Purification and crystallization of mammalian lens gamma-crystallins

Influence of glutathione fructosylation on its properties

Incubation of fructose and glutathione leads to the formation of N-2-deoxy-glucos-2-yl glutathione as the major glycation product, with characteristic positive ion at 470 Th in LC-MS spectra. Glutathione disulfide and fructose generate two compounds: N-2-deoxy-glucos-2-yl glutathione disulfide (m/z=775 Th) and bis di-N,N'-2-deoxy-glucos-2-yl glutathione disulfide (m/z=937 Th). N-2-deoxy-glucos-2-yl glutathione is 2.5-fold less effective than glutathione in reducing dehydroascorbic acid. Glutathione peroxidase and glutahione-S-transferase exhibit marginal activity toward N-2-deoxy-glucos-2-yl glutathione, while glyoxalase I shows 44.9% of the enzyme's specific activity. Glutathione reductase demonstrates 6.9% of the enzyme's specific activity with bis di-N,N'-2-deoxy-glucos-2-yl glutathione, while with mono-N-glucosyl glutathione disulfide retained 5 6.1% of the original activity. Glutathione reductase could not reduce N-2-deoxy-glucos-2-yl glutathione in mixed disulfide with gammaS-crystallin, but reduced glutathione in mixed disulfide with gammaS-crystallin by 90%. The presence of N-2-deoxy-glucos-2-yl glutathione in mixed disulfide with gammaS-crystallin makes this molecule more susceptible to unfolding than native gammaS-crystallin.

Chaperone-like activity of a synthetic peptide toward oxidized gamma-crystallin

alphaA-Crystallin can function like a molecular chaperone. We recently reported that the alphaA-crystallin sequence, KFVIFLDVKHFSPEDLTVK (peptide-1, residues 70-88) by itself possesses chaperone-like (anti-aggregating) activity during a thermal denaturation assay. Based on the above data we proposed that the peptide-1 sequence was the functional site in alphaA-crystallin. In this study we investigated the specificity of peptide-1 against gamma-crystallin aggregation in the presence of H2O2 and CuSO4. Peptide-1 was able to completely protect against the oxidation-induced aggregation of gamma-crystallin. Removal of N-terminal Lys or the replacement of Lys with Asp (DFVIFLDVKHFSPEDLTVK, peptide-2) did not alter the anti-aggregation property of peptide-1. However, deletion of KF residues from the N-terminus of peptide-1 resulted in a significant loss of its anti-aggregation property. Bio-gel P-30 size-exclusion chromatography of gamma-crystallin incubated with peptide-2 under oxidative conditions revealed that a major portion of the peptide elutes in the void volume region along with gamma-crystallin, suggesting the binding of the peptide to the protein. Peptide-1 and -2 were also able to prevent the UV-induced aggregation of gamma-crystallin. These data indicate that the same amino acid sequence in alphaA-crystallin is likely to be responsible for suppressing the heat-denatured, oxidatively modified and UV-induced aggregation of proteins.

Purification of gamma-crystallin from human lenses by acetone precipitation method

PURPOSE

The aim of this study was to develop a new purification method for human lens gamma-crystallin by utilizing its unique property of remaining soluble during acetone precipitation of water soluble (WS) proteins.

METHODS

The WS protein fractions from lenses of donors of different ages were precipitated with 50% acetone (v/v) and the supernatant and precipitated protein fractions were collected following centrifugation. Among lens crystallins, gamma-crystallin remained soluble (recovered in the supernatant following centrifugation) while other crystallins were precipitated. To determine the recovery of maximal levels of gamma-crystallin as soluble protein during acetone precipitation, the WS proteins were precipitated under different conditions, and both supernatant and precipitated fractions were quantified for proteins and analyzed by size-exclusion chromatographic and Western blot methods. Based on these results, a three-step purification procedure for gamma-crystallin was developed which consisted of acetone precipitation followed by preparative isoelectric focusing (IEF) and size-exclusion HPLC of the soluble fraction.

