Some aspects of the phosphorylation of alpha-crystallin A

Alpha-crystallin can function as a molecular chaperone

The alpha-crystallins (alpha A and alpha B) are major lens structural proteins of the vertebrate eye that are related to the small heat shock protein family. In addition, crystallins (especially alpha B) are found in many cells and organs outside the lens, and alpha B is overexpressed in several neurological disorders and in cell lines under stress conditions. Here I show that alpha-crystallin can function as a molecular chaperone. Stoichiometric amounts of alpha A and alpha B suppress thermally induced aggregation of various enzymes. In particular, alpha-crystallin is very efficient in suppressing the thermally induced aggregation of beta- and gamma-crystallins, the two other major mammalian structural lens proteins. alpha-Crystallin was also effective in preventing aggregation and in refolding guanidine hydrochloride-denatured gamma-crystallin, as judged by circular dichroism spectroscopy. My results thus indicate that alpha-crystallin refracts light and protects proteins from aggregation in the transparent eye lens and that in nonlens cells alpha-crystallin may have other functions in addition to its capacity to suppress aggregation of proteins.

Structural organization and stability of a thermoresistant domain generated by in vivo hydrolysis of the alpha-crystallin B chain from calf lens

A protein fragment (M(r) approximately 9000) isolated from the cortex of nonpathological calf lenses has been structurally characterized. The polypeptide structure was well organized (39% alpha-helix, 33% beta-structure, and 28% remainder) according to the far-ultraviolet circular dichroism. The fluorescence was heterogeneous for the presence of two tryptophan classes. Structure perturbation by pH and denaturant revealed cooperative structural transitions which are characteristics of a globular organization. A single-step unfolding curve induced by Gdn-HCl (midpoint = 1.38 M Gdn-HCl) was monitored by emission maximum shift as well as by far-ultraviolet circular dichroism. This transition was analyzed as a two-state process. The standard free energy of unfolding in the absence of the denaturant, delta Go (H2O), was found to be 10.80 +/- 0.25 kJ/mol at 20 degrees C and pH 7.4. The fragment also shows an unusual thermal resistance. Its structure was unperturbed up to 90 degrees C according to the fluorescence and dichroism. This last property, its peculiar amino acid composition, and the sequence of a small segment are shared, among crystallins, only with the N-terminal region of the alpha-crystallin B chain. A search for proteolysis sites along the alpha-crystallin B chain sequence revealed that it possesses specific points for proteinase attack. These sites are particularly exposed and clustered in a very flexible region in the middle of the protein sequence. They are also well represented in the C-terminal extension of the molecule while a few are buried in the N-terminal region.(ABSTRACT TRUNCATED AT 250 WORDS)

Site-specific glycation of lens crystallins by ascorbic acid

The oxidation of ascorbic acid leads to the formation of several compounds which are capable of reacting with protein amino groups via a Maillard reaction. Radioactivity from [1-14C]ascorbic acid was linearly incorporated into lens crystallins over a 10 day period in the presence of NaCNBH3. This rate of incorporation was 6-7-fold more rapid than that obtained with [14C]glucose under the same conditions. SDS-PAGE showed a linear incorporation into all the crystallin subunits. [1-14C]Ascorbic acid-label led alpha-crystallin was separated into its component A and B subunits, and each was digested with chymotrypsin. HPLC peptide analysis showed a differential labelling of the various lysine residues. Analysis of the peptides by mass spectrometry allowed the identification of the sites and the extent of modification. These values ranged from 6% for Lys-78 to 36% for Lys-11 in the A subunit and from 5% for Lys-82 to an average of 38% for the peptide containing Lys-166, Lys-174 and Lys-175 in the B subunit. Amino acid analysis demonstrated a single modification reaction producing N epsilon-(carboxymethyl)lysine. This agreed with the mass increase of 58 observed for each modified peptide.

