Cleavage from the N-terminal region of beta Bp crystallin during aging of the human lens

N-terminal extension of beta B1-crystallin: identification of a critical region that modulates protein interaction with beta A3-crystallin

The human lens proteins beta-crystallins are subdivided into acidic (betaA1-betaA4) and basic (betaB1-betaB3) subunit groups. These structural proteins exist at extremely high concentrations and associate into oligomers under physiological conditions. Crystallin acidic-basic pairs tend to form strong heteromolecular associations. The long N-terminal extensions of beta-crystallins may influence both homo- and heteromolecular interactions. However, identification of the critical regions of the extensions mediating protein associations has not been previously addressed. This was studied by comparing the self-association and heteromolecular associations of wild-type recombinant betaA3- and betaB1-crystallins and their N-terminally truncated counterparts (betaA3DeltaN30 and betaB1DeltaN56) using several biophysical techniques, including analytical ultracentrifugation and fluorescence spectroscopy. Removal of the N-terminal extension of betaA3 had no effect on dimerization or heteromolecular tetramer formation with betaB1. In contrast, the level of self-association of betaB1DeltaN56 increased, resulting in homotetramer formation, and heteromolecular association with betaA3 was blocked. Limited proteolysis of betaB1 produced betaB1DeltaN47, which is similar to intact protein formed dimers but in contrast showed enhanced heteromolecular tetramer formation with betaA3. The tryptic digestion was physiologically significant, corresponding to protease processing sites observed in vivo. Molecular modeling of the N-terminal betaB1 extension indicates structural features that position a mobile loop in the vicinity of these processing sites. The loop is derived from residues 48-56 which appear to be critical for mediating protein interactions with betaA3-crystallin.

BetaB2-crystallin undergoes extensive truncation during aging in human lenses

Based on the present literature, it is unclear whether betaB2-crystallin undergoes age-related truncation in human lenses. To answer this question, the purpose of this study was to determine in vivo truncation of betaB2-crystallin in human lenses during aging by examining its fragments in the beta(H)-crystallin fraction. The WS-protein fraction was isolated from lenses of desired ages and separated by a size-exclusion Agarose A 1.5m column to recover alpha-, beta(H)-, beta(L)-, and gamma-crystallin fractions. The beta(H)-crystallin fractions, isolated from lenses of 24- and 70-year-old donors, were utilized for two-dimensional (2D) gel electrophoresis (isoelectric focusing in the first dimension followed by SDS-PAGE in the second dimension). The partial N-terminal sequences of the desired fragments (Molecular weights [M(r)]<18-19kDa) from a 2D-gel of WS-proteins from lenses of a 70-year-old donor were determined. More than 37 crystallin fragments with M(r) between 4 and 19kDa were observed on a 2D-gel. Nine fragments in beta(H)-crystallin fraction were from betaB2-crystallin but additional single fragments of alphaA-, gammas-, betaA4, and of either gammaB-, gammaC- or gammaD-crystallins were also observed. Seven cleavage sites in the betaB2-crystallin were identified, which included two sites at Q(7)-A(8) and A(8)-G(9) bonds in the N-terminal extension, two sites at E(46)-K(47) and G(49)-S(50) bonds in the motif 1, one site at S(94) -S(95) in the motif 2, and two sites at N(115)-F(116) and Q(135)-Y(136) in motif 3. No fragments with cleavage in the motif 4 and C-terminal extension of betaB2-crystallin were seen. Apparently, three betaB2-crystallin fragments with only N-terminal cleavage and five with both N- and C-terminal cleavages were observed. Additional fragments with cleavage sites at Q(54)-Y(55) in alphaA-crystallin, at E(112)-N(113) in betaA4-crystallin, at G(4)-T(5) in gammas-crystallin, at M(69)-G(70) in either gammaB-, gammaC- or gammaD-crystallins (three have identical sequences at the cleaved bond), and at G(1)-K(2) in gammaB or gammaC (both have identical sequences at the cleavage site) were observed.Conclusions. The results showed that betaB2-crystallin undergoes age-related truncation producing fragments with M(r) between 4 and 19kDa that existed in the beta(H)-crystallin oligomer. The beta(H)-crystallin fraction also contained single fragments of alpha-, betaA4-, gammas-, and other gamma-crystallins.

