Isolation and partial characterization of the human alpha A-crystallin gene

Degradation of C-terminal truncated alpha A-crystallins by the ubiquitin-proteasome pathway

PURPOSE

Calpain-mediated C-terminal cleavage of alpha A-crystallins occurs during aging and cataractogenesis. The objective of the present study was to explore the role of the ubiquitin-proteasome pathway (UPP) in degrading C-terminal truncated alpha A-crystallins.

METHODS

Recombinant wild-type (wt) alpha A-crystallin and C-terminal truncated alpha A(1-168)-, alpha A(1-163)-, and alpha A(1-162)-crystallins were expressed in Escherichia coli and purified to homogeneity. The wt and truncated alpha A-crystallins were labeled with (125)I, and proteolytic degradation was determined using both lens fiber lysate and reticulocyte lysate as sources of ubiquitinating and proteolytic enzymes. Far UV circular dichroism, tryptophan fluorescence intensity, and binding to the hydrophobic fluorescence probe Bis-ANS were used to characterize the wt and truncated alpha A-crystallins. Oligomer sizes of these crystallins were determined by multiangle light-scattering.

RESULTS

Whereas wt alpha A-crystallin was degraded moderately in both lens fiber and reticulocyte lysates, alpha A(1-168)-crystallin was resistant to degradation. The susceptibility of alpha A(1-163)-crystallin to degradation was comparable to that of wt alpha A-crystallin. However, alpha A(1-162)-crystallin was much more susceptible than wt alpha A-crystallin to degradation in both lens fiber and reticulocyte lysates. The degradation of both wt and C-terminal truncated alpha A(1-162)-crystallins requires adenosine triphosphate (ATP) and was stimulated by addition of a ubiquitin-conjugating enzyme, Ubc4. The degradation was substantially inhibited by the proteasome inhibitor MG132 and a dominant negative mutant of ubiquitin, K6W-Ub, indicating that at least part of the proteolysis was mediated by the UPP. Spectroscopic analyses of wt and C-terminal truncated alpha A-crystallins revealed that C-terminal truncation of alpha A-crystallin resulted in only subtle changes in secondary structures. However, C-terminal truncations resulted in significant changes in surface hydrophobicity and thermal stability. Thus, these conformational changes may reveal or mask the signals for the ubiquitin-dependent degradation.

CONCLUSIONS

The present data demonstrate that C-terminal cleavage of alpha A-crystallin not only alters its conformation and thermal stability, but also its susceptibility to degradation by the UPP. The rapid degradation of alpha A(1-162) by the UPP may prevent its accumulation in the lens.

Hydrophobicity and flexibility of alpha A- and alpha B-crystallin are different

Since the discovery that the lens protein alpha-crystallin is also found in non-lenticular tissues and can function as a chaperone, relatively little attention has been paid to differences in properties between alpha A- and alpha B-crystallin, which form mixed aggregates in the lens but have so far never been found together in other tissues. In this study hydrophobicity and flexibility, properties that are thought to be relevant for chaperone function, are compared for alpha A- and alpha B-crystallin. Hydrophobicity was monitored from sodium dodecylsulphate polyacrylamide gel electrophoresis in the absence and presence of (methyl-substituted) ureas. Flexibilities were calculated from primary structures. Based on literature data also some other properties are compared. The results indicate significant difference in hydrophobicity profile, flexibility of the terminal parts and stability of alpha A- and alpha B-crystallin.

A comparison of structural relationships among alpha-crystallin, human Hsp27, gamma-crystallins and beta B2-crystallin

The 3D structures of alpha-crystallin, a major eye lens protein, and related small heat shock proteins are unresolved. It has been assumed that alpha-crystallin is primarily a beta-sheet globular protein similar to alpha-crystallin (Siezen and Argos, Biochim. Biophys. Acta, 1983, 748, 56-67) containing sequence repeats in its two domains (Wistow, FEBS Lett. 1985, 181, 1-6). Positional flexibility of amino acid residues and far UV-circular dichroism spectroscopy were used to investigate structural relationships among these proteins. The utility of flexibility plots for predicting protein structure is demonstrated by the excellent correlation of these plots with the known 3D X-ray structures of beta/gamma-crystallins. Similar analyses of alpha-crystallin subunits, alpha A and alpha B, and human heat shock protein 27 show that the C-terminal domains and connecting segments of these proteins are very similar while the N-terminal domains have significant structural differences. Unlike beta/gamma-crystallins, both Hsp27 and alpha-crystallin subunits are asymmetrical with highly flexible C-terminal domains. Flexibility is considered essential for protein functional activity. Therefore, the C-terminal region may play an active role in alpha-crystallin and small heat shock protein function. Differences in flexibility profiles and estimated secondary structure distribution in alpha-crystallin by three recent/updated algorithms from far UV-CD spectra support our predicted 3D structure and the concept that alpha-crystallin and members of beta/gamma superfamily are structurally dissimilar.