RESULTS

During precipitation of WS proteins by 50% (v/v) acetone, only gamma-crystallin remained soluble. The identity of gamma-crystallin was based on its Mr of 20 kDa on SDS-PAGE, co-elution with lens homogenate gamma-crystallin during a size-exclusion Agarose chromatography, immunoreactivity with anti-gamma-crystallin antibody on a Western blot and an overlap of its partial N-terminal sequence with gammaC-crystallin. A three-step procedure, as described above, provided a highly purified preparation of gammaC-crystallin from the WS protein fraction. The three-step procedure was also utilized to recover a highly purified human lens recombinant gammaD-crystallin preparation from E. coli lysate.

CONCLUSIONS

The unique property of human lens gamma-crystallin of remaining soluble during acetone precipitation can be utilized to purify this crystallin by a three-step procedure. This procedure is also applicable in the purification of recombinant gammaD-crystallin from E. coli lysate.

Towards a molecular understanding of phase separation in the lens: a comparison of the X-ray structures of two high Tc gamma-crystallins, gammaE and gammaF, with two low Tc gamma-crystallins, gammaB and gammaD

gamma-Crystallins, although closely related in sequence, show intriguing differences in their temperature-dependent interactions: those that have a high or intermediate Tc for phase separation are cryoproteins whereas low Tc gamma-crystallins are not. To address the molecular basis of phase separation, X-ray crystallography has been used to define the structural differences between high and low Tc gamma-crystallins. A pre-requisite for this study was to clarify the assignment of bovine gene sequences to bovine gamma-crystallin proteins used for biophysical measurements. Based on nucleotide sequence analyses of gamma E and gamma F bovine crystallin genes, gamma F corresponds to the previously crystallised high Tc protein bovine gamma IVa and gamma E corresponds to the high Tc bovine protein fraction previously known as gamma IIIa. The gamma F sequence has enabled the completion of the refinement of the bovine gamma F crystal structure which shows that the molecule has an additional surface tryptophan explaining why gamma F has different spectroscopic properties from gamma B. A high Tc protein from rat lens, gamma E crystallin, has been crystallised and the X-ray structure solved at 2.3 A resolution. Comparison of the X-ray structures of two high Tc proteins, rat gamma E and bovine gamma F, with the structures of two low Tc proteins, bovine gamma B and bovine gamma D, shows that the main conformational change between high and low Tc proteins is in the cd surface loop of motif 3. All four structures have numerous ion pairs on their surfaces leading to a high surface charge density, yet with low overall charge. Comparison of the lattice contacts of the two high Tc proteins with the two low Tc gamma-crystallins indicates that these high Tc proteins utilise more amino-aromatic interactions such as between histidine and arginine. Comparison of the sequences of all the gamma-crystallins which have been characterised for phase separation temperature indicates that only residue Arg/Lys 163 uniquely distinguishes cryo from non-cryo gamma-crystallins and it is close to the altered surface loop. Although this region probably contributes to phase separation, Tc is likely to be a function of an overall global property that is responsive to overall charge distribution. Calculated dipole moments of native gamma-crystallins, low Tc gamma-crystallin sequences threaded into high Tc gamma-crystallin structures, and vice versa, show how both sequence and 3D structure contribute to this overall property. High Tc gamma-crystallins have on average higher Arg/Lys ratios and higher histidine content. It is hypothesised that this increases the proportion of surface static paired charged networks which thus reduces the repulsive hydration force and so increases the attractive interactions of the protein-rich phase in binary liquid phase separation.

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.

