The amino acid sequence of -crystallin (fraction II) from calf lens

Primary sequence contribution to the optical function of the eye lens

The crystallins have relatively high refractive increments compared to other proteins. The Greek key motif in βγ-crystallins was compared with that in other proteins, using predictive analysis from a protein database, to see whether this may be related to the refractive increment. Crystallins with Greek keys motifs have significantly higher refractive increments and more salt bridges than other proteins with Greek key domains. Specific amino acid substitutions: lysine and glutamic acid residues are replaced by arginine and aspartic acid, respectively as refractive increment increases. These trends are also seen in S-crystallins suggesting that the primary sequence of crystallins may be specifically enriched with amino acids with appropriate values of refractive increment to meet optical requirements. Comparison of crystallins from five species: two aquatic and three terrestrial shows that the lysine/arginine correlation with refractive increment occurs in all species investigated. This may be linked with formation and maintenance of salt bridges.

The gradient index lens of the eye: an opto-biological synchrony

The refractive power of a lens is determined largely by its surface curvatures and the refractive index of its medium. These properties can also be used to control the sharpness of focus and hence the image quality. One of the most effective ways of doing this is with a gradient index. Eye lenses of all species, thus far, measured, are gradient index (GRIN) structures. The index gradation is one that increases from the periphery of the lens to its centre but the steepness of the gradient and the magnitudes of the refractive index vary so that the optics of the lens accords with visual demands. The structural proteins, the crystallins, which create the index gradient, also vary from species to species, in type and relative distribution across the tissue. The crystallin classes do not contribute equally to the refractive index, and this may be related to their structure and amino acid content. This article compares GRIN forms in eye lenses of varying species, the relevance of these forms to visual requirements, and the relationship between refractive index and the structural proteins. Consideration is given to the dynamics of a living lens, potential variations in the GRIN form with physiological changes and the possible link between discontinuities in the gradient and growth. Finally, the property of birefringence and the characteristic polarisation patterns seen in highly ordered crystals that have also been observed in specially prepared eye lenses are described and discussed.

Ca2+-loaded spherulin 3a from Physarum polycephalum adopts the prototype gamma-crystallin fold in aqueous solution

Spherulin 3a is the most abundantly expressed cytosolic protein in spherulating plasmodia of the slime mold Physarum polycephalum. High yields of unlabeled, uniformly 15N and uniformly 13C/15N-labeled recombinant spherulin 3a from Escherichia coli could be produced by a simple protocol described here. The three-dimensional solution structure of Ca2+-loaded spherulin 3a was determined by homo- and heteronuclear NMR spectroscopy. The structure of monomeric spherulin 3a consists of two pleated beta-sheets plus a short alpha-helix arranged into the gamma-crystallin fold. The beta-sheets comprise two intertwined Greek-key motifs. An additional N-terminal beta-strand is unique to spherulin 3a. Complexation of calcium ions greatly enhances overall conformational stability of the protein. The average atomic root-mean-square deviations (r.m.s.d.) for heavy atoms in beta-strands were 0.34(+/-0.16) A for the backbone atoms and 0.73(+/-0.40) A for all atoms. The corresponding r.m.s.d. values for heavy atoms in the whole protein were 0.62(+/-0.42) A for the backbone atoms and 0.99(+/-0.65) A for all atoms. We show the structural relationship between spherulin 3a, a myxomycete dormancy protein, and crystallins from the vertebrate eye lens. Since spherulin 3a has a structure corresponding to one domain of bovine gammaB(II)-crystallin, it represents a hypothetical ancestral gamma-crystallin precursor structure.

The reaction of methylglyoxal with human and bovine lens proteins

Methylglyoxal is an endogenous metabolite that increases in diabetes and has been implicated in some of its long-term complications such as retinopathy, neuropathy and cataract. We investigated the reaction of methylglyoxal with isolated human and bovine lens crystallins (alpha, beta H, beta L and gamma). After 7 days incubation at 37 degrees C and pH 6.9, the reaction of methylglyoxal with lens proteins yielded stable adducts that exhibited fluorescent properties. SDS-polyacrylamide gel electrophoresis was used to monitor aggregation and crosslinking of the modified protein and autoradiography showed that [14C]methylglyoxal was incorporated into all the protein bands. Bovine gamma-crystallin was the most reactive towards methylglyoxal. Reaction of methylglyoxal with bovine gamma II-crystallin, which is found mainly in the lens nucleus, could alter the change surface network of the molecule, resulting in aggregation, increased light scattering and hence cataract. Modification of gamma II-crystallin by methylglyoxal produced an overall loss of positive charge and an increase in molecular weight and non-disulfide covalent crosslinking. Amino acid analysis of the modified gamma II-crystallin showed a loss of 47% of arginine residues.

Identification of the site of glycation of gamma-II-crystallin by (14C)-fructose

Cataract formation in diabetes may be via non-enzymic glycosylation (glycation) of lens proteins due to increased concentrations of sugars present in the lenses of diabetic patients. The objective of this project was to identify the site(s) of glycation of bovine gamma-II-crystallin by [14C]fructose. gamma-II-crystallin was isolated from soluble lens nucleus proteins by gel chromatography, followed by ion-exchange chromatography and was then glycated by incubation with [14C]fructose. Radioactively labelled gamma-II-crystallin was cleaved with trypsin. Affinity chromatography of the tryptic peptides gave a single main peak containing the majority of the radioactivity. This indicated that fructose had reacted at a single site on the protein. Amino acid analysis of this peptide showed it to contain only lysine and a trace amount of glycine. By relating the results of the amino acid analysis to the amino acid sequence of gamma-II-crystallin, it was concluded that the labelled peptide corresponded to the N-terminal dipeptide. The site of glycation of bovine gamma-II-crystallin by fructose was thereby identified as the alpha-NH2 group of the N-terminal glycine.