Identification of the posttranslational modifications of bovine lens alpha B-crystallins by mass spectrometry

A combination of mass spectrometric techniques has been used to investigate the amino acid sequence and post-translational modifications of alpha B-crystallin isolated from bovine lenses by gel filtration chromatography and reversed-phase high performance liquid chromatography. Chromatographic fractions were analyzed by electrospray ionization mass spectrometry to determine the homogeneity and molecular weights of proteins in the fractions. The alpha B-crystallin primary gene product, its mono- and diphosphorylated forms, its N- and C-terminal truncated forms, as well as other lens proteins unrelated to the alpha B-crystallins were identified by their molecular weights. Detailed information about the sites of phosphorylation, as well as evidence supporting reassignment of Asn to Asp at position 80, was obtained by analyzing proteolytic digests of these proteins by fast atom bombardment mass spectrometry. Results of this investigation indicate that alpha B-crystallin is phosphorylated in vivo at Ser 45, Ser 59, and either Ser 19 or 21. From the specificity of phosphorylation of alpha-crystallins, it appears that there may be two different kinases responsible for their phosphorylation.

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

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

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

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

Progressive modifications of mouse lens crystallins in cataracts induced by buthionine sulfoximine

L-buthionine-S,R-sulfoximine (BSO), a specific inhibitor of GSH biosynthesis, was administered four times daily to mouse pups on post-natal days 7 and 8, inducing initiation of opacification on day 9. The initial progression of the cataract (less than 24 hr) was divided into four stages: (1) developing floriform; (2) mature floriform; (3) degenerate floriform; and (4) amorphous translucent cataract. Following this, dense corticonuclear opacities developed within several days. Two-dimensional gel electrophoresis of water-soluble whole lens extracts indicated that the most rapid early cataractous changes, occurring mainly during stage 2, were loss of the two major components of the heavy beta-crystallin fraction, a 31-kDa basic polypeptide and an acidic component at 27 kDa, concomitant with the appearance of new species at 30 and 25 kDa. This was followed by more extensive modification of both alpha and beta-crystallins during stages 3 and 4 and the appearance of abnormal species at 26, 19 and 18 kDa, which were slightly more acidic than the major normal alpha A-crystallin polypeptide. The gamma-crystallin components, relatively unaffected at stage 4, were then lost rapidly as dense opacities ensued. By contrast with the water-soluble fraction, the normal day 9 urea-soluble fraction was deficient in gamma-crystallin polypeptides and enriched in anodic components whose relative electrophoretic mobilities were similar to those reported previously for phosphorylated bovine alpha A-crystallin and several cytoskeletal polypeptides. At stage 4 of the cataract, the modifications of normal alpha and beta-crystallin components in the urea-soluble fraction paralleled those in the water-soluble fraction, but the products seen were more numerous. In addition, the cytoskeletal proteins were no longer detectable. Substantial increases in lens Ca2+ that precede all of the above changes in lens polypeptide composition suggest that Ca(2+)-activated proteolysis may play a major role in development of BSO cataracts.

Determination of alpha B crystallin aggregation: a new alternative method to assess ischemic damage of the heart

alpha B crystallin, a heat-shock-like protein, is a major component of the soluble protein fraction of the heart and is thought to play a protective role in stress situations. During an ischemic episode, the cytosol of cardiomyocytes acidifies, thus causing the aggregation of the protein with cytoskeletal elements. After homogenization of the tissue, alpha B crystallin can then be recovered with the insoluble cell components. This study investigated the change of the solubility properties of crystallin in the ischemic heart. The distribution of crystallin in the soluble and insoluble cellular fractions was determined by centrifugation of heart homogenates and immunoblot analysis with anti-alpha B crystallin antibodies. The proportion of aggregated alpha B-crystallin increased in hearts reperfused after total normothermic ischemia of increasing severity. alpha B crystallin aggregation was proportional to the amount of lactate dehydrogenase activity released by the hearts and was inversely correlated to the ability of the hearts to recover contractile activity after the ischemic episode. This study shows that the amount of aggregated crystallin can be used as a new marker for the ischemic damage of the heart. Biopsies of a few milligrams are sufficient for the analysis.