Partial physical map of human chromosome 21 from fibroblast and lymphocyte DNA

A partial physical map of the human chromosome 21 including 26 genes and anonymous sequences was established by pulsed-field gel electrophoresis analysis of restriction fragments obtained from lymphocyte and fibroblast DNAs. The sizes of the restriction fragments obtained by total digestion with eight different enzymes were compared in these two tissues. Differences resulting from the variations in the methylation state of the restriction sites were frequently observed. These differences and partial digestions were used to estimate the order and the distances between genes and sequences. Six linkage groups were defined: D21S13-D21S16, D21S1-D21S11, D21S65-D21S17, (D21S55,ERG)-ETS2, BCEI-D21S19-D21S42-D21S113-CBS-CRYA1, and COL6A2-S100B. For six intergenic distances the resolution of previous maps was significantly increased.

Primary structure of rabbit lens alpha-crystallins

The primary structure and posttranslational modifications of rabbit lens alpha-crystallins were examined using electrospray ionization mass spectrometry to determine the molecular weights of the intact proteins and fast atom bombardment mass spectrometry to analyze proteolytic digests of the alpha A- and alpha B-crystallins. The previously determined primary structure of alpha A-crystallin was confirmed. Posttranslational modifications detected included one phosphorylation site and the presence of a truncated form minus the five C-terminal residues. The previously undetermined amino acid sequence of rabbit alpha B-crystallin was determined to be the same as the bovine alpha B-crystallin sequence except at three residues: Thr 40, Thr 132, and Pro 153. Rabbit alpha B-crystallin showed evidence of phosphorylation at the same three sites as bovine alpha B-crystallin. The molecular weights of the intact proteins indicated that any one molecule had a maximum of two phosphorylations. Also, there was a truncated form which did not include the five C-terminal residues.

Two-dimensional gel electrophoretic analysis of human lens proteins

Human lens proteins from clear lenses were separated and identified using two-dimensional polyacrylamide electrophoresis. Isoelectric focusing, both equilibrium and non-equilibrium, was performed in the first dimension and SDS electrophoresis in the second dimension. Proteins were identified by Western blotting and sequencing techniques and by comparison with patterns obtained with purified crystallin fractions. Analyses were performed on total urea soluble proteins of lenses varying in age from fetal to 73 yr. Several hundred protein spots representing crystallins, cytoskeletal proteins and enzymes were resolved in the fetal lens. In the older lenses there was a dramatic increase in the number of protein species in the molecular weight range of the crystallins and a reduced number of discrete protein species visible at molecular weights greater than 50,000. Conversely, a number of proteins below approximately 15 kDa were visible even in the fetal lens. The number and amount of polypeptides in this molecular weight range were increased in the older lenses. Many of these low molecular weight species could be assigned to either the alpha-, beta- or gamma-crystallin fractions. An age dependent increase in the number of acidic species of both crystallins and other proteins, such as, glyceraldehyde 3-phosphate dehydrogenase was observed as well as the loss or mobility change of gamma-crystallin. Two-dimensional gel electrophoresis provides a sensitive and practical technique for characterizing all of the proteins of the human lens.

A map of the distal region of the long arm of human chromosome 21 constructed by radiation hybrid mapping and pulsed-field gel electrophoresis

We have used radiation hybrid (RH) mapping and pulsed-field gel electrophoresis (PFGE) to determine the order and positions of 28 DNA markers from the distal region of the long arm of human chromosome 21. The maps generated by these two methods are in good agreement. This study, combined with that of D. R. Cox et al. (1990, Science 250:245-250), results in an RH map that covers the long arm of chromosome 21 (21q). We have used a subtelomeric probe to show that our map includes the telomere and have identified single-copy genes and markers within 200 kbp of the telomere. Comparison of the physical and RH maps with genetic linkage maps shows "hot spots" of meiotic recombination in the distal region, one of which is close to the telomere, in agreement with previous cytogenetic observations of increased recombination frequency near telomeres.