On the stability of bovine gamma II crystallin

Bovine gamma II crystallin, undergoes structural changes when lyophilized. The lyophylized powder does not readily dissolve in buffer, although it can be taken up in distilled water. The lyophylized sample, as opposed to a sample concentrated by vacuum distillation at 30 degrees C, does exhibit different migration on isoelectric focusing gels. The hydration and denaturation properties of the two preparations are different. The lyophylized sample possesses a higher non-freezable water content as a function of concentration than its counterpart. The lyophylized sample also shows less denaturation in differential scanning calorimetry scans, up to 110 degrees C, than its counterpart. This indicates that lyophylization may induce a slight denaturation, due to structural-conformational change. On the other hand, the CD spectra of lyophylized and non-lyophylized gamma II crystallins do not differ significantly. This implies that the conformational changes upon lyophylization do not involve the secondary structure of gamma II crystallin.

Protein interactions in the calf eye lens: interactions between beta-crystallins are repulsive whereas in gamma-crystallins they are attractive

Non-specific interactions in beta- and gamma-crystallins have been studied by solution X-ray scattering and osmotic pressure experiments. Measurements were carried out as a function of protein concentration at two ionic strengths. The effect of temperature was tested between 7 degrees C and 31 degrees C. Two types of interactions were observed. With beta-crystallin solutions, a repulsive coulombic interaction could be inferred from the decrease of the normalized X-ray scattering intensity near the origin with increasing protein concentration and from the fact that the osmotic pressure increases much more rapidly than in the ideal case. As was previously observed with alpha-crystallins, such behaviour is dependent upon ionic strength but is hardly affected by temperature. In contrast, with gamma-crystallin solutions, the normalized X-ray scattering intensity near the origin increases with increasing protein concentration and the osmotic pressure increases less rapidly than in the ideal case. Such behaviour indicates that attractive forces are predominant, although we do not yet know their molecular origin. Under our experimental conditions, the effect of temperature was striking whereas no obvious contribution of the ionic strength could be seen, perhaps owing to masking by the large temperature effect. The relevance of the different types of non-specific interactions for lens function is discussed.

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.

Binary-liquid phase separation of lens protein solutions

We have determined the coexistence curves (plots of phase-separation temperature T versus protein concentration C) for aqueous solutions of purified calf lens proteins. The proteins studied, calf gamma IIIa-, gamma IIIb-, and gamma IVa-crystallin, have very similar amino acid sequences and three-dimensional structures. Both ascending and descending limbs of the coexistence curves were measured. We find that the coexistence curves for each of these proteins and for gamma II-crystallin can be fit, near the critical point, to the function /(Cc-C)/Cc/ = A [(Tc - T)/Tc]beta, where beta = 0.325, Cc is the critical protein concentration in mg/ml, Tc is the critical temperature for phase separation in K, and A is a parameter that characterizes the width of the coexistence curve. We find that A and Cc are approximately the same for all four coexistence curves (A = 2.6 +/- 0.1, Cc = 289 +/- 20 mg/ml), but that Tc is not the same. For gamma II- and gamma IIIb-crystallin, Tc approximately 5 degrees C, whereas for gamma IIIa- and gamma IVa-crystallin, Tc approximately 38 degrees C. By comparing the published protein sequences for calf, rat, and human gamma-crystallins, we postulate that a few key amino acid residues account for the division of gamma-crystallins into low-Tc and high-Tc groups.

Suppression of phase separation in solutions of bovine gamma IV-crystallin by polar modification of the sulfur-containing amino acids

The calf lens protein gamma IV-crystallin, a strong determinant of the net phase-separation temperature of the lens, was chemically modified with N-bromoacetylethanolamine phosphate. The phase-separation temperatures of solutions of the modified protein were measured and found to be dramatically reduced with respect to those of the native protein. At neutral pH the reagent alkylates only the cysteine and methionine residues and introduces a doubly charged phosphate anion at a maximum distance of 10-12 A from the sulfur atoms. At a protein concentration of 38 g/liter, and with 30% of the cysteines and 40% of the methionines alkylated, the phase-separation temperature is lowered from approximately 25 +/- 2 degrees C to approximately 12 +/- 2 degrees C. The ascending limbs of the coexistence curves for the native and modified proteins were determined at two different degrees of modification. The coexistence curve of the protein with 35% of the cysteines and 40% of the methionines modified shows that as protein concentration approaches the critical concentration of 289 g/liter, there is a much larger suppression of the critical temperature, from approximately 38 +/- 2 degrees C in the native protein to approximately 16 +/- 2 degrees C. Incubation of intact calf lenses in vitro with the reagent results in the suppression of the phase-separation temperature by 3-9 degrees C. These results are consistent with the view that the observed suppression in the critical temperature is due to an increase in the hydrophilicity of the protein in the vicinity of the sulfur-containing residues.