Local variations in protein structure in the human eye lens: a Raman microspectroscopic study

Confocal Raman microspectroscopy was used to monitor local and age-related changes in protein conformation in human eye lenses. In clear human lenses of varying age (range 17-80 years) spectra were recorded along the visual axis, using laser light of 660 nm wavelength. The Raman vibrations in the 650-1750 cm-1 spectral region were analyzed. Difference spectra between central core and different positions along the visual axis were calculated after calibration for protein content using the I(1450) cm-1 CH2/CH3 vibration peak. Tryptophan content was quantified using the peak at 760 cm-1 calibrated for protein. Changes in the 'exposed' vs. 'buried' position of tryptophan were analyzed using the peak heights at I(880) and I(760) cm-1. The difference spectra revealed an excess of tryptophan, tyrosine, phenylalanine, beta-sheet conformation and molecules or molecular groups responsible for a 1425 cm-1 peak in the core region in all lenses investigated. The excess peaks disappeared at about 0.6-0.9 mm below the surface. The tryptophan content increased from superficial to deep layers, levelling off between 0.4-0.8 mm below the surface. Upon aging, the tryptophan content increases in the core not in the cortex. No changes in the 'exposed' vs. 'buried' position of tryptophan were observed. Changes in tryptophan and tyrosine probably reflect the maturational shift from cortex to core in the relative content of alpha, beta and gamma crystallines. The age-related increase in tryptophan in the core may reflect the preferential breakdown by endo- and exopeptidases of alpha-crystallins damaged upon aging. The increase in beta-sheet conformation may indicate a post-translational shift in secondary conformation upon aging. These changes in protein conformation are largely completed in a small superficial zone, i.e., in the early life span of the crystallins.

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.

Intermolecular protein interactions in solutions of bovine lens beta L-crystallin. Results from 1/T1 nuclear magnetic relaxation dispersion profiles

We report the magnetic field dependence of 1/T1 of solvent water protons and deuterons (nuclear magnetic relaxation dispersion, or NMRD, profiles) for solutions of steer lens beta L-crystallin. Such data allow the study of intermolecular protein interactions over a wide concentration range, here 1-34% vol/vol, by providing a measure of the rotational relaxation time of solute macromolecules. We conclude that, for approximately less than 5% protein, the solute particles are noncompact, with a rotationally averaged volume approximately three times that of a compact 60-kD sphere. (Earlier results for alpha-crystallin, approximately 1,000 kD, from optical and osmotic measurements (Vérétout and Tardieu, 1989. J. Mol. Biol. 205:713-728), show a similar, approximately twofold, effect). At intermediate concentrations, to approximately 20% protein, there is evidence for limited association or oligomerization, as found for the structurally related gamma II-crystallin (Koenig et al. 1990. Biophys. J. 57:461-469), to a limiting size about two-thirds that of alpha-crystallin. The difference in NMRD behavior of the three classes of crystallins is consonant with their differing osmotic properties (Vérétout and Tardieu. J. Mol. Biol. 1989, 205:713-728; Kenworthy, McIntosh, and Magid. Biophys. J. 1992. 61:A477; Tardieu et al. 1992. Eur. Biophys. J. 21:1-12). We indicate how the unusual structures and interactions of these three classes of proteins can be combined to optimize transparency and minimize colloid osmotic difficulties in eye lens.

Primary structure of beta s-crystallin from human lens

The complete primary structure of beta s-crystallin from human lens is reported. The sequence was elucidated by automatic Edman degradation of tryptic and CNBr peptides. The blocked N-terminal dipeptide was identified by fast-atom-bombardment mass spectroscopy. The sequence comparison with other members of crystallin family reveals a closer relationship to human gamma-crystallin (53% identity) than with beta A3/A1 crystallin (37% identity). The structure, evolutionary characteristics and role of beta s-crystallin in lens are discussed.

The glycation and cross-linking of isolated lens crystallins by ascorbic acid

Individual lens crystallins were isolated from calf lens extracts and incubated in the presence of ascorbic acid for 3 weeks under aerobic conditions. Both alpha-crystallin and beta H-crystallin rapidly cross-linked to form high molecular weight proteins, which did not enter the resolving gel on SDS-PAGE. Beta L-crystallin was somewhat less reactive, but gamma-crystallin showed little or no crosslinking. Gamma-crystallin, however, was almost equivalent to the other crystallins as a substrate for glycation. This was measured by: (a) the binding of protein to a boronate affinity column; (b) the incorporation of 3H from NaB3H4 into protein; (c) amino acid analysis of the modified proteins to estimate the extent of lysine modification; and (d) the incorporation of [1-14C]ASA into individual crystallins. When the separated crystallins were combined with [125I]gamma-crystallin and incubated with ascorbic acid, radioactivity was readily incorporated into the cross-linked products with other crystallins, but again not with gamma-crystallin itself. Gel filtration chromatography of a mixture of [125I]gamma-crystallin and alpha-crystallin showed the formation of a complex between gamma- and alpha-crystallins. These data suggest that all crystallins are glycated, but that cross-linking occurs preferentially between proteins, which are already bound together non-covalently.

Analysis of photo-oxidized amino acids in tryptic peptides of calf lens gamma-II crystallin

Insoluble and crosslinked proteins and increased pigmentation in the eye lens are features of aging and cataracts. Determining the amino acids which are involved in insolubilization, crosslinking and visible light scattering will shed light on the mechanisms by which cataracts form. Calf lens gamma-II crystallin was irradiated at 295 nm, digested and separated into tryptic peptides. Additional tryptic peptides were found in the digest of irradiated gamma-II which were not present in the dark control digest. These peptides were identified by amino acid sequencing and shown to correspond to expected tryptic fragments of the protein, indicating more facile digestion in the UV-irradiated protein than in dark controls. Amino acid analysis of the irradiated protein and peptides showed losses of histidine, methionine and cysteine residues as compared to control samples. Tryptophan, which is not detected by amino acid analysis, was also found to be reactive since losses in its fluorescence intensity were observed after irradiation. Some of the photochemically active amino acids had lower than expected responses in amino acid sequencing experiments. This suggested specific sites of photochemical activity in the various peptides. The evidence for peptide crosslinks is also discussed.