Resistance of human betaB2-crystallin to in vivo modification

Post-translational modifications and/or structural changes induced by modifications are likely causes of the decrease in crystallin solubility associated with aging and the development of cataract. Characterization of human lens crystallins by mass spectrometry has demonstrated that betaB2-crystallin undergoes less modification than any of the other crystallins. As the lens ages, betaB2-crystallin retains its hydrophilic N-terminus while the hydrophilic C-termini of alpha-crystallins and large portions of the N-termini of betaA3/A1 and betaB1 are truncated. The hydrophilic terminal regions of crystallins contribute to their solubility. Furthermore, deamidation and disulfide bond formation, other modifications that may affect solubility by altering conformation, are less extensive in betaB2 than in the other crystallins. This resistance to modification results in higher levels of betaB2 compared with the other crystallins in the water-soluble fraction of older lenses. The solubility of betaB2 and its propensity to form non-covalent associations with less soluble beta-crystallins may contribute to the solubility of the other beta-crystallins. A current hypothesis is that the chaperone-like properties of alpha-crystallins contribute to lens crystallin solubility, particularly in younger lenses. In older lenses, where most of the alpha-crystallins have become water-insoluble, betaB2-crystallins may play a dominant role in lens crystallin solubility.

Studies on trypsin-modified bovine and human lens acylpeptide hydrolase

Acylpeptide hydrolase removes the N -acetylated amino acids from the peptide substrates but not from intact proteins. Cleavage between amino acid residues 203--204 of the native acylpeptide hydrolase results in the formation of a 55 kDa truncated active enzyme in the bovine lens, in vivo. In this study we explored the hydrolytic properties of the truncated enzyme using lens beta- and gamma-crystallins as substrates. SDS--PAGE analysis indicated that the beta B2-crystallin was cleaved by truncated acylpeptide hydrolase into several protein fragments (10--26 kDa). No cleavage of the gamma-crystallins was observed under similar conditions. Both the acylpeptide hydrolase activity and the protease activity of the 55 kDa enzyme were completely inhibited by diisopropylfluorophosphate, p -chloromercuribenzoate and ebelactone, and moderately inhibited by N -tosyl phenylalanine chloromethyl ketone. SDS--PAGE analysis followed by fluorography of ((3)H) diisopropylfluorophosphate labeled human lens acylpeptide hydrolase preparation showed the presence of the 55 kDa truncated form of the enzyme, as observed in the bovine lens. The peptide (d)-AIKGDQFL-NH(2)--the amino acid sequence 200--207 of the native bovine acylpeptide hydrolase with an in vivo cleavage site of native protein--was hydrolysed by the lens protease(s) suggesting that the in vivo generation of the 55 kDa acylpeptide hydrolase may be mediated through a proteolytic processing. The protease(s) responsible for the cleavage of this peptide was inhibited by diisopropylfluorophosphate and p -chloromercuribenzoate.

The major in vivo modifications of the human water-insoluble lens crystallins are disulfide bonds, deamidation, methionine oxidation and backbone cleavage

This investigation of the water-insoluble crystallins from human lenses has used multiple chromatographic separations to obtain proteins of sufficient purity for mass spectrometric analysis. Each fraction was analysed to determine the molecular masses of the constituent proteins as well as peptides in tryptic digests of these proteins. The major components of the water-insoluble crystallins were identified as alphaA- and alphaB-crystallins. In addition, gammaS-, betaB1-, gammaD-, betaA3/A1- and betaB2-crystallins were found, in order of decreasing abundance. Although there was evidence of some backbone cleavage, the predominant forms of alphaA-, alphaB, betaB2-, gammaS- and gammaD-crystallins were the intact polypeptide chains. The major modifications distinguishing the water-soluble crystallins were increased disulfide bonding, oxidation of Met, deamidation of Gln and Asn and backbone cleavage. Of the many reactions hypothesized to lead to crystallin insolubility and cataract, these results most strongly support metal-catalysed oxidation, deamidation and truncation as initiators of conformational changes that favor aggregation.