Alpha B-crystallin exists as an independent protein in the heart and in the lens

Alpha B-crystallin, a polypeptide of molecular mass 22 kDa, is considered to be one of two subunits (alpha A and alpha B) of the multimeric lens-specific protein, alpha-crystallin. Recent demonstrations of the extra-lenticular presence of alpha B-crystallin have suggested that outside of the lens, this polypeptide may have functions independent of alpha A. Within the lens however, as part of the protein alpha-crystallin, its function is assumed to be structural. In an effort to investigate the functional status of alpha B-crystallin in the lens, we have characterized this polypeptide in the rat heart and the human lens. Unequivocal identity of alpha B-crystallin in the rat heart and the rat lens was established by the sequence analyses of the respective cDNA clones. Size exclusion chromatography (FPLC) and immunoblotting showed that in the rat heart, alpha B-crystallin exists as an aggregate of 300-400 kDa average molecular mass, similar to that of purified alpha B-crystallin isolated from bovine lens. Interestingly, analysis of the human lens proteins by immunoblotting showed that, with age, unlike alpha A-crystallin, the alpha B subunit remains detectable in the soluble fractions derived from normal lenses as old as 82 years. Importantly, the average molecular mass of the alpha B subunit in the soluble fractions prepared from 60-80-year-old human lens nuclei was also found to be 300-400 kDa. These data lead to the conclusion that alpha B-crystallin may exist as an independent protein not only in non-lens tissues (e.g. heart) but in the lens as well.

Alpha B-crystallin is a small heat shock protein

Sequence similarity between alpha B-crystallin and small heat shock proteins (HSPs) has prompted us to investigate whether alpha B-crystallin expression is induced by heat shock. Indeed, accumulation of alpha B-crystallin was detected immunologically in NIH 3T3 cells after incubation at elevated temperatures and after addition of Cd2+ or sodium arsenite to these cells. Two-dimensional gel electrophoresis revealed identity between alpha B-crystallin from eye lenses and from heat-treated fibroblasts. The promoter of the alpha B-crystallin gene was fused to the bacterial chloramphenicol acetyltransferase gene and was shown to confer heat inducibility on this reporter gene in transient transfection assays. A perfect heat shock element within the promoter region is likely to mediate this response. Small HSPs and alpha B-crystallin were shown to share the following two physical properties: (i) they form supramolecular structures with sedimentation values around 17 S and (ii) they are associated with the nucleus at high temperatures and are localized in the cytoplasm under normal conditions. We conclude that alpha B-crystallin has to be considered a member of the class of small HSPs.

Reverse-phase HPLC analysis of human alpha crystallin

A rapid and highly sensitive reverse-phase HPLC (RP-HPLC) method was used to separate crystallin subunits from human alpha crystallin. Three distinct peaks were separated; by electrophoretic and immunological analyses the first and second peaks were identified as alpha B and alpha A respectively. On the other hand, peak 3 appeared to be a modified form of alpha crystallin. The ratio of alpha A and alpha B proteins was 3:1 in 1 day old lenses which gradually changed to 2:1 in 17 year old lenses and to 1:1 in the 50 and 82 year old whole lenses and 82 year old lens cortex, with a concomitant increase in the modified alpha, suggesting that alpha A subunits are relatively more involved in aggregation. Analysis of the 82 year old lens nucleus also supported this conclusion. The RP-HPLC analysis of the HMW aggregate fraction showed substantial enrichment of the modified alpha. The alpha A and alpha B subunits independently reassociated to form polymeric alpha crystallin whereas the modified alpha reassociated to form HMW aggregates as shown by molecular sieve HPLC. Hence it appears that the HMW aggregate peak was constituted by modified alpha crystallin. Only in the peak 3 material the 280 nm absorbance was about 2-fold higher than what was expected from the actual protein content. The data suggest that the changes induced by post-translational modifications may have some role in the formation of modified alpha. The present RP-HPLC method is useful in separating these modified alpha from the unmodified alpha A and alpha B subunits.