Assignment of the alpha B-crystallin gene to human chromosome 11

Using a human alpha B-crystallin genomic probe and human-mouse somatic cell hybrids, the human alpha B-gene was assigned to chromosome 11 and further corroborated by in situ hybridization to normal metaphase chromosomes. This assignment confirmed and regionally mapped the locus to q22.3-23.1.

A pseudo-exon in the functional human alpha A-crystallin gene

The frequent correspondence of exons to structural or functional domains in proteins has suggested that many proteins have evolved by modular assembly. This idea is supported by examples of apparent exon duplication and by shared domains among both alternatively spliced and completely separate genes. During this process it is probable that some combinations of exons would not prove advantageous and would therefore be lost. Here we report that within the active single-copy human gene for alpha A-crystallin there is a 'pseudo-exon' in the early stages of being extinguished, perhaps the result of a failed experiment in the evolution of this specialized, lens-specific protein.

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.

Confirmation of assignment of the human alpha 1-crystallin gene (CRYA1) to chromosome 21 with regional localization to q22.3

The crystallins are highly conserved structural proteins universally found in the eye lens of all vertebrate species. In mammals, three immunologically distinct classes are present, alpha-, beta-, and gamma-crystallins, and each class represents a multigene family. The alpha-crystallin gene family consists of alpha 1-crystallin (CRYA1) and alpha 2-crystallin (CRYA2) genes (previously designated alpha A- and alpha B-crystallin, respectively), which show extensive sequence homology. We constructed a synthetic oligonucleotide probe of 25 bases corresponding to a specific region of the human alpha 1-crystallin gene sequence. This 25-mer probe bears little sequence homology to human alpha 2-crystallin gene and does not cross-hybridize to alpha 2-crystallin sequences in Southern blot analysis. Using this unique synthetic probe, we have demonstrated the identity of the alpha 1-crystallin gene in human genomic DNA. In addition, we have also confirmed its chromosomal location on human chromosome 21. Finally, we have regionally localized the gene to q22.3 by using both Southern blot analysis of a panel of cell hybrids containing different parts of human chromosome 21, and in situ hybridization to metaphase chromosomes. The use of synthetic oligonucleotide probes specific for individual genes should be useful in identifying and mapping members of multigene families.

Ontogeny of alpha A and alpha B crystallin polypeptides during Rana temporaria lens development

The ontogeny and localization of alpha A and alpha B polypeptide chains of alpha-crystallin were investigated in the developing lens of Rana temporaria, an anuran amphibian, using the indirect immunofluorescence staining method with heterologous antibodies directed against these two polypeptides. alpha A and alpha B crystallins are primary gene products and are translated by different mRNAs in mammals. Although they show about 6000 amino-acid sequence homology (Bloemendal, 1977), the alpha A cDNA of rat and mouse does not hybridize to alpha B mRNA (Dodemont et al., 1981; King and Piatigorsky, 1983). Antigenically too, alpha A and alpha B polypeptides have been shown to be different. These two polypeptides were isolated from mouse lens native alpha-crystallin by SDS-gel electrophoresis and were injected into young rabbits to raise antibodies. These antibodies were tested by immunoblotting against R. temporaria total lens soluble proteins before their use in the present investigation. Results presented here show that in the developing lens of R. temporaria, alpha A appears earlier than alpha B, suggesting a differential gene activation. In addition, these two polypeptides could not be detected either in the developing lens epithelium or in the epithelium of young froglets (2-3 weeks post-metamorphosis).

Complete nucleotide sequence of the chicken alpha A-crystallin gene and its 5' flanking region

The chicken alpha A-crystallin gene and 2.6 kb of its 5' flanking sequence have been isolated and characterized by electron microscopy and sequencing. The structural gene is 4.5 kb long and contains two introns, each approx. 1 kb in length. The first intron divides codons 63 and 64, and the second intron divides codons 104 and 105, as in rodents. There is little indication that the insert exon of rodents (an alternatively spliced sequence) is present in complete form in the chicken alpha A-crystallin gene; small stretches of similarity to this sequence were found throughout the gene. The 5' flanking sequence of the chicken alpha A-crystallin gene shows considerable sequence similarity with other mammalian alpha B-crystallin genes. In addition, one consensus sequence (GCAGCATGCCCTCCTAG) present in the 5' flanking region of the chicken alpha A-crystallin gene was found in the 5' flanking region of most reported crystallin genes.