Age-related degradation of betaA3/A1-crystallin in human lenses

The aim of this study was to determine age-related degradation of betaA3/A1-crystallin in human lenses. The betaA3/A1-crystallin fragments were identified by Western blot analysis using two site-specific anti-betaA3/A1-crystallin antibodies. The first antibody was raised against a N-terminal region (residues 35-66), and the second to the C-terminal (residues 203-214) region of the crystallin. During the analyses, either preparative SDS-PAGE-separated fragments from betaH-crystallin fraction or water-soluble (WS) protein fractions from lenses of different aged donors were used. In lenses from 27- to 30-year-old donors, four major crystallin fragments of about 5, 16, 17, and 18 kDa immunoreacted with the anti-betaA3/A1-N-terminal antibody, suggesting their intact N-terminus but cleaved C-terminus. A similar analysis with the anti-betaA3/A1-C-terminal antibody identified 15-, 18-, 19-, and 20-kDa species and also five species between 4 and 11 kDa that had intact C-terminus but cleaved N-terminus. In lenses from a 5-year-old donor only two crystallin species, a major 15-kDa and a minor 18-kDa species, showed an intact N-terminus and cleaved C-terminus, whereas, eight species with Mr's between 4 and 19 kDa exhibited intact C-terminus but cleaved N-terminus. Upon two-dimensional gel electrophoresis of a betaH-crystallin fraction from the lenses of a 70-year-old donor, a degradation profile almost similar to the crystallin mentioned above was observed. However, the existence of multiple spots with identical Mr's of truncated betaA3/A1-crystallin species on the 2D-gel suggests their existence as isoforms (identical size species with different charges) because of post-translational modifications. Five species of 4, 6, 11, 15, and 18 kDa showed an identical partial N-terminal sequence of N-F-Q-G, suggesting cleavage at the E39-N40 bond during their production. Together, the data suggest that the majority of age-related cleavages in betaA3/A1-crystallin occur at the N-terminal region, with a major cleavage site at the E39-N40 bond generating some of these fragments.

Age-related changes in human lens crystallins identified by HPLC and mass spectrometry

Analysis of water-soluble crystallins from human lenses, ages 32 week fetal to 55 years has led to identification of the major modifications of the proteins comprising the lens. These modifications were identified by the masses of the proteins determined by electrospray ionization mass spectrometry after the proteins were separated by gel filtration and reversed phase high performance liquid chromatography. Examination of all the proteins isolated from the water soluble portion demonstrated that the major age-related modifications causing significant alteration in the molecular weights of the lens crystallins include truncation of the N-termini of beta B1, beta A3 and beta A1, and partial phosphorylation and C-terminal degradation of alpha-crystallins. N-terminal degradation of beta B1, beta A3 and beta A1 was evident in human lenses less than one year old, and the proportion of these truncated proteins became greater with age. Phosphorylation of alpha A- and alpha B-crystallins increased from the fetal to the 3 year old lens, but did not change with further aging. Minor components indicating truncation of the C-termini of alpha-crystallins were found in older lenses. In contrast to beta B1, beta A3 and beta A1, the masses of the major species of alpha A, alpha B, beta B2, beta A4, gamma S, gamma C, and gamma D did not change with aging. This suggested that the major modifications to these crystallins are limited to deamidation and possibly intra-molecular disulfide bonds. These data, in conjunction with the data in the accompanying manuscript, established deamidation as a common modification, since deamidation, which causes only a one dalton change in mass, is the only modification that is consistent with the absence of a detectable change in molecular weight and the observed increased acidity demonstrated in the two-dimensional gels of the accompanying paper. Other age related changes included a decrease in beta B3 (M(r) 24224), a major component of the fetal lens, which was not detected in lenses older than 3 years, and increases in the ratios of alpha B:alpha A and gamma S:gamma C.

Size of human lens beta-crystallin aggregates are distinguished by N-terminal truncation of betaB1

The aggregates formed by the interactions of the human lens beta-crystallins have been particularly difficult to characterize because the beta-crystallins comprise several proteins of similar structure and molecular weight and because their sequences were not known until recently. Previously, it could not be ascertained whether the species of various acidities were different proteins or modifications of the same proteins. The recent determination of the sequences permits calculation of molecular weights and unambiguous identification of the various beta-crystallins and their modified forms by mass spectrometry. In this investigation, the components of the three sizes of beta-crystallin aggregates, beta1 (approximately 150,000), beta2 (approximately 92,000), and beta3 (approximately 46,000), were determined. The principal differences among the different beta-crystallin aggregates was the presence of betaA4 in beta1 and beta2, but not beta3, and the length of the N-terminal extension of betaB1. The size of the beta-crystallin aggregate correlated with the length of the N-terminal extension of betaB1, indicating that the flexible N terminus of betaB1 is critical to the formation of higher molecular weight aggregates of beta-crystallins. Separation of the components by ion exchange under non-denaturing conditions showed that betaB2 occurs as homo-dimers and homo-tetramers as well as contributing to hetero-oligomers. Other beta-crystallins were present only as hetero-oligomers.