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.

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.

The molecular localization of non-tryptophan chromophores in calf lens crystallins

A single-step separation of calf lens gamma-crystallin into six protein components is described. UV absorption spectra, characterized by the presence of high absorbance in the 240-250 nm and 310-360 nm spectral regions as well as by fluorescence emission above 400 nm, are shown by six components. alpha-, beta and beta S crystallins have been compared with the gamma-fraction for the presence of non-tryptophan fluorescence. The chromophores responsible for this non-tryptophan fluorescence were found to be associated with gamma-crystallin components only. The spectral features of one selected gamma-crystallin component (characterized by an isoelectric point of 7.68) have been examined. Results seem to suggest the presence of oxidative products of tryptophan. Implications of these findings for the expression of human and bovine genes are also considered.

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)

Raman spectroscopic evidence for a disulfide bridge in calf gamma II crystallin

Laser Raman spectroscopy has been applied to native and dithiothreitol-treated bovine cortical gamma II crystallin to examine the state of the thiol groups and the presence of a putative disulfide bridge. The data reveal significant differences in two key spectral regions. In the thiol stretching region (2500-2600 cm-1), the dithiothreitol-reduced form shows a 25% increase in the integrated Raman signal as compared to the native form. The magnitude of this increase corresponds to the presence of 1 mol of disulfide/mol of gamma II as determined both by the Raman data and the previous biochemical analysis from this laboratory. In the disulfide stretching region (500-540 cm-1), the native form shows a line near 511 cm-1 which is absent in the reduced form. Both native and reduced forms show a triple-banded thiol signal with one or more distinct shoulders, suggesting at least three and perhaps five different environments for the cysteine residues. The difference spectrum, obtained by a 1:1 computer subtraction of the native from the reduced form, indicates that the increase in thiol signal is centered around 2572 cm-1. In every other spectral region, both native and reduced gamma II forms are closely similar. These results strongly support the biochemical data reported earlier and indicate that the reduction of the single disulfide bridge is accompanied by minimal changes in secondary structure in solution.

Optimal resolution of eye lens gamma-crystallins by cation-exchange high-performance liquid chromatography on SynChropak CM300

Cation-exchange high-performance liquid chromatography on SynChropak CM300 in Tris-acetate buffers of pH 5-7, using sodium acetate gradients, produces an excellent separation of the various gamma-crystallin gene products and their post-synthetically modified forms from eye lens. With a single analysis of total lens extract, the gamma-crystallins can be resolved, quantified and collected for amino acid analysis. Experimental conditions are presented for optimal resolution of individual human, rat, bovine and dogfish shark gamma-crystallins. Applications presented include determinations of different synthesis of gamma-crystallins and chemical modification (oxidation by hydrogen peroxide) in situ.

