gamma IVa-crystallin have been inferred from an analysis of the lattice interactions in the crystalline state, where the protein packing density is similar to that of the intact lens. It appears that the point mutation at position 103 from a serine residue in gamma II to a valine in gamma IVa gives rise to a lattice contact formed by two four-stranded beta-sheets in gamma IVa. A group-specific mutation at position 118 from leucine to phenylalanine induces subtle differences in core packing, leading to a reorganization around residue 103. However, the final phase separation determinant may be a complex combination of many side-chain functions.

Phosphorescence measurements of calf gamma-II, III, and IV crystallins at 77 and 293 K

Structural and dynamical features of bovine gamma-crystallin tryptophan residues were investigated by phosphorescence measurements at 77 and 293 K. The low temperature phosphorescence spectra and lifetimes of calf gamma-II, III, and IV crystallins did not reflect heterogeneity among the gamma-crystallins. The 0-0 bands were all at 414 +/- 1 nm and the emission lifetimes were all single-exponential with lifetimes of 5.1, 5.3 and 5.3 +/- 0.3 sec, respectively. In contrast, phosphorescence measurements at room temperature were sensitive to subtle differences in exposure, accessibility, and flexibility of gamma-crystallin tryptophan residues. Thorough deoxygenation allowed for measurement of the normally-quenched room-temperature phosphorescence, and we report the first native phosphorescence measurements of lens crystallins at ambient temperature. The emission maxima for gamma-II, III and IV were 446, 442, and 440 +/- 2 nm, respectively. The intensity decay curves were all non-single exponential, and the decays were fit to a sum of two exponentials with lifetimes of 9.1 and 93 msec (gamma-II), 11 and 75 msec (gamma-III), and 4.2 and 68 msec (gamma-IV), +/- 10%. The components of the gamma-II emission were assigned to the four tryptophans based on X-ray structural information. Quantum yields of the phosphorescence emission were in the ratio of 20:7:1 for gamma-II, III and IV, and comparison of lifetimes and quantum yields suggests that tryptophan rigidity increases in the order gamma-IV less than III less than II. Acrylamide quenching constants for the long-lived components of gamma-II and III were roughly equal, while the short-lived tryptophans of gamma-III were an order of magnitude more accessible than those of gamma-II. The wide range of phosphorescence lifetimes and quenching constants allowed for discrimination of distinct contributions to the phosphorescence emission, and we suggest that room-temperature phosphorescence measurements will be an effective tool for studying conformational changes of lens crystallins.

Light scattering and photocrosslinking in the calf lens crystallins gamma-II, III and IV

The calf lens proteins gamma-II, -III and -IV crystallin have been photolyzed in pH 7.5 phosphate buffer solution at 25 degrees C. The photolysis light source was either a xenon arc lamp/monochromator system set to pass 290 +/- 5 nm or a nitrogen laser operating at 337.1 nm. Photolysis experiments at 337.1 nm were done both in the presence and absence of added 1.0 x 10(-4) M N-formylkynurenine (NFK). In addition, 1 x 10(-5) M riboflavin was added as a photosensitizer in a few of the experiments. All solutions were 1.0 mg ml-1 protein, and 1.0 ml of solution was irradiated for periods ranging from 10 min to 3 hr. During the 337.1 nm irradiations, the turbidity of the protein solutions was continuously monitored using a He-Ne laser at 632.8 nm. Progress of the 290 nm irradiations was monitored by observing the loss of tryptophan fluorescence for each of the gamma crystallin proteins. The rate of growth of light scattering, upon 337.1 nm irradiation, was greatest for gamma-IV. Addition of NFK caused the rates of growth of UV-induced light scattering of all three gamma crystallins to increase significantly. These rates were in the order: gamma-IV much greater than gamma-III greater than gamma-II. Following UV exposure, the protein solutions were analyzed using UV-visible absorption spectroscopy and SDS-PAGE. Irradiated gamma crystallin solutions showed increased optical density throughout the visible region, resulting from solution turbidity.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative studies of beta s-crystallins from human, bovine, rat and rabbit lenses

Soluble extracts from young bovine, human, rat and rabbit lenses were fractionated by high resolution size-exclusion chromatography to demonstrate the existence of three discrete size-classes of monomeric crystallins in each species. These were identified by ion exchange chromatography, amino acid analysis, SDS electrophoresis and isoelectric focusing as the beta s-, gamma A- and gamma B-crystallins. Conventional SDS electrophoretic analysis of these proteins revealed apparent Mr values of about 23kD, 22kD and 19kD, respectively. Similar analysis in the presence of 6 M urea showed the proteins all co-migrated with an apparent Mr of about 20,500, which is far more consistent with the molecular weights calculated from beta s- and gamma-crystallin sequence data. Amino acid compositions of all the beta s samples indicate a high degree of homology to the bovine protein, whose sequence is known. The different species beta s-crystallins showed other general similarities in size, charge, thiol content and secondary structural properties. On the other hand, near UV CD and fluorescence emission and energy transfer measurements indicate that these proteins have subtle yet significant differences in their tertiary structures. Unlike the gamma-crystallins, the secondary structure of all of the beta s samples is completely denatured in the presence of 8 M urea at 20 degrees C.