Solution conformation of bovine lens alpha- and betaB2-crystallin terminal extensions

alpha- and betaB2-Crystallin are the major proteins in the mammalian lens. Each of these crystallins has short, flexible terminal extensions from its domain core; the two alpha-crystallin subunits have C-terminal extensions of eight and ten amino acids whilst betaB2-crystallin has N- and C-terminal extensions of 15 and 11 amino acids, respectively. The solution conformations of these chemically synthesised extensions have been examined by two-dimensional 1H NMR spectroscopy. The N-terminal extension of betaB2-crystallin and the C-terminal extensions of alpha-crystallin adopt little ordered structure. In the membrane-mimicking solvent trifluoroethanol, the alpha-crystallin extensions are also unstructured. In contrast, the C-terminal extension of betaB2-crystallin in water has a structural preference towards turn-like structures, creating a hydrophobic region involving G198, F200 and P202. In the lens, the C-terminal extension of betaB2-crystallin is the only one of these extensions that interacts to any large extent with other crystallins. The structural preference of the C-terminal extension of betaB2-crystallin may therefore have implications for the role of this extension in crystallin-crystallin interactions.

Age-dependent cleavage at the C-terminal region of lens beta B2 crystallin

A previous study has demonstrated that in vivo, the peptide bonds at the C-terminal region of the alpha-A crystallins were cleaved in an age-dependent manner, between the two hydroxyl-containing amino acids in the sequence -PS(T)S (Takemoto, 1995). Bovine beta B2 crystallin also contains the sequence-PSS at its C-terminus. To determine if this specific site of cleavage occurred in other lens proteins besides alpha-A crystallin, beta B2 crystallin was prepared from total proteins of young versus adult bovine lens fiber cells. After cleavage by cyanogen bromide, the C-terminal fragments were characterized by mass spectrometry. The results showed that peptide cleavage also occurred between the two hydroxyl-containing amino acids in the sequence -PSS of beta B2 crystallin from older fiber cells, demonstrating that age-dependent cleavage of this peptide bond occurs in multiple proteins of the aging lens.

Interaction of an altered beta-crystallin with other proteins in the Philly mouse lens

An altered beta B2-crystallin is synthesized in the lens of the Philly mouse. This beta B2 has a more acidic isoelectric point than the beta B2 that is isolated from normal mouse lens. The altered beta B2 is immunologically reactive with antibody to the amino terminal of the beta B2-crystallin, but appears to be present in only very small quantities in the Philly lens. When the soluble proteins are isolated from the Philly lens and chromatographed by gel exclusion chromatography, the beta B2 can be found primarily in the heavy molecular weight fraction. Some immunoreactive material was also found throughout the higher molecular weight beta-crystallin region, beta H, and the lower molecular weight region, beta L. These results would indicate that the altered beta B2-crystallin in the Philly lens can interact with the other beta-crystallins in the lens; however, interactions of the beta B2-crystallin with the other proteins of the lens may cause rapid aggregation of the cellular proteins leading to the formation of the heavy molecular weight material. The increased number of these aggregates may eventually lead to the cataract formation in the Philly mouse.

Immunochemical characterization of the major low molecular weight polypeptide (10K) from human cataractous lenses

Polyclonal antisera to whole crystallins and to synthetic peptides corresponding to various sequences of these crystallins have been used to probe Western blots that contain a low molecular weight component of approximately 10,000 daltons found in the water-soluble fractions from human cataractous lenses. This 10K component binds only to antiserum made against human gamma crystallin. Incubation of human cataractous lens homogenates with alpha chymotrypsin or trypsin will produce low molecular components of similar molecular weight, and identical specificity of binding to the gamma crystallin antiserum. Together, these results suggest that the gamma crystallins constitute a class of macromolecules that are susceptible to in vivo proteolysis during cataractogenesis of the aged human lens.

Age-related changes in a fiber cell-specific extrinsic membrane protein

Western blot analysis using a monoclonal antibody raised against a lens fiber cell-specific, extrinsic membrane protein reveals several immunologically related bands in fractions derived from bovine lens. Previous work suggests that the parent molecule is the Mr 115 species, and that lower molecular weight bands represent the products of a progressive, step-wise, post-translational degradation. In this report we compare the extent of proteolytic degradation in extracts prepared from the lens cortex and lens nucleus, using both protease-suppressive and protease-permissive isolation protocols. The results suggest that the observed degradation is a result of in vivo post-translational modification of the Mr 115 antigen, and thus represents physiologic aging of this protein. This analysis also suggests that degradation alters the solubility and/or membrane affinity of this antigen, resulting in a progressive shift to the insoluble phase.