The disulfide content of calf gamma-crystallin

The disulfide content of calf gamma-crystallin polypeptides has been investigated. The gamma-crystallin fraction of the soluble lens proteins was separated into five distinct polypeptides and characterized by isoelectric focusing, amino acid composition, and N-terminal sequence analysis to 25 residues. It has been demonstrated that 7 cysteines are present in gamma II, 4 to 5 cysteines in gamma IIIa, gamma IIIb, and gamma IV, and 6 cysteines in gamma I (beta s). Reduction of the total gamma-crystallin fraction with DTT resulted in an increase of approximately 1 to 1.5 mol of free SH per mole of protein. This increase in sulfhydryls was demonstrated to be contributed primarily by gamma II, the major polypeptide representing 50% of the total gamma-crystallin, which showed an increase of approximately 2.5 mol of sulfhydryl per mole of protein upon reduction. Insignificant disulfide content was present in gamma III and gamma IV and only a slight amount of disulfide was found in gamma I (beta s). The observed increase in sulfhydryl content upon reduction was not due to the presence of mixed disulfides of 2-mercaptoethanol, glutathione, or cysteine. The data are consistent with approximately 1 mol of intramolecular disulfide per mole of protein being present in gamma II. X-ray crystallography of gamma II has shown that the spatial location of Cys18 and Cys22 in the tertiary structure permits disulfide bond formation. Sequence analysis of the four major polypeptides of gamma-crystallin, gamma II, gamma IIIa, gamma IIIb, and gamma IV indicates that only gamma II has both Cys18 and Cys22. Cys18 is present in gamma IIIa, gamma IIIb, and gamma IV but Cys22 is replaced by His22. It is probable that the lack of disulfide in gamma IIIa, gamma IIIb, and gamma IV is due to the absence of Cys22.

Variation in the relative abundance of gamma-crystallin gene transcripts during development and ageing

The transcripts of gamma-crystallin mRNA were examined during adulthood. The mRNA transcripts were detected by Northern blot technique. Although the total RNA per lens measured remains constant during adulthood, the mRNA transcript size was observed to decline with ageing, specifically for the gamma-crystallin and not for alpha- and beta-crystallins. We could show that the relative amount of mRNA transcripts detected with the four probes decreased with ageing, with gamma 1 and gamma 2 transcripts being found at a higher level than the gamma 3 and gamma 4 transcripts.

Rat lens gamma-crystallins. Characterization of the six gene products and their spatial and temporal distribution resulting from differential synthesis

We have isolated, purified and characterized six individual gamma-crystallin polypeptides present in the rat lens. Comparison of their amino acid compositions with the known structure of the six gamma-crystallin genes permits a one-to-one correspondence to be made between each protein synthesized and the encoding gene. This demonstrates that each of the six genes is actually expressed in vivo. Two classes of three gamma-crystallins each, which we have designated classes gamma ABC and gamma DEF, are known to exist, on the basis of internal sequence homology. We have measured the temperature-dependent phase-separation characteristics of solutions of the six purified gamma-crystallins, and find that the three members of the gamma DEF class (gamma 2-2, gamma 3-1 and gamma 4-1) are all cryo-proteins with relatively high phase-separation temperatures, whereas the three gamma ABC crystallins (gamma 1-1, gamma 1-2 and gamma 2-1) do not show phase separation above -7 degrees C. We have measured the spatial distribution in rat lens of each of the alpha-, beta- and gamma-crystallins as a function of age from 1 to 420 days, using size-exclusion and ion-exchange high-pressure liquid chromatography (HPLC). Our findings in the cortical layer permit us to establish the differential synthesis of each of the crystallins during lens development. Particular attention has been devoted to the spatial and temporal distribution of the six individual gamma-crystallins. Up to birth, synthesis of the three components of the gamma DEF class predominates, and in particular that of gamma 2-2. In subsequent development the three components of the gamma ABC class assume a greater proportion of monomeric crystallins synthesized, while beta s-crystallin synthesis predominates in late development. Our analysis of different layers within single lenses provides novel information on spatial gradients of the water-soluble and water-insoluble protein fractions as a function of age. We consider the consequences of these findings for lens transparency and opacity in both rat and mouse lens. We show that the high concentrations of gamma DEF-crystallins appear to be responsible for the opacity known to occur in young rat lenses. We conclude from these observations that close control of the differential synthesis of gamma-crystallins plays an important role in maintaining lens transparency during development.

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.