Ageing and changes in protein conformation in the human lens: a Raman microspectroscopic study

Using confocal Raman microspectroscopy with laser light of 660 nm the secondary and tertiary conformation of crystallins was studied in human lenses of varying age (20-75 years). Differences in cortical and nuclear proteins in individual lenses and among lenses of different age and differences between small equatorial opacities and adjacent clear sites were analysed using a difference spectrum approach. Intensity calibration allows assessment of local variations in protein content. The main findings and conclusions are as follows. (1) Irrespective of the age of the lens nuclear proteins proved to contain more aromatic amino acids, i.e. tryptophan, tyrosine and phenylalanine. This change most probably reflects differences in crystallin composition between nucleus and cortex as described in biochemical studies. (2) Changes in the amide bands indicate a more pronounced beta-sheet conformation of the nuclear proteins. Taking into account available biochemical evidence this most probably reflects a true post-translational change in the secondary conformation of the crystallins. (3) Cortical proteins in 'old' and 'young' lenses are largely identical indicating that ageing is not accompanied by gross alterations in the transcription/translation of the crystallin genes. (4) 'Old' and 'young' nuclear proteins deviate with respect to the amount of aromatic amino acids, being more abundant in 'young' nuclear proteins. (5) Proteins in small early opacities do not exhibit alterations in conformation. (6) A pronounced peak in the difference spectra in the region 1425-1435 cm-1 for nuclear proteins especially when compared with equatorial cortical proteins may be considered as evidence for the advanced photooxidation of tryptophan and/or deamidation of asparagine from superficial to deep regions of the lens.

Mass spectrometric analysis of the structure of gamma II bovine lens crystallin

The amino acid sequence of bovine gamma II-crystallin has been verified by a combination of electrospray and fast atom bombardment mass spectrometry. The molecular weight of gamma II, isolated by gel filtration and ion exchange chromatography, was determined to be 20,967 +/- 3 by electrospray mass spectrometry. Another aliquot of gamma II was completely digested by trypsin in a medium of 20% CH3CN and 0.1 M Tris, pH 8.2. The tryptic peptides were separated by reversed phase HPLC and identified by their molecular weights, as determined by fast atom bombardment mass spectrometry (FABMS). The identification of each peptide was confirmed by digesting the peptide further to give new peptides whose molecular weights were also determined by FABMS and related to the proposed amino acid sequences. The data from both types of mass spectrometric analyses were consistent with the sequence previously proposed by Hay et al. (J. Biol. Chem. 1987, 146, 332-338), including threonine at position 119. The FAB mass spectrum of one HPLC fraction suggested that disulfide bonding between Cys 18 and Cys 22 was present in at least half the protein preparation. Whether the Cys 18/Cys 22 disulfide bond was present in native gamma II or was produced during isolation or enzymic digestion could not be determined from these studies. Samples that had been stored for several weeks showed that several of the cysteines had become disulfide bonded. These studies illustrate the power of mass spectrometric techniques to accurately confirm the primary structure of proteins and to identify post-translational modifications.

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.

Differential glycation of rat alpha-, beta- and gamma-crystallins

Crystallin glycation seems to play an important role in the development of diabetic cataract. In order to understand the role of glycation in cataractogenesis, levels of glycation of different crystallins were determined by in vitro glycation of rat lens soluble fraction with 50 mM glucose or glucose-6-phosphate (G6P) for up to 5 days and in streptozotocin-diabetic rats during various stages of cataract development. All samples were reduced with [3H]NaBH4 and the tritium incorporation was taken as a measure of glycation. Proteins were routinely separated by molecular sieve HPLC. In vitro studies with glucose showed that gamma-crystallin was readily glycated and reached a plateau by 3 days, while alpha- and beta-crystallins were glycated slowly initially up to 3 days followed by a steep increase as seen on the fifth day. Incubation with 50 mM G6P resulted in an approximately two fold increase in glycation compared to glucose of all crystallins. In the diabetic animals also gamma-crystallin glycation increased approximately twofold within 15 days after the onset of diabetes and an additional threefold within the next 45 days followed by a slight decrease during the following 90-120 days. Increase in glycation, on the contrary, was very slow up to 30 days for alpha-crystallin and up to 60 days for beta-crystallin, followed by a steep increase during the remainder of the experimental period. The high molecular weight (HMW) aggregates had higher levels of glycation than other proteins; the insoluble HMW aggregates contained higher levels of glycation than the soluble HMW aggregates.(ABSTRACT TRUNCATED AT 250 WORDS)

Oligomerization and conformation change in solutions of calf lens gamma II-crystallin. Results from 1/T1 nuclear magnetic relaxation dispersion profiles

From analyses of the magnetic field dependence of 1/T1 (nuclear magnetic relaxation dispersion [NMRD] profiles) of water protons in solutions of highly purified calf lens gamma II-crystallin, we find that monomers form oligomers at relatively low concentrations, which increase in size with increasing concentration and decreasing temperature. At approximately 16% by volume and -4 degrees C, the mean oligomeric molecular weight is approximately 120-fold greater than the monomeric value of 20 kD. Below this concentration, there is no indication of any substantive change in conformation of the monomeric subunits. At higher concentrations, the tertiary structure of the monomer appears to reconfigure rather abruptly, but reversibly, as evidenced by the appearance of spectra-like 14N peaks in the NMRD profiles. The magnitudes of these peaks, known to arise from cross-relaxation of water protons through access to amide (NH) moieties of the protein backbone, indicate that the high concentration conformation is not compact, but open and extended in a manner that allows enhanced interaction with solvent. The data are analogous to those found for homogenates of calf and chicken lens (Beaulieu, C. F., J. I. Clark, R. D. Brown III, M. Spiller, and S. H. Koenig. 1988. Magn. Reson. Med. 8:47-57; Beaulieu, C. F., R. D. Brown III, J. I. Clark, M. Spiller, and S. H. Koenig. 1989. Magn. Reson. Med. 10:62-72). This unusually large dependence of oligomeric size and conformation on concentration in the physiological range is suggested as the mechanism by which osmotic equilibrium is maintained, at minimal metabolic expense, in the presence of large gradients of protein concentration in the lens in vivo (cf Vérétout and Tardieu, 1989. Eur. Biophys. J. 17:61-68). Finally, the results of the NMRD data provide a ready explanation of the low temperature phase transition, and "cold-cataract" separation of phases, observed in gamma II-crystallin solutions; we suggest that the phases that separate are the two major conformers detected by NMRD.