Elucidation of the primary structures of proteins by mass spectrometry

A combination of new mass spectrometric methods that can be used to determine the primary structures of proteins, including post-translational modifications, with unprecedented speed and accuracy is described. Structural characterization of alpha-crystallins from bovine lenses has been used to illustrate the methods. The molecular weights of alpha-crystallins fractionated, but not to homogeneity, by reversed-phase HPLC were determined with an uncertainty of 0.01% which is at least 100 times more accurate than is possible using conventional methods. This information was used to identify the primary gene product as well as its phosphorylated and truncated forms. Molecular weight maps of proteolytic digests of these proteins were determined by directly coupled capillary HPLC fast atom bombardment-mass spectrometry. From these maps, the entire amino acid sequence was confirmed, and the phosphorylated peptide was identified. The MS/MS daughter ion mass spectrum of the phosphorylated peptide provided sufficient information to determine which residue was phosphorylated. Because protein structure, including post-translational modifications, is determined on the basis of molecular weight, this method has broad application and will be useful for a variety of diverse and challenging problems in protein structure elucidation.

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

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

Differential synthesis of crystallins in the developing rat eye lens

The patterns of protein synthesis in rat lenses ranging in age from newborn to 4 months were compared. After incubation of lenses in [35S]methionine-containing medium it was possible to identify the de novo synthesized crystallins by two-dimensional gel electrophoresis and fluorography, in combination with peptide mapping and immunoblotting. It was found that the relative synthesis of alpha A and beta A3 stays fairly constant in rat lenses of all investigated ages. The relative synthesis of beta B2 and gamma s shows a pronounced increase with age in these post-natal lenses. A differential decrease can be observed in the relative synthesis of the other six gamma-crystallins (gamma A-gamma F). There appears to be a good correlation between the changes in relative synthesis of the various crystallins and previously reported alterations in mRNA levels, although certain mRNAs exhibit marked differences in translational efficiency.

Specificity of alpha crystallin binding to the lens membrane

The A1, A2, B1, and B2 species of bovine alpha crystallin have been purified and renatured to form high molecular weight aggregates comprised of only one species, and the aggregated forms of each of these species have been tested for their ability to bind to lens membrane in vitro. The aggregated forms of alpha-A1 and alpha-A2 bound to membrane in a saturable manner while those of alpha-B1 and alpha-B2 bound in much lower amounts, in a manner inconsistent with saturable binding. Together, these results demonstrate specific and saturable binding of aggregated alpha-A1 and alpha-A2 to the lens membrane, suggesting that these species are responsible for the previously observed interaction between alpha crystallin and the lens fiber cell membrane.

The alternative splicing product alpha Ains-crystallin is structurally equivalent to alpha A and alpha B subunits in the rat alpha-crystallin aggregate

In rodents and some other unrelated mammals, alternative splicing of the alpha A-crystallin gene transcript results in the synthesis of the elongated alpha Ains-crystallin chain. This polypeptide is identical to the normal alpha A-crystallin chain of 173 residues, but contains an additional sequence of 23 amino acid residues inserted between positions 63 and 64. To determine the effects of this insert peptide, the structure of the rat alpha-crystallin aggregate and its subunits alpha A-, alpha Ains- and alpha B-crystallin was studied using fluorescence spectra, partial urea dissociation, and lactoperoxidase-catalysed iodination of surface residues. The data suggest that all alpha-crystallin subunits occupy equivalent positions in the protein aggregate, and that the insert peptide merely elongates the connecting peptide between the putative amino- and carboxyl-terminal domain of the alpha A-crystallin subunit.

Synthesis of alpha-crystallin by a cell line derived from the lens of a transgenic animal

Cultured cells derived from transgenic animals have not generally been utilized to investigate questions in cell biology; however, to study the properties of proteins from the lens of the eye, a cell line which synthesizes alpha-crystallin was established from a transgenic mouse. All the alpha-crystallins, alpha A, alpha B, and alpha Insert, accumulate in the cell line. The alpha-crystallin, 1.6% of the cellular protein, is found in large molecular weight aggregates similar to the aggregates found in the normal mouse lens. The alpha-crystallins in the lens cells correspond exactly to both the unmodified and the phosphorylated alpha-crystallins found in the lens of the mouse, suggesting that some post-translational modification of the mouse alpha-crystallin may be important to the structure of this protein.

