Preferential conservation of the globular domains of the beta A3/A1-crystallin polypeptide of the chicken eye lens.

The primary structure of the beta 19/26-crystallin 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 beta 23 polypeptide, which is the homologue of the chicken beta 19/26 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 beta 19/26 and murine beta 23 to beta A3/A1, which is the name of the homologous bovine polypeptide. The deduced sequence of the chicken beta A3/A1 polypeptide fits the predicted three-dimensional structure involving two homologous domains, each folded into two 'Greek key' motifs, common to the beta gamma-crystallin superfamily of proteins. Comparison of the amino acid sequence of the chicken and mammalian beta A3/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 beta A3/A1 polypeptide. Hybrid-selected translation coupled with primer extension experiments suggest that a single chicken beta A3/A1 mRNA synthesizes two polypeptides, beta A3 (25 kDa) and beta A1 (23 kDa) by utilization of different translation initiation sites.