The rates of photolysis of the four individual tryptophan residues in UV exposed calf gamma-II crystallin

Aqueous buffer solutions of the lens protein bovine gamma-II crystallin were irradiated at 295 nm in the presence of dithiothreitol to determine the individual photolysis susceptibilities of the four tryptophan residues. Reverse-phase high performance liquid chromatography was utilized to compare the tryptic peptide maps before and after irradiation. Sequence analysis of collected tryptic peptides showed that the four tryptophans in calf gamma-II crystallin. TRP-42, TRP-68, TRP-131, and TRP-157 appeared in four distinct tryptic peptides. Fluorescence and absorption (diode array) monitoring of the eluting peptides allowed assessment of the changes in peptide absorbance and fluorescence following irradiation. Tryptophan fluorescence losses of (40 +/- 15)%, (17 +/- 4)%, (35 +/- 5)% and (15 +/- 4)% were observed for the peptides containing TRP-42, TRP-68, TRP-131 and TRP-157, respectively. Thus the four tryptophans in calf gamma-II crystallin did not all photolyze at the same rate. The rate differences are presumably related to the microenvironments of the individual tryptophan residues, and this is discussed in terms of the known crystal structure of calf gamma-II crystallin.

Comparative study of lens proteins of gray squirrel and human

1. The four crystallins of the gray squirrel lens have been characterized using gel filtration chromatography, polyacrylamide gel electrophoresis, and immunoblotting. Alpha, beta-heavy, beta-light, and gamma crystallins of squirrel lenses have been identified immunologically, and they cross-react strongly with rabbit polyclonal antibodies. The gamma-24 crystallin of the squirrel lens also reacts strongly with monoclonal anti-human lens gamma-24, as shown by its inhibition of the ELISA reaction by 85%. 2. The water-insoluble urea soluble proteins represent non-covalently associated species of soluble crystallins and the lens cytoskeletal proteins. The membrane intrinsic protein in the urea insoluble pellet has a mol. wt of 27,000 but other lower and higher mol. wt components are also present, which were removed by washing with 0.1 NaOH. The N-terminal 30 amino acid of squirrel lens gamma crystallin was found to be identical to that of the bovine (and human) lens. 3. Measurements of the distribution and state of SH and SS compounds in the squirrel lens have shown greater similarities to those of primates than those of rodents. The findings show that on the basis of both protein and sulfur chemistry the squirrel lens is a representative model for studies of oxidative lens changes in diurnal animals, including man.

Site of carbamoylation of bovine gamma-II-crystallin by potassium [14C]cyanate

One possible route to cataract formation may be via the carbamoylation of lens proteins due to increased concentrations of cyanate in the body resulting from uraemia associated with renal failure and with severe diarrhoea. Carbamoylation of gamma-II-crystallin, which is found in the lens core, could alter the surface charge network of the molecules, resulting in aggregation, increased light-scattering and hence cataract. We have attempted to locate the site(s) of carbamoylation in gamma-II-crystallin. gamma-II-Crystallin was isolated by gel chromatography and ion-exchange chromatography. gamma-II-Crystallin was then carbamoylated by incubation with potassium [14C]cyanate, followed by citraconylation and digestion with trypsin to give peptides that were separated by high-resolution ion-exchange chromatography. The amino acid compositions of the radioactive peptides were compared with the expected peptide composition for gamma-II-crystallin. The radioactive peptide compositions, which agreed with the theoretical peptides, all matched with the N-terminal region of gamma-II-crystallin and had in common the presence of the N-terminal glycine residue. It appears that the alpha-amino group of the N-terminal glycine was the main site of carbamoylation. This site forms part of the charge network on the surface of gamma-II-crystallin.

The effect of near-UV light on Na-K-ATPase of the rat lens

The influence of in vitro near-UV radiation exposure on the physical state of the rat lens and on its membrane-bound Na-K-ATPase activity was investigated. Lens swelling was correlated to the appearance of opacities and the inactivation of the enzyme. The results show a significant decrease in the Na-K-ATPase activity which may be an early change leading to osmotic type cataracts. The dose-effect curves obtained for cortical and epithelial enzymes were different. Since the data do not follow a monoexponential function, the existence of two forms of Na-K-ATPase in the lens is discussed.

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.

Physicochemical characterization of gamma-crystallins from bovine lens--hydrodynamic and biochemical properties

A detailed hydrodynamic study has been made on the gamma-crystallin of the bovine lens. Sedimentation study indicates that gamma-crystallin shows a nearly gaussian peak throughout the course of sedimentation at high speed, using a synthetic boundary cell. The diffusion and sedimentation coefficients are 10.3 x 10(-7) cm2/sec and 2.51 S, respectively. The weight-average molecular weight of the unfractionated gamma-crystallin calculated from sedimentation equilibrium is 21,800. The four major subfractions of gamma-crystallin show similar hydrodynamic properties with an intrinsic viscosity of 2.50 ml/g and a Stokes radius of 21 A. The distinct electrophoretic mobilities exhibited by the four subfractions show gel-concentration dependence and similar slopes in the Ferguson plot, indicative of being charge isomers of the same molecular species. Amino acid analysis of these four subfractions corroborated the conclusions that these gamma-crystallin polypeptides are closely related and comprise a multigene family of crystallins. Based on the sedimentation and intrinsic viscosity data, gamma-crystallin can be modeled as a prolate ellipsoid with an axial ratio of approximately 3.0 and a hydration factor of 0.27 g water per gram protein. The circular dichroism data for gamma-crystallins showed a minimum at about 217 nm, characteristic of a beta-sheet conformation. These structural characteristics are in good accord with those derived from X-ray diffraction data for gamma-crystallin II.