The effect of phosphorylation on the structure of alpha-crystallin

Homopolymers were constructed from the highly purified phosphorylated (A1 and B1) and non-phosphorylated (A2 and B2) polypeptides of alpha-crystallin. These were examined using electron microscopy, light scattering, fluorescence spectroscopy and sulphydryl probing to determine the effects of phosphorylation on the structure of alpha-crystallin. Each reconstituted aggregate consisted of uniform particles with circular cross-sections and diameters of 9.3-9.5 nm. Some of these were associated in chain-like structures. The molecular mass of the homopolymers varied from 360-507 kDa; for a single particle, it was estimated to be 216 +/- 10 kDa. Tryptophan residues in the alpha B2 homopolymers were more accessible to the solvent and to quenchers than those in alpha A2. Phosphorylation increased this accessibility in the alpha A homopolymer but decreased it in alpha B. This decrease could be attributed to phosphorylation of the serine adjacent to tryptophan 60 in the B chain. The kinetics of the reaction with DTNB indicated that the single cysteine in the A chain was buried in the alpha A2 homopolymer (k1' = 0.013 min-1) but more accessible in alpha A1 (k1' = 0.046 min-1). It was concluded that phosphorylation significantly alters the conformation of the alpha A subunits but probably has little effect on the B subunits. The alterations do not affect the ability of the subunits to associate into alpha-crystallin-like particles.

The in vivo phosphorylation sites of bovine alpha B-crystallin

Phosphate content determinations established that in alpha B-crystallin two phosphate groups can be present in vivo in bovine lenses. Comparison of tryptic digests of phosphorylated and unphosphorylated alpha B chains, revealed the location of the two phosphorylation sites in tryptic peptides T2 and T3. Thermolytic digestion and gas-phase sequencing demonstrated that Ser-19 and Ser-45 are the in vivo phosphorylation sites of bovine alpha B-crystallin. This pattern of phosphorylation differs from the previously reported in vitro obtained results.

The dephosphorylation of lens alpha-crystallin A chain

The present communication reports the presence of a phosphoprotein phosphatase activity in bovine lens preparations which dephosphorylates alpha Ap, the phosphorylated form of alpha A, one of the alpha-crystallin polypeptides, in a Ca2+/calmodulin dependent manner. The activity was found in soluble preparations from epithelial cells but it could not be detected in similar preparations from fiber cells. A 60,000 Mr calmodulin binding polypeptide and a 15,000 Mr polypeptide found in the epithelial cell preparations comigrated in SDS-PAGE with the A and B subunits of bovine brain calcineurin (phosphoprotein phosphatase 2B) respectively. The 15,000 Mr was specifically recognized by an anti-bovine brain calcineurin antiserum. Bovine brain calcineurin was as effective in dephosphorylating alpha Ap as the lens preparations. Thus, it is likely that the activity present in the lens is related to this enzyme. The results indicate that the lens specific polypeptide alpha A may be subject to metabolic control through phosphorylation and dephosphorylation pathways regulated by cAMP and calcium respectively. Changes in the activities of these pathways appear to occur during differentiation of the lens epithelial cell and may be related to gene regulation during the differentiation process.

Expression of the murine alpha B-crystallin gene is not restricted to the lens

The murine alpha B-crystallin gene was cloned and its expression was examined. In the mouse, significant levels of alpha B-crystallin RNA were detected not only in lens but also in heart, skeletal muscle, kidney, and lung; low and trace levels were detected in brain and spleen, respectively. The RNA species in lung, brain, and spleen was 400 to 500 bases larger than that in the other tissues. Transcription in lens, heart, skeletal muscle, kidney, and brain initiated at the same position. A mouse alpha B-crystallin mini-gene was constructed and was introduced into the germ line of mice, and its expression was demonstrated to parallel that of the endogenous gene. Transgene RNA was always detected in lens, heart, and skeletal muscle, while expression in kidney and lung was variable; it remains uncertain whether there is transgene expression in brain and spleen. These results demonstrate that regulatory sequences controlling expression of the alpha B-crystallin gene lie between sequences 666 base pairs upstream of the transcription initiation site and 2.4 kilobase pairs downstream of the poly(A) addition site and are not located within the introns. Transfection studies with a series of alpha B-crystallin mini-gene deletion mutants revealed that sequences between positions -222 and -167 were required for efficient expression in primary embryonic chick lens cells; sequences downstream of the poly(A) addition signal were dispensable for expression in this in vitro system.