Surface interactions of gamma-crystallins in the crystal medium in relation to their association in the eye lens

A comparative study of intermolecular interactions in crystals of two homologous low molecular weight proteins, gamma-II and gamma-IIIb crystallins, from calf eye lens was carried out. Crystal packings for these proteins are very different: intermolecular contact areas compose about 33% of the total accessible surface area of gamma-II as compared with 13% in gamma-III. Two key residues seem to be mainly responsible for the differences in protein association in the crystal medium. These are Ser 103 and Leu 155 in gamma-II, which are replaced by Met 103 and His 155 in gamma-IIb. A similar substitution of these residues is observed in different gene products of gamma-crystallins from a number of vertebrates. This is consistent with the existence of a genetically controlled mechanism for determining intermolecular association of gamma-crystallins in the native medium of the lens.

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.

Human lens gamma-crystallins: isolation, identification, and characterization of the expressed gene products

We have isolated the individual gamma-crystallins expressed in young human lenses and identified with which of the six known human gamma-crystallin genes they each correspond. We find that at least 90% of the gamma-crystallins synthesized in the young human lens are the products of genes gamma G3 and gamma G4. We demonstrate that gamma G4-crystallin undergoes a temperature-dependent phase separation, and we have measured the low-concentration branch of its coexistence curve (phase separation temperature vs. concentration) up to about 40 mg/ml. By comparison, we found no evidence of gamma G3-crystallin phase separating, even at lower temperatures and higher concentrations. This is consistent with predictions based on sequence homology between human and rat gamma-crystallins. The implications of these findings for human inherited and senile cataracts are considered.

Differential expression of crystallin genes during development of the rat eye lens

The concentrations of alpha A-, beta B1-, and total gamma-crystallin mRNAs were measured during development of the rat eye lens, using Northern blot and dot blot analysis. After 14 days of fetal growth a sharp increase in the concentration of all three mRNA species was observed. After birth, the concentration of alpha A-crystallin transcripts remains high until 6 months of age, the concentration of gamma-crystallin transcripts decreases gradually, while the concentration of beta B1-crystallin transcripts decreases sharply. The composition of the gamma-crystallin mRNA pool was determined by measuring the relative amount of the transcripts of each of the six gamma-crystallin genes using primer extension and S1-nuclease mapping. The transcripts of all six genes are found until the third month after birth. Thereafter the transcripts of the gamma 1-1, the gamma 3-1, and gamma 4-1 crystallin genes are no longer detectable. Later on the transcripts of the gamma 2-1 and gamma 2-2 genes also disappear leaving only the transcripts of the gamma 1-2 crystallin gene at the age of 8 months. The concentration of the six different gamma-crystallin mRNAs is thus regulated differentially.

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.

In situ detection of beta-galactosidase in lenses of transgenic mice with a gamma-crystallin/lacZ gene

Transgenic mice carrying the gamma 2-crystallin promoter fused to the coding region of the bacterial lacZ gene were generated. The offspring of three founder mice expressed high levels of the enzyme solely in the central nuclear fiber cells of the lens as measured by an in situ assay for the detection of beta-galactosidase activity. These results suggest that gamma 2-crystallin sequences between -759 to +45 contain essential information required for appropriate tissue-specific and temporal regulation of the mouse gamma 2-crystallin gene. In a broader context, this study also demonstrates the utility of beta-galactosidase hybrid gene constructs for monitoring the activity of gene regulatory elements in transgenic mice.

Differential regulation of gamma-crystallin genes during mouse lens development

Using gene-specific probes derived from four mouse gamma-crystallin cDNAs, we have examined the regulation of different members of the mouse gamma-crystallin gene family during lens development. Our analysis revealed that, while the different gamma-crystallin genes appear to be coordinately activated during embryogenesis, the steady-state levels of their corresponding transcripts are differentially regulated, resulting in variations in the relative abundance of individual species at different stages of development. This complex pattern of gene regulation presumably accounts for one of the mechanisms determining the spatial distribution of different gamma-crystallins within the lens.