Binary liquid phase separation and critical phenomena in a protein/water solution

We have investigated the phase diagram of aqueous solutions of the bovine lens protein gamma II-crystallin. For temperatures T less than Tc = 278.5 K, we find that these solutions exhibit a reversible coexistence between two isotropic liquid phases differing in protein concentration. The dilute and concentrated branches of the coexistence curve were characterized, consistently, both by measurements of the two coexisting concentrations, c(T), and by measuring the cloud temperatures for various initial concentrations. We estimate that the critical concentration, cc, is 244 mg of protein per ml solution. The coexistence curve is well represented by the absolute value of (c - cc)/cc = 5.2 square root (Tc - T)/Tc. Using the temperature dependence of the scattered light intensity along isochores parallel to the critical isochore, we estimated the location of the spinodal line and found it to have the form (c - cc)/cc = 3.0 square root (Tc - T)/Tc. The ratio of the widths of the coexistence curve and the spinodal line, (5.2/3.0), is close to the mean-field value square root 3. We have also observed the growth of large crystals of gamma II-crystallin in some of these aqueous solutions and have made preliminary observations as to the factors that promote or delay the onset of crystallization. These findings suggest that selected protein/water systems can serve as excellent model systems for the study of phase transitions and critical phenomena.

Covalent changes at the N- and C-terminal regions of gamma crystallin during aging of the normal human lens

Polyclonal antisera have been made to synthetic peptides corresponding to the N- and C-terminal regions of the putative gamma 1-2 gene from human lens. These antisera are specific for gamma crystallin, showing no cross-reactivity with any polypeptides of the alpha- and beta-crystallin fractions. Western blot analysis demonstrates a dramatic decrease in the binding of these antisera to gamma crystallin during aging of the normal human lens, while identical analysis with polyclonal antisera to the major cyanogen bromide fragment (19,000 MW) of human gamma crystallin shows little, if any, change during aging. Together, these demonstrate that antisera to synthetic peptides of the N- and C-terminal regions of the gamma 1-2 gene are very specific probes that can demonstrate extensive covalent modification from both ends of the gamma crystallin molecule during aging of the normal human lens.

The amino-terminal sequences of four major carp gamma-crystallin polypeptides and their homology with frog and calf gamma-crystallins

Four major gamma-crystallin subfractions have been isolated from the carp (Cyprinus carpio) and their N-terminal sequences determined by Edman protein sequencing. Extensive homologies indicative of close relatedness in their primary structure were found in these four gamma-crystallin polypeptides. Comparison of the carp N-terminal sequences with those of mammalian and amphibian gamma-crystallins also showed a high degree of homology present in their N-terminal segments despite the dissimilarity of amino acid compositions of fish gamma-crystallins to those of higher classes of vertebrates. The distinct yet closely-related partial sequences of carp gamma-crystallins could account for the profound microheterogeneity detected in the characterization of carp crystallins, suggesting the presence of a multigene family for gamma-crystallin in the lowest class of vertebrates, i.e. the fish.

A computer graphics model of frog gamma-crystallin based on the three-dimensional structure of calf gamma-II crystallin

Molecular models for Rana gamma-1 and gamma-2 crystallins have been constructed using computer graphics on the basis of the protein primary structure derived from the complementary DNA sequence and the three-dimensional structure of calf gamma-II crystallin that has been defined at high resolution by X-ray analysis. The models show that the cores of the two domains are conserved as hydrophobic, with the polypeptide chain arranged as a four Greek-key motif structure. Although many lysines replace arginines at equivalent positions in mammalian proteins, the Rana crystallins also have an extensive series of ion pairs on their surface; these are strongly implicated in their function as stable structural molecules, which are highly conserved in the evolution of the vertebrate eye lens.

Spectroscopic studies on human lens crystallins

Human lens crystallins were studied by absorption, circular dichroism and fluorescence spectroscopy. The absorption spectra in the near-ultraviolet region show some differences in intensity, but spectral features are similar, except for the alpha-crystallin, which gives a fine structure due to phenylalanine between 250 and 270 nm. Tryptophan fluorescence and near-ultraviolet circular dichroism indicate that tryptophan residues are more exposed in alpha-crystallin than in either beta- or gamma-crystallin, and that the degree of exposure decreases in the order of alpha less than beta 1 greater than beta 2 greater than beta 3 greater than gamma. The far ultraviolet CD suggests that these proteins exist mainly in a beta-sheet conformation and that the amount does not vary much among them. The greater exposure of the tryptophan residues in the high-molecular-weight crystallins may reflect greater unfolding in their protein domains. Spectroscopic measurements are thus useful in predicting protein tertiary structure in the absence of the complete sequence and X-ray data. The fact that the high-molecular-weight proteins exist in a more unfolded state may render them more vulnerable to exogeneous insults, and these effects may be studied by spectroscopic measurements.

Comparison of the crystallin mRNA populations from rat, calf and duck lens. Evidence for a longer alpha A2-mRNA and two distinct alpha B2-mRNAs in the birds

Total cytoplasmic poly(A)-containing RNA from rat, calf and duck lens was fractionated by electrophoresis in methylmercury hydroxide-containing agarose gels. RNA electrophoresed in parallel lanes was either transferred onto nitrocellulose and hybridized with total cDNA synthesized on the initial mRNA or was recovered from individual gel fractions for in vitro translation in a reticulocyte cell-free system. This allowed the identification and size-characterization of individual mRNA species encoding alpha-, beta-, gamma- and delta-crystallin polypeptides. The 14 S mRNA fraction of rat lens comprises two alpha A2-mRNAs of approximately 1250 and 1350 nucleotides and the alpha AIns-mRNA with a size similar to that of the largest alpha A2-mRNA. The calf lens 14 S mRNA fraction harbors a heterogeneous population of alpha A2-mRNA. In the same fraction another mRNA encoding a polypeptide, designated X, has been found sharing no homology with alpha A sequences. The duck lens alpha A2-mRNA appears to be 400-450 bases longer than the rat and calf lens alpha A2-mRNAs. Furthermore, in contrast to the single alpha B2-mRNA in rat and calf lens, two alpha B2-mRNAs have been identified in duck lens, one, the major species, similar in size to the alpha B2-mRNA in rat and calf lens (800 bases), and the other species 700 nucleotides longer. The large size differences among the alpha A2- and alpha B2-mRNAs most likely reside in their 3'-untranslated sequences.