Presence of nontryptophan fluorophores specifically bound to gamma 2-crystallin

alpha-, beta-, and gamma-crystallins have been purified from nonpathological lenses of calves. The pure proteins have been examined for nontryptophan fluorescence and fluorescent compounds have been found specifically bound to gamma 2-crystallin. The protein has been unfolded by 6 M guanidine hydrochloride (Gdn-HCl) and a separation of the fluorescent compounds has been obtained by gel chromatography in the presence of 6 M Gdn-HCl. The spectroscopic features (absorbance, fluorescence) of the protein returned to normal following removal of the chromophores. The low-molecular-weight separated fluorescent compounds have been fractionated and extracted from the Gdn-HCl solution by ethyl acetate. TLC chromatography has shown the presence of kynurenine, 3-OH-kynurenine, and free tryptophan. These data suggest that direct involvement of the intrinsic protein tryptophans in the photochemical processes leading to formation of fluorescent compounds has to be excluded. Free tryptophan and intrinsic metabolic factors are probably more relevant in determining the cataractous insult.

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.

High-molecular-weight protein aggregates of calf and cow lens: spectroscopic evaluation

To gain insight into the molecular features of the high-molecular-weight (HMW) fraction of soluble lens proteins and their changes in aging, we isolated this fraction from the nucleus of calf and cow lenses and measured fluorescence and circular dichroism (CD) properties of the samples. Not only was there an increase in the HMW fraction in the older lens, but there was also an age-related difference in tertiary structure that was clearly manifested in the fluorescence and CD parameters. The far-u.v. CD of low- and high-molecular-weight proteins do not differ significantly in band position and magnitude, but the near-u.v. CD of HMW protein does differ distinctly from that of all other crystallins (alpha, beta and gamma); the entire CD spectrum of this protein is displayed in the negative region. Millipore filtration further revealed that HMW aggregates are essentially a polydisperse population of different conformation (tertiary structure) and that these aggregates are associated by non-convalent interactions. This association is caused mainly by the apolar (hydrophobic) nature of the constituent protein. alpha-Crystallin has more hydrophobic domain along the peptide chain that do other crystallins and thus is likely to be the predominant protein in HMW aggregates.

Fluorescence quenching studies of the structures of calf gamma-II, III, and IV crystallins

The structures of the calf lens crystallin fractions gamma-II, gamma-III, and gamma-IV have been investigated using the fluorescence quenching method. The three crystallin fractions showed very large differences in the quenching rates of their fluorescent tryptophan residues, for quenching by acrylamide or iodide in pH 7.5 phosphate buffer solutions. The experimentally measured quenching rate constants were kq(II) = 3.2 x 10(8), kq(III) = 9.9 x 10(8), and kq(IV) = 1.8 x 10(9) M-1 sec-1. Smaller rate constants were obtained for iodide quenching of the three crystallins, but the values were in approximately the same ratios as the ones found for acrylamide quenching. The conclusion is that the tryptophan residues in gamma-II crystallin are 6-10 times less easily quenched than those of gamma-IV crystallin and 3-6 times less easily quenched than those of gamma-III. These conclusions are in accord with those reached by Mandal et. al. based on fluorescence and CD data, who found the following order of Trp hydrophobicities: gamma-II greater than gamma-III greater than gamma-IV. The significance of these structural differences for lens function and stability remains to be elucidated.

Structure and stability of gamma-crystallins. II. Differences in microenvironments and spatial arrangements of cysteine residues

The gamma-crystallin fractions II, III and IV from calf eye lens were treated with the thiol-specific fluorescent probe 2-(4'-maleimidylanilino)naphthalene-6-sulfonate (MIANS), in order to determine the reactivity of the seven (gamma-II) or six (gamma-III, gamma-IV) cysteine residues. Two classes of reactive cysteines were distinguished by variations in fluorescence intensity with increasing molar excess of the probe, and approximately three cysteines were nonreactive in each gamma-crystallin. From the position of the emission maximum, it is apparent that MIANS-labeled cysteines of gamma-IV are in the least hydrophobic environment. Fluorescence energy transfer was observed from tryptophan to MIANS-labeled cysteines in both gamma-II and gamma-III crystallins, with efficiencies of 86% and 89%, respectively, but not in gamma-IV crystallin. We suggest that the spatial arrangements and microenvironments of cysteine residues of gamma-crystallins are sufficiently different from each other to account for the variations in fluorescence characteristics of the MIANS-labeled proteins and the lack of energy transfer in gamma-IV crystallins.

Acrylamide and iodide fluorescence quenching as a structural probe of tryptophan microenvironment in bovine lens crystallins

Fluorescence quenching using acrylamide and iodide quenchers has been used to investigate the microenvironments of tryptophan residues in bovine alpha-, beta-, and gamma-crystallin fractions. Acrylamide quenching is very sensitive to the degree of tryptophan accessibility to the solvent containing the acrylamide. Since acrylamide is able to diffuse into the interior of the protein, accessibility to acrylamide may result from Trp residues lying at the surface of the protein or from the existence of channels leading to the interior of the protein. Iodide ion is hydrated and is limited by its large size and charge to quenching of tryptophan residues lying at or near the surface of proteins. Tryptophan residues in the lens crystallin fractions were found to be highly accessible to acrylamide, yet the rate of quenching by acrylamide was very low, indicating that the tryptophan residues of the lens crystallin fractions occupy predominately hydrophobic environments. The high accessibility to acrylamide likely results from diffusion of acrylamide into the interior of the protein. Accessibility to iodide was much lower, as was the rate of quenching by iodide, adding support to the conclusions from acrylamide quenching.