Sequence and expression of chicken beta A2- and beta B3-crystallins

Crystallins are a diverse group of proteins that contribute to the transparency and refractive properties of the eye lens. Previously, the chicken orthologs of four out of the six known bovine beta-crystallin genes have been cloned and sequenced. In the present study, cDNAs corresponding