Characterization of the rat gamma-crystallin gene family and its expression in the eye lens

Rat genomic clones, which together contain all of the rat genomic gamma-crystallin sequences, have been characterized. Five gamma-crystallin genes are located on a contiguous DNA region, 63 X 10(3) base-pairs long. These genes, named (5') gamma 1-1, gamma 1-2, gamma 2-2 and gamma 3-1 (3'), are all oriented head to tail. A sixth gamma-crystallin gene, named the gamma 4-1 gene, could not be linked to the gamma-crystallin gene cluster with our present set of genomic clones. Mapping experiments using single copy sequences which form the extreme 5' or 3' region of the gene cluster showed that, if the gamma 4-1 gene is located on the same chromosome, then it must be separated from the gene cluster by at least 25 X 10(3) base-pairs of DNA. All gamma-crystallin genes have a similar mosaic structure. They contain a large (0.9 X 10(3) to 1.88 X 10(3) base-pairs) intron in the middle of the gene and are further interrupted close to the 5' end of the gene. The length of the first exon varies from about 40 to about 50 base-pairs. The complementary DNA clone pRL-gamma-3 used in this study is a copy of the transcript of the gamma 3-1 gene, while the second complementary DNA clone, pRL-gamma-2, is most likely a copy of the transcript of the gamma 2-1 gene. It is further shown that rat lens messenger RNA protects fragments from the 3' ends of the four other gamma-crystallin genes against degradation by S1 nuclease, hence all six gamma-crystallin genes present in the rat genome must be transcribed in the lens. Repetitive sequences were found to be present between and around the gamma-crystallin genes. Mapping with cloned repetitive sequences showed that three different repeats, designated A, B and C, occur more than once in the gamma-crystallin gene cluster. Repeat C is also found in the gamma 4-1 region. A repetitive region 3' to the gamma 3-1 gene contains members of all three repeat families.

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.

A second polymorphic lens crystallin (LEN-2) in the mouse: genetic and biochemical analysis of LEN-1 and LEN-2

Two electrophoretic polymorphisms affecting lens crystallins, designated LEN-1 and LEN-2, have been discovered among inbred strains of mice. Analysis by isoelectric focusing demonstrated that both crystallins are monomeric proteins with isoelectric points at or above pH 7. Both proteins eluted in the low molecular weight (LM) fraction upon Sephadex G-200 gel filtration but LEN-2 was shown to be larger than LEN-1 by G75SF gel filtration and denaturing gel electrophoresis. Linkage analysis demonstrated that the genes encoding LEN-1 and LEN-2 assort independently. Amino acid analysis of the allelic products of the two genes revealed that genetic variants of each respective crystallin were very similar in amino acid compositions but that LEN-1 and LEN-2 were dissimilar crystallins.

Bovine beta-crystallin complementary DNA clones. Alternating proline/alanine sequence of beta B1 subunit originates from a repetitive DNA sequence

A library of recombinant plasmids carrying complementary DNA sequences synthesized from bovine lens messenger RNAs was constructed. Clones coding for five different beta-crystallin subunits: beta B1, beta B3, beta Bp, beta s, beta A3 (and beta A1), were identified by means of hybridization selection, followed by one- and two-dimensional gel electrophoresis of the translational products. Under rather stringent conditions each of these clones hybridizes with its corresponding mRNA and does not show significant cross-hybridization with mRNAs coding for other beta-crystallins, except in the case of the homologous beta A3 and beta A1-crystallins. The beta A3 and beta A1 subunits seem to be encoded by one mRNA using two different AUG codons as start position for translation. We have also determined the nucleotide sequence of a beta B1-crystallin cDNA (pBL beta B1) which enabled us to deduce the complete amino acid sequence of the protein. The beta B1-crystallin, a characteristic component of the high molecular weight crystallin aggregate (beta H), is internally homologous both at DNA and protein level as has been reported for gamma- and other beta-crystallins. This is in agreement with the idea that these proteins had a common ancestral precursor gene that internally duplicated. The G + C content of the coding sequence of beta B1 is very high: 67% overall and even 84.2% for the first 170 nucleotides, due to a remarkable non-random codon usage. A proline/alanine repetition in the N-terminal domain of the protein is encoded by a repetitive "simple" DNA sequence.

Nonenzymatic glycation of human lens crystallin. Effect of aging and diabetes mellitus

We have examined the nonenzymatic glycation of human lens crystallin, an extremely long-lived protein, from 16 normal human ocular lenses 0.2-99 yr of age, and from 11 diabetic lenses 52-82-yr-old. The glucitol-lysine (Glc-Lys) content of soluble and insoluble crystallin was determined after reduction with H-borohydride followed by acid hydrolysis, boronic acid affinity chromatography, and high pressure cation exchange chromatography. Normal lens crystallin, soluble and insoluble, had 0.028 +/- 0.011 nanomoles Glc-Lys per nanomole crystallin monomer. Soluble and insoluble crystallins had equivalent levels of glycation. The content of Glc-Lys in normal lens crystallin increased with age in a linear fashion. Thus, the nonenzymatic glycation of nondiabetic lens crystallin may be regarded as a biological clock. The diabetic lens crystallin samples (n = 11) had a higher content of Glc-Lys (0.070 +/- 0.034 nmol/nmol monomer). Over an age range comparable to that of the control samples, the diabetic crystallin samples contained about twice as much Glc-Lys. The Glc-Lys content of the diabetic lens crystallin samples did not increase with lens age.

