Preferential conservation of the globular domains of the βA3/A1-crystallin polypeptide of the chicken eye lens

The primary structure of the β19/26-cry stallin polypeptide of the chicken lens has been determined by cDNA sequencing and primer extension experiments. In addition, a primer extension experiment has corrected the sequence for the N-terminal arm of the murine β23 polypeptide, which is the homologue of the chicken β19/26 polypeptide.We also show that, in the chicken and mouse, the N-terminal arm of the polypeptide is encoded on two separate exons. For simplicity, we have changed the names of both chicken β19/26 and murine β23 to βA3/A1,which is the name of the homologous bovine polypeptide. The deduced sequence of the chicken βA3/A1 polypeptide fits the predicted three-dimensional structure involving two homologous domains, each folded into two 'Greek key' motifs,common to the βγ-crystallin superfamily of proteins. Comparison of the amino acid sequence of the chicken and mammalian βA3/A1 polypeptides indicates that different regions of the protein, which are encoded on different exons, are diverging at different rates.The N-terminal extension is the fastest evolving region of the βA3/A1 polypeptide. Hybrid-selected translation coupled with primer extension experiments suggest that a single chicken βA3/A1 mRNA synthesizes two polypeptides, βA3 (25 kDa) and βA1(23 kDa) by utilization of different translation initiation sites.