Antisera to synthetic peptides as probes of structural changes during aging of alpha-crystallin from the bovine lens

Polyclonal antisera have been made to synthetic peptides of 11-15 residues that correspond to nine different regions of the alpha A crystallins. These antisera have been used in a radioimmunoassay to quantitatively probe for structural and/or covalent changes of alpha-crystallins in the nucleus versus cortex of the adult bovine lens. Antisera specific for the C-terminal and N-terminal regions of the alpha-crystallins bind more to alpha-crystallins from cortex. Antisera to three out of the seven internal sequences (residues 75-89, 87-101 and 135-149) bind better to alpha-crystallins from the bovine lens nucleus, suggesting a greater accessibility of these sequences to antisera binding. Together, these studies demonstrate that antisera against synthetic peptide sequences of alpha A crystallins are very specific probes that can detect structural and/or covalent changes in specified regions of the alpha-crystallins during the process of aging in the bovine lens.

In vitro and in vivo phosphorylation of chicken beta B3-crystallin

Incubations of chicken lens homogenates with [32P]-ATP revealed the phosphorylation of a 28 kDa protein, and phosphoamino acid analysis of the phosphorylated protein showed the presence of phosphoserine. The protein is present in the beta-crystallin fraction and after purification and partial sequence determination, by way of peptide mapping and subsequent amino acid analyses and Edman degradation, this 28 kDa protein was identified as the beta B3-crystallin subunit, based on its homology with the bovine and rat orthologue. From phosphate content determination it could be concluded that this chicken beta B3 subunit contains in vivo 2 mol phosphate/mol polypeptide.

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

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

Protein folding and aggregation studied by isoelectric focusing across a urea gradient and isoelectric focusing in two dimensions

Isoelectric focusing across a concentration gradient of urea was used to study the folding-unfolding and association-dissociation processes of proteins. Myoglobulin, albumin, RNase, papain, beta L- and alpha-crystallin were analyzed with this technique, and examples are given of visualized dissociation steps and of equilibrium-unfolding intermediates. Furthermore, a two-dimensional isoelectric focusing technique is presented that is useful to deduce whether a transition of a protein aggregate observed upon urea-gradient isoelectric focusing must be attributed to a change in the protein's tertiary or quaternary structure.

Expression of the murine alpha B-crystallin gene is not restricted to the lens

The murine alpha B-crystallin gene was cloned and its expression was examined. In the mouse, significant levels of alpha B-crystallin RNA were detected not only in lens but also in heart, skeletal muscle, kidney, and lung; low and trace levels were detected in brain and spleen, respectively. The RNA species in lung, brain, and spleen was 400 to 500 bases larger than that in the other tissues. Transcription in lens, heart, skeletal muscle, kidney, and brain initiated at the same position. A mouse alpha B-crystallin mini-gene was constructed and was introduced into the germ line of mice, and its expression was demonstrated to parallel that of the endogenous gene. Transgene RNA was always detected in lens, heart, and skeletal muscle, while expression in kidney and lung was variable; it remains uncertain whether there is transgene expression in brain and spleen. These results demonstrate that regulatory sequences controlling expression of the alpha B-crystallin gene lie between sequences 666 base pairs upstream of the transcription initiation site and 2.4 kilobase pairs downstream of the poly(A) addition site and are not located within the introns. Transfection studies with a series of alpha B-crystallin mini-gene deletion mutants revealed that sequences between positions -222 and -167 were required for efficient expression in primary embryonic chick lens cells; sequences downstream of the poly(A) addition signal were dispensable for expression in this in vitro system.

Physicochemical characterization of alpha-crystallins from bovine lenses: hydrodynamic and conformational properties