A locus for a human hereditary cataract is closely linked to the gamma-crystallin gene family

Within the human gamma-crystallin gene cluster polymorphic Taq I sites are present. These give rise to three sets of allelic fragments from the gamma-crystallin genes. Together these restriction fragment length polymorphisms define eight possible haplotypes, three of which (Q, R, and S) were found in the Dutch and English population. A fourth haplotype (P) was detected within a family in which a hereditary Coppock-like cataract of the embryonic lens nucleus occurs in heterozygotes. Haplotype P was found only in family members who suffered from cataract, and all family members who suffered from cataract had haplotype P. The absolute correlation between the presence of haplotype P and cataract within this family shows that the gamma-crystallin gene cluster and the locus for the Coppock-like cataract are closely linked [logarithm of odds (lod) score of 7.58 at its maximum at phi = 0]. This linkage provides genetic evidence that the primary cause of a cataract in humans could possibly be a lesion in a crystallin gene.

Structural variation in mammalian gamma-crystallins based on computer graphics analyses of human, rat and calf sequences. 1. Core packing and surface properties

A comparison of mammalian gamma-crystallins has been made by computer-graphics model building of several gamma-crystallin sequences based on the atomic co-ordinates of the X-ray determined structure of calf gamma-II crystallin. The complete family of rat gamma-crystallins is compared together with the orthologous protein, gamma 1-2 crystallin, from rat, human and calf lens, and the orthologous protein, gamma 2-1 crystallin, from rat and human lens. In human gamma-crystallins, a major structural difference, the replacement of an arginine by a cysteine, occurs in one of the four-fold repeated folded hairpins, which may affect stability. Sequence variations involving buried residues were observed, leading to small differences in core packing of the different sequences which may be related to their regional location in the lens. Model-building studies also indicate that the surfaces of the different gamma-crystallins vary in number of exposed hydrophobic residues and ion pairs. These differences would affect protein-water interactions and therefore contribute to refractive index. A major variable region of the gamma-crystallin structures involves polar residues surrounding the inter-domain contact and the length of the polypeptide connecting the two domains. An attempt is made to correlate bovine gamma-crystallins which are known to be responsible for cold cataract with the corresponding sequences from rat lens.

Further studies on low molecular weight crystallins: relationship between the bovine beta s, the human 24kD protein and the gamma-crystallins

Three classes of monomeric crystallins separable by SDS gel electrophoresis or by gel filtration have been demonstrated in human lens. In previous studies we have concluded from physico-chemical and immunological data that all three polypeptides are related and should be classified as gamma-crystallins. The present paper presents further evidence supporting this conclusion, but also demonstrates that the 24,000 dalton (24kD) polypeptide corresponds to the beta s-crystallin. beta s-crystallin was purified by classical techniques from bovine lens and was shown to cross-react with a monoclonal antibody specific for the human 24kD polypeptide. This antibody exhibited no reactivity to other crystallin fractions from either bovine or human lenses. The identification of the 24kD polypeptide as beta s was further supported by analysis of the tertiary structures of the molecules by near UV-circular dichroism and by the finding of a blocked amino terminus on the 24kD polypeptide. Our finding that the human beta s (24kD polypeptide) should actually be classified as a gamma-crystallin is fully consistent with recently reported sequence data on bovine beta s-crystallin.