Inhibition of cysteine proteinases and dipeptidyl peptidase I by egg-white cystatin

The interactions between egg-white cystatin and the cysteine proteinases papain, human cathepsin B and bovine dipeptidyl peptidase I were studied. Cystatin was shown to be a competitive reversible inhibitor of cathepsin B (Ki 1.7 nM, k-1 about 2.3 X 10(-3) s-1). The inhibition of dipeptidyl peptidase I was shown to be reversible (Ki(app.) 0.22 nM, k-1 about 2.2 X 10(-3) s-1). Cystatin bound papain too tightly for Ki to be determined, but an upper limit of 5 pM was estimated. The association was a second-order process, with k+1 1.0 X 10(7) M-1 X s-1. Papain was shown to form equimolar complexes with cystatin. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis of complexes formed between papain or cathepsin B and an excess of cystatin showed no peptide bond cleavage after incubation for 72 h. The reaction of the active-site thiol group of papain with 5,5'-dithiobis-(2-nitrobenzoic acid) at pH 8 and 2,2'-dithiobispyridine at pH 4 was blocked by complex-formation. Dipeptidyl peptidase I and papain were found to compete for binding to cystatin, contrary to a previous report. The two major isoelectric forms of cystatin were found to have similar specific inhibitory activities for papain, and similar affinities for papain, cathepsin B and dipeptidyl peptidase I. This, together with specific oxidation of the N-terminal serine residue with periodate, showed the N-terminal amino group of cystatin 1 to be unimportant for inhibition. General citraconylation of amino groups resulted in a large decrease in the affinity of cystatin for dipeptidyl peptidase I. It is concluded that the interaction of cystatin with cysteine proteinases has many characteristics similar to those of an inhibitor such as aprotinin with serine proteinases.

Complete nucleotide sequence of a cDNA derived from calf lens gamma-crystallin mRNA: presence of Alu I-like DNA sequences

The nucleotide sequence of a cloned cDNA derived from gamma-crystallin mRNA of calf lens was determined. The cloned cDNA contains the entire coding region 522 bp long, 30 nucleotides of the 5' noncoding region, and 67 residues in the 3' noncoding region followed by a poly(A) tail of 25 nucleotides. The deduced amino acid sequence directly demonstrates for the first time that the calf gamma-crystallin contains 174 residues. The nucleotide sequence contains a number of interesting features including a 32-bp sequence in the 3' region with 70% complementarity to the 3' end of the first monomer unit of the consensus Alu I DNA. Within this region, a 32-bp sequence shows about 80% homology with a segment of hamster 4.5S RNA. The possible evolutionary and regulatory significance of these sequences is discussed.

Analysis of the mouse gamma-crystallin gene family: assignment of multiple cDNAs to discrete genomic sequences and characterization of a representative gene

Blot hybridization analysis of mouse DNA with gamma-crystallin-specific cDNAs has detected the presence of a multigene family comprised of at least four related genes. The detailed structure of one of these genes, mouse gamma 4-crystallin (M gamma 4.1), and its corresponding cDNA has been determined. The gene spans approximately 2.6 kilobases (kb) and contains two introns. The gene predicts a polypeptide of 174 amino acids that shares extensive sequence homology with gamma-crystallin polypeptides of other species. The two similar structural domains of the protein correspond exactly to the second and third exons of the gene, supporting an exon-duplication model of gene evolution. The similarity in structure of this gene to that recently reported for a gamma-crystallin gene of the rat (1) suggests that a common structure may exist for all gamma-crystallin genes of the two species. Moreover, a highly conserved region, 50 nucleotides in length, immediately precedes the TATA box of both the mouse and rat genes, suggesting that this sequence may be important in gene regulation.

Purification and characterization of a gamma crystallin from mouse lenses

A gamma-crystallin has been purified by a two-step column chromatography from an extract of water soluble lens proteins from (101/ E1xC3H /E1)F1 mice. About 17% of the water soluble lens protein in normal mice is represented by this gamma-crystallin. The protein has been shown to be absent in cataractous lenses of Nop /+ mice after isoelectric focusing of water soluble lens proteins. It has a MW of 20,000. Amino acid analysis reveals the occurrence of eight cystein residues, which is considered to be high compared to other crystallins. The protein might play an important role in cataractogenesis.

Homology between the primary structures of the major bovine beta-crystallin chains

Partial amino acid sequences of six major subunits of bovine beta-crystallin have been determined by automatic liquid-phase Edman degradation and the dansyl-Edman procedure, complemented by amino acid analyses of peptides. The results show that, including the previously established beta Bp sequence [H. P. C. Driessen et al. (1981) Eur. J. Biochem. 121, 83-91], there exist at least seven primary gene products in bovine beta-crystallin, which exhibit 40% or more sequence homology. Two of the gene products are completely identical except for the presence in one of them of 17 additional residues at the N terminus, possibly caused by differential splicing of the same primary RNA transcript. The rate of evolutionary change of the beta chains (4% sequence change per 100 X 10(6) years) is about equally slow as that of alpha-crystallin, and the gene duplications giving rise to the different chains must have occurred very early in vertebrate evolution. The beta chains can be divided into two groups, according to sequence homology and presence of deletions/insertions and C-terminal extension, on which basis a new, rational nomenclature for the beta subunits is introduced. The N-terminal extensions of all beta chains are very different in length and sequence, even between homologous beta chains in different species. Possible explanations for this finding are discussed.

Multiple genes coding for the frog eye lens gamma-crystallins

Three new recombinant cDNA clones coding for gamma-crystallins have been identified in the frog (Rana temporaria) clonotheque by hybrid-selected translation/immunoprecipitation experiments, in addition to the gamma-1-crystallin clone that was isolated and sequenced previously [ Tomarev et al., Gene 17 (1982) 131-138; FEBS Letters 146 (1982) 315-318]. mRNA species coding for all these gamma-crystallins are about 650 nucleotides in length, but differ in structure, as follows from restriction and sequence analysis of the cloned cDNAs. The conclusion is that the R. temporaria genome contains a family of at least four similar but not identical gamma-crystallin genes. The complete nucleotide sequence has been determined for the cDNA of one of these clones coding for gamma-2-crystallin. It is 69% homologous with that of R. temporaria gamma-1-crystallin and contains four regions of partial internal homology corresponding to the four structural folding units of the gamma-crystallin molecules. An unusual feature of the gamma-2-crystallin amino acid sequence is the high lysine/arginine ratio equal to 1.1, in contrast to 0.05-0.16 for other known gamma-crystallins.