A detailed investigation of hydrodynamic and conformational behavior has been made of the HM alpha-crystallin and alpha-crystallins of bovine lens. Results from this study indicated that HM alpha (high-molecular-weight alpha-crystallin) and alpha (low-molecular-weight alpha-crystallin) possess considerable size and charge heterogeneities in their native structures and subunit polypeptides, respectively. Sedimentation velocity showed a heterogeneous polydisperse system of HM alpha with an average sedimentation coefficient of about 50S and a more homogeneous system of alpha-crystallin of 20 S. Viscosity and circular dichroism studies pointed to a compact and globular shape of dominant beta-sheet conformation for alpha-crystallin, yet a highly asymmetrical and aggregated form for HM alpha. The conformational stability of alpha-crystallin was investigated in the presence of various denaturants. The evidence presented shows that hydrogen bonding is the main force in maintaining the quaternary structure of compact native alpha-crystallin. Conformational flexibility of alpha-crystallin demonstrated in the equilibrium unfolding study indicated a multistep transition that made the extraction of thermodynamic data from the heat denaturation study difficult. Temperature perturbation on alpha-crystallin suggested the possible involvement of hydrophobic interaction in the aggregation process, leading to the formation of HM alpha from alpha-crystallin. The comparison of conformational properties between HM alpha and alpha-crystallin strongly indicated that HM alpha is a denatured form of alpha-crystallin.

Differential synthesis and phosphorylation of the alpha-crystallin A and B chains during bovine lens fiber cell differentiation

[14C]-amino acids and [32P]-orthophosphate incorporation experiments were carried out in bovine lenses in culture to study the synthesis and phosphorylation of alpha-crystallin A and B polypeptides during differentiation of the lens fiber cells. Following culture, the [14C] or [32P]-labelled alpha-crystallin was isolated by gel filtration chromatography from four regions of the lens corresponding to: A) quiescent epithelial cells, B) dividing epithelial cells and early stages of elongation, C) young elongating fibers, and D) mature fibers from the superficial cortex. The incorporation of label into the alpha-crystallin primary gene products alpha A2 and alpha B2 and their respective phosphorylated forms alpha A1 and alpha B1 was determined by isoelectric focusing and radioautography. Different synthesis and phosphorylation patterns were observed in alpha A and alpha B polypeptides. Synthesis and phosphorylation of the alpha B chain occurs most actively in the epithelial cells, both processes decrease during differentiation and there is no net accumulation of the phosphorylated form alpha B1 in the mature fiber cell. In contrast to the B chain, the A chain synthesis, minimal in the epithelial cell, increases with differentiation. Most striking, the A chain phosphorylation, not detectable in the epithelial cells, increases with differentiation. In the mature fiber cell, the phosphorylated form alpha A1 accounts for one third of the A chain. These observations indicate that the two chains may have different functions. the synthesis and phosphorylation patterns of alpha A suggest a lens-specific function of this polypeptide in the fiber cell and in the terminal differentiation process.(ABSTRACT TRUNCATED AT 250 WORDS)

Definition and comparison of the phosphorylation sites of the A and B chains of bovine alpha-crystallin

The major phosphorylation sites of bovine alpha-crystallin Ser122 in the A chain, Ser59 and Ser43 and/or Ser45 in the B chain have been previously characterized. Further analysis of total alpha-crystallin, isolated from the cortex of calf lenses incubated in the presence of [32P]orthophosphate, demonstrated the presence of additional phosphorylation sites in both chains. At least three additional phosphorylation sites were found in the A chain and at least one in the B chain. These additional sites accounted for approximately 25% of the radioactivity incorporated in the protein. Two general sequences were found in most phosphorylation sites of both chains of alpha-crystallin: (Arg/Lys)-(X)-Pro-Ser and Ser-(X)-Ser-Leu-Ser. In spite of the 57% homology in the sequences of the A and B chains, the phosphorylation sites are located, in the A polypeptide, at the C-terminal third and in the B polypeptide, at the N-terminal third. The alignment of the regions containing the phosphorylation sites of both chains (C-terminal third of the A and N-terminal third of the B chain) revealed an unexpected similarity in the relative positions of the sites in each chain.

MP17, a fiber-specific intrinsic membrane protein from mammalian eye lens

A major protein with a molecular weight of 17,000, designated as MP17, has been identified in mammalian eye lens plasma membranes. Hydrophobic photolabeling experiments revealed that MP17 is a genuine intrinsic membrane protein. By using monoclonal antibodies we demonstrated that MP17 is not detectable in liver, heart, muscle, spleen and kidney, and thus can be considered, like MP26, as a lens-specific membrane protein. Furthermore, we showed that MP17 is a substrate for cAMP-dependent protein kinase and that it is a calmodulin-binding protein.