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

The three major bovine gamma-crystallin fractions (gamma-II, gamma-III and gamma-IV) are known to have closely related (80-90%) amino acid sequences and three-dimensional folding of the polypeptide backbone. Their chiroptical and emission properties, as measured by circular dichroism (CD) and fluorescence, are now shown to differ distinctly. The far-ultraviolet CD spectra indicate that all three gamma-crystallins have predominantly beta-sheet conformation (45-60%) with only subtle differences in secondary structure. The fluorescence emission maxima of gamma-II, gamma-III and gamma-IV, due to the four tryptophan residues, appear at 324, 329 and 334 nm, respectively, suggesting that tryptophan residues are buried in environments of decreasing hydrophobicity. Corresponding differences in quantum yield may be due to fluorescence quenching by neighboring sulfur-containing residues. Titratable tyrosines are maximal for gamma-III, as manifested from difference absorption spectra at alkaline pH. The near-ultraviolet CD spectra differ in position, magnitude and sign of tryptophan and tyrosine transitions. In addition, a characteristic CD maximum at 235 nm, presumably due to tyrosine-tyrosine exciton interactions, differs in magnitude for each gamma-crystallin. This study shows that the environment and interactions of the aromatic residues of the individual gamma-crystallin fractions are quite different. These variations in tertiary structure may be significant, in terms of stability of gamma-crystallins towards aggregation and denaturation, for understanding lens transparency and cataract formation in general.

The reaction of glutathione with the eye-lens protein gamma-crystallin

Lens cells contain high concentrations of thiol-rich proteins, gamma-crystallins and reduced glutathione. Solutions of bovine gamma-crystallin react avidly with either reduced or oxidized glutathione to form protein-glutathione mixed disulphides. A method of purification of a gamma-II crystallin-glutathione adduct containing two mixed disulphide groups is described.

Opacification of gamma-crystallin solutions from calf lens in relation to cold cataract formation

To determine the molecular mechanisms for cold cataract formation in the nucleus of the young mammalian lens, we have investigated the thermally reversible opacification of gamma-crystallin solutions isolated from calf lens. Coexistence curves (plots of opacification temperature Tc versus protein concentration) were determined for the individual gamma-crystallin fractions II, III, and IV as well as for the unfractionated gamma-crystallin mixtures isolated from the nucleus and cortex. The coexistence curve of gamma IV-crystallin is remarkably elevated above those of gamma II- and gamma III-crystallin and the gamma-crystallin mixtures. The gamma IV-crystallin fraction is the major determinant of the opacification temperature within the whole lens or isolated cytoplasm. Quasielastic light-scattering spectroscopy of gamma IV-crystallin solutions indicates that above Tc there are two populations of protein aggregates of distinctly different mean size. As the temperature is lowered towards Tc, both populations increase in size. Opacification occurs when the population of large scatterers, which is composed of less than 0.1% protein by weight, reaches an average radius of about 20,000 A.

Superior resolution of gamma-crystallins from microdissected eye lens by cation-exchange high-performance liquid chromatography

A novel procedure is presented for the rapid quantitative analysis of eye lens gamma-crystallins and beta s-crystallin by ion-exchange high-performance liquid chromatography on Synchropak CM300. At least six different gamma-crystallin gene products can be resolved from the soluble fraction of calf lens extract. This method is applicable to the analysis of microsections from individual lenses, and can be used to rapidly characterize spatial variations in gamma-crystallin composition which occur with aging and cataractogenesis.

The molecular structures and interactions of bovine and human gamma-crystallins

Knowledge of the three-dimensional structure of bovine gamma II-crystallin has provided the basis for building molecular models using computer graphics of two human gamma-crystallins, the sequences of which have recently been determined. The tertiary structures of these gamma-crystallins are predicted to be highly conserved. They have extensive networks of interacting charges on their surfaces, which may contribute to their thermodynamic stability and partially define the degree of water retention in the lens. The human crystallins appear to be more hydrophobic than the bovine molecule. All have arrangements of cysteine thiols which may be important as electron sinks and reserve redox potential in the normal lens but which may contribute to protein aggregation in cataract.

Fish eye-lens reagents: a possible new class of reagents for molecular and cellular identification

Initial studies have shown that fish lens protein can be modified in vitro to acquire a specific affinity for selected molecules, e.g., human hemoglobins, and cells, e.g., human erythrocytes. These studies were extended by preparing lens reagents that can distinguish bovine serum albumin from ovalbumin, and major groups of human erythrocytes in the ABO system from each other. Lens reagents appear to be potentially useful tools for the identification of specific molecules, either in solution or on cell surfaces.