CDNA and deduced protein sequence for the beta B1-crystallin polypeptide of the chicken lens. Conservation of the PAPA sequence.

Abstract

The nucleotide sequence of two cDNA clones corresponding to the beta B1-crystallin mRNA (formerly beta 35) of the chicken eye lens has been determined. The derived amino acid sequence of the chicken beta B1 polypeptide fits well with the two-domain, four "Greek Key" motif structure common to the beta gamma-crystallin superfamily of proteins. The calculated molecular weight of the encoded chicken beta B1 protein is 27,267. The beta B1 polypeptide has both an N- and C-terminal extension and is highly homologous to the mammalian beta B1-crystallin polypeptide. There is a 72% homology between the core regions of the chicken and bovine beta B1 polypeptides; by contrast, there is only 27% homology between the N-terminal extensions of these polypeptides. The N-terminal extension of chicken beta B1 contains a short alternating proline-alanine (PAPA) sequence, like that in the mammalian beta B1, and has some homology with the N-terminal region of histone H1.4, myosin light chain, prokaryotic outer membrane protein A, and adenovirus 24/28-kDa early protein. At the nucleic acid level, the chicken beta B1 crystallin gene has an atypical polyadenylation signal, AATTAAA. This appears to be associated with microheterogeneity of the polyadenylation site by comparison of two cDNA clones, suggesting an additional level at which diversity in crystallin gene expression may arise.