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

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.

Melittin-induced conformational changes in human lens protein

Circular dichroism and fluorescence measurements showed a reduced conformational order in proteins of a normal human lens when they were incubated in vitro with melittin, a bee venom peptide. Since melittin is also known to react with lipids to induce a breakdown of vesicular structure, the observed denaturation of water-soluble proteins of a human lens that developed a cataract due to multiple bee stings may be accounted for by the effects of melittin to some extent. The melittin-induced decrease of conformational order, as observed in our in-vitro studies could thus be of physiological significance.

The calf gamma crystallins--a Raman spectroscopic study

The solution structures of the four major components of bovine lens gamma-crystallin, gamma s, gamma II, gamma III and gamma IV are compared using Raman spectroscopy. The spectral region sensitive to the vibrational frequencies of aromatic and sulfur containing residues and to the backbone skeletal stretching modes (500-1000 cm-1), and that reflecting secondary structure (1,000-1,700 cm-1) are strikingly similar in all four gamma-crystallin fractions. These similarities are indicative of the dominant anti-parallel beta sheet structure common to all the gamma-crystallins. A comparison of the ratios of the Raman intensities at 850 cm-1 and 830 cm-1 (I850/I830), an empirical measure of the degree of hydrogen bonding of phenolic hydroxyl groups, suggests that the tyrosine residues in all the gamma-crystallin fractions are moderately hydrogen bonded. Distinct differences in the solution structures of the gamma-crystallins were observed in the higher energy end of the vibrational Raman spectra. The sulfhydryl stretching frequencies for the gamma-crystallins exhibit complex splitting patterns in the 2,500-2,600 cm-1 region. These patterns are due to the competing effects of hydrogen bonding and S-pi interactions with neighboring aromatic residues. All five proteins exhibit multiple, but distinct, thiol frequencies, suggesting that the microenvironments of the cysteine residues in these proteins are significantly different.

Effect of acetylation by aspirin on the thermodynamic stability of lens crystallins

To assess the effect of aspirin on cataractogenesis, we compared the stability of individual, native protein fractions alpha L, beta H, beta L, beta s, beta B2, gamma-II, gamma-III and gamma-IV with that of their acetylated counterparts. The conformational stabilities of native fractions beta B2 and beta s, which were not reported earlier, were determined first from their thermal and a thermal denaturation behaviour. Since alpha L, beta H and beta L fractions are oligomeric, no thermodynamic analysis of these fractions was attempted. The thermal stability of beta s and beta B2 is rather low; their melting temperature (T1/2) range is 58-60 degrees C compared with 67-75 degrees C for the gamma-crystallins. Furthermore, except for alpha L, which remains stable even at 100 degrees C, and beta B2, all crystallins aggregate at temperatures slightly above T1/2. The Gibbs free energy of unfolding, delta GH2OD, calculated from guanidine HCl (GdnHCl) denaturation, is surprising low (3-9 kcal mol-1) for all crystallin fractions. The low values of delta GH2OD indicate that the structural destabilization of these proteins, which may lead to cataract formation, could result from a slight disturbance of a particular kind (sugar, UV light, oxidation, and other factors). The overall effect of acetylation on the individual crystallin fractions is mixed. The thermal stability of beta B2 increased, tended to decrease in the case of gamma-crystallins, but remained virtually unchanged for other proteins. Delta GH2OD values of the native crystallin fractions do not differ significantly from those of their acetylated counterparts.

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.

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.

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.

Heterogeneity of gamma-crystallins from spiny dogfish (Squalus acanthias) eye lens

Mammalian lenses contain multiple gamma-crystallin gene products, which are differentially synthesized during lens development. We now report the isolation and characterization of multiple gamma-crystallins from lenses of adult spiny dogfish (Squalus acanthias) aged about 20-30 years. About 50% of total lens protein solubilized in 50 mM phosphate, pH 7.0; about 25% of this soluble fraction consists of gamma-crystallins as determined by gel filtration. These gamma-crystallins appear homogeneous with respect to molecular weight (approximately equal to 20,000) on SDS-polyacrylamide gels, but their isoelectric points range from below pH 6 to above 10. Preparative cation-exchange chromatography on SP-Sephadex at pH 4.8 resolves four major subfractions, while anion-exchange on DEAE-cellulose at pH 9.5 resolves seven subfractions. Although these procedures separate basic from acidic polypeptides, most of these gamma-crystallin subfractions still consist of polypeptide mixtures, as determined by ion-exchange HPLC and isoelectric focusing. Analytical cation-exchange HPLC on SynChropak CM300 at pH 6.0 resolves at least 10 different gamma-crystallin components. Amino acid compositions of all the subfractions are similar, yet distinct in the sense that three subclasses can be distinguished. Sulfhydryl residues range from three to six per chain, most of which are buried. The large heterogeneity of gamma-crystallins in adult lens may result from different gene products in combination with post-translational modification.