Determination of the solvent accessibility of specific aromatic residues in gamma-crystallin by photo-CIDNP NMR measurements

The surface or solvent accessibility of certain individual aromatic residues of calf-gamma II crystallin in solution (1 mM) were measured by the dramatic intensity enhancements of NMR lines generated by the interactions of cyclic radical pair formation of the 3-N-carboxymethyl lumiflavin (flavin I) dye excited (488nm) by an argon laser (5 watts) with the protein. This effect is called photo-chemically induced dynamic nuclear polarization: photo-CIDNP. The "light" and "dark" NMR spectra were taken in alternating scans in the pulsed Fourier transform mode on a Bruker 360 MHz instrument. Subtraction results in the photo-CIDNP difference spectrum containing lines of the polarized residues. With flavin dyes only tyrosine, histidine, and tryptophan can be polarized. The respective theoretical static accessibility of these residues based upon van der Waal's contact radii have been calculated from the atomic coordinates and provide a basis for evaluating the dynamic NMR photo-CIDNP results and for assigning the resonances. These results suggest that while the four tryptophan residues are completely buried, His-113 and His-14 of the five histidines; and Tyr-165 and Tyr-62 of the fifteen tyrosines are sufficiently exposed to elicit a photo-CIDNP effect. These results confirm and extend the observations previously obtained with theoretical electrostatic programs and FT-NMR measurements.

Delta crystallins and their nucleic acids

delta-Crystallin is a major structural protein of avian and reptilian lenses that is absent from the lenses of fish, amphibia and mammals. It appears to be a tetrameric protein with a native molecular weight near 200 000 (200K) and polypeptide molecular weight near 50K and 48K) (see Note added in proof). The alpha-crystallin polypeptides are extremely similar, associate in various combinations of four and are held together by hydrophobic interactions. Although principally cytoplasmic, delta-crystallin may associate with the cell membrane. delta-Crystallin differs from other lens crystallins in its alpha-helical content, native and subunit molecular weights, antigenicity, low wavelength of maximum fluorescence emission (315 nm) after excitation at 280 nm and amino acid composition (high in leucine; low in aromatic residues en no cysteine). Analyses of peptides, native and subunit molecular weights, and circular dichroism spectra indicate that the primary, secondary, tertiary and subunit structures of delta-crystallin have been generally conserved during evolution. There are at least two tandemly arranged delta-crystallin containing 13-15 introns in the chicken; a similar structure exists for a cloned delta-crystallin gene in the duck. Experiments with chicken show that delta-crystallin synthesis occurs principally in the embryo, especially during lens fiber cell differentiation. delta-Crystallin synthesis also takes place during lens fiber cell differentiation in culture. There is evidence for both transcriptional and post-transcriptional regulation of delta-crystallin synthesis. Current studies on the crystallographic and primary structures of delta-crystallin, on the structure, evolution and expression of the delta-crystallin genes, and on the translation of delta-crystallin mRNAs make this specialized lens protein an active area of investigation.

Strict co-linearity of genetic and protein folding domains in an intragenically duplicated rat lens gamma-crystallin gene

Two complementary DNA clones pRL gamma-2 and pRL gamma-3 of different rat lens gamma-crystallin messenger RNAs have been used to identify gamma-crystallin gene sequences in rat genomic DNA. Subsequently, the DNA present in the 18,000 to 20,000 bases region of the EcoRI digest, giving rise to a strong doublet hybridization signal, was cloned in lambda phage Charon-4A. One of the clones, lambda RCH gamma-3, carrying an insert of 17,500 bases has been characterized in detail. From analysis at the restriction enzyme level with 5'-, "middle" and 3'-specific subprobes of pRL gamma-3 it could be deduced that lambda RCH gamma-3 contains only one gamma-crystallin gene. The coding sequences of this gene are interrupted by intronic DNA. The primary structure of this gene and its flanking regions have been established by sequencing the relevant regions of a subclone of lambda RCH gamma-3, designated pRCH gamma-3 . 1. The sequence data show that the gamma-crystallin gene extends over 2700 bases of rat genomic DNA. The gene is split by two introns, one of 87 base-pairs after the third translation codon and a large one of 1880 base-pairs after codon 84. The mosaic structure of the gene is strictly co-linear with the structure of the gamma-crystallin polypeptide in that the large intron is positioned in a region which specifies the so-called "connecting peptide" and which links the two highly symmetrical and homologous protein domains. Although expected from the cDNA and protein sequence no introns were observed between the coding regions in the DNA specifying the two homologous folding motifs present in each protein domain. The relevance of this phenomenon in terms of the evolution of the mature gamma-crystallin gene is discussed.

Purification and crystallization of mammalian lens gamma-crystallins

Monomeric crystallins, beta s and several members of the gamma-crystallin family, occur in different relative proportions in a way which is related to the age of the lens cells. Methods of purification of large amounts of these different low molecular weight protein components from young and old bovine lenses are described along with details of crystallization of several gamma-fractions. Purification procedures have been developed for rabbit gamma-crystallins. The chromatographic methods achieve separation of several bovine and rabbit gamma-crystallins which have very similar electrophoretic mobilities. However, on storage, many electrophoretic variants are generated from some of these fractions.

Aggregation behavior of the bovine beta-crystallin Bp chain studied by limited proteolysis

The bovine beta-crystallin Bp chain is organized into two very similar domains, with short extensions at both N- and C-termini, and two alternative models for the beta Bp dimer have been proposed (Wistow, G., Slingsby, C., Blundell, T., Driessen, H.P.C., De Jong, W.W. and Bloemendal, H. (1981) FEBS Lett. 133, 9-16). By limited proteolysis the C-terminal arms can be cleaved off rapidly from the beta Bp dimer, while the N-terminal arms are more difficult to remove. Trypsin divides the beta Bp chain into two fragments which approximately correspond to the two structural domains. Dissociation and reassociation of the different products of limited proteolysis indicated that: the C-terminal arm extends freely from the surface and is not involved in subunit-contact; at least one N-terminal arm seems required for dimer formation; the N-terminal domains have a greater tendency to associate than the C-terminal domains and, when mixed, the purified domains reassociate partially to a Mr 50 000 structure like native beta Bp. These findings support the more extended dimer model of beta Bp.