Ontogeny of alpha-crystallin polypeptides during the lens development of a mutant mouse

The ontogeny of alpha A, alpha B and alpha Ains polypeptides of the alpha-crystallin was investigated by the indirect immunofluorescence staining method with antibodies directed against these three polypeptides in a mutant mouse strain called dyl (27). In this strain cataractogenesis starts around day 16 of lens development but the early development of the lens and the ontogeny of lens crystallins do not differ from the normal genotype (29). The polypeptides were fractionated from normal adult Swiss albino mice total native alpha-crystallin by SDS gel electrophoresis, extracted, lyophilized and injected into young rabbits for production of the antibody. The isolated polypeptides were controlled by SDS gel re-electrophoresis and the antibodies were tested against rat lens native alpha-crystallin by immunoblotting. alpha A and alpha Ains antibodies cross reacted, while alpha B did not show any cross reaction. Results presented here show that alpha A and alpha B appear simultaneously while alpha Ains was detectable at a later stage of lens development. These results are discussed.

Nucleotide sequence of a bovine lens alpha A-crystallin cDNA

We have determined the nucleotide sequence of a bovine lens alpha A2-crystallin cDNA clone, designated pBL alpha A2-1. The 793 bp cDNA insert contains coding information for the entire 173 amino acid alpha A2-crystallin polypeptide, as well as non-translated sequences located both upstream and downstream from the coding region. The coding sequences contained in pBL alpha A2-1 are at least 89% homologous with the corresponding sequences from other mammalian alpha A-crystallin genes, and are 78% homologous to the frog alpha A-crystallin coding region. In contrast, the downstream nontranslated sequences of the mammalian alpha A-crystallin transcripts show much greater sequence divergence, with the bovine sequences averaging 47% homology with the corresponding sequences from other mammalian species.

Age-dependent deamidation of chicken alpha A-crystallin

The major posttranslational modification product of alpha A-crystallin from chicken eye lenses has one more negative charge than the corresponding primary gene product. These polypeptides were compared by peptide mapping after tryptic digestion and cyanogen bromide cleavage, and the charge difference could be located in a peptide, comprising residues 146-150 of the amino acid sequence of alpha A-crystallin. Subsequent enzymatic hydrolysis with aminopeptidase showed that asparagine at position 149 of the primary gene product is replaced by aspartic acid. Two-dimensional gel electrophoresis of total lens homogenates from chickens of different ages revealed an age-dependent increase of the deamidated alpha A-subunit.

The phosphorylation sites of the B2 chain of bovine alpha-crystallin

The B2 chain of bovine lens alpha-crystallin is phosphorylated in a cAMP-dependent reaction. By analysis of 32P-labelled chymotryptic peptides isolated from alpha-crystallin obtained from lenses labelled in organ culture, two phosphorylated B2 chain fragments were found. Sequence analysis of the fragments gave the following results: Arg-Ala-Pro-Ser-Trp-Ile-Asp-Thr-Gly-Leu and Ser-Leu-Ser-Pro-Phe corresponding to residues 56 to 65 and 43 to 47, respectively. It is established by this work that B1 is a phosphorylated post-translational product of B2. Both the A2 and B2 chains of alpha-crystallin are phosphorylated at a similar site with the sequence Arg-(X)-Pro-Ser. This is an unusual site for cAMP-phosphorylation since the phosphorylated serine is preceded by a proline residue. It may also be of significance that the other B2 chain phosphorylation site even more radically differs from previously reported cAMP-dependent phosphorylation sites.

The eye lens crystallins: ambiguity as evolutionary strategy

Comparative studies of the different families of lens-specific proteins of the vertebrates, the crystallins, and their genes reveal several interesting evolutionary features. The origin of alpha-crystallin can be traced back to the small heat shock proteins, while the superfamily of beta gamma-crystallins shows structural similarities with a bacterial spore coat protein. The crystallins display a great diversity within and between species, as well as during development. Ambiguous transcription, mRNA-processing, and translation contribute to this diversity of the crystallins and their expression. These mechanisms include the occurrence of atypical poly-A addition signals, alternative splicing, and the use of two initiation codons on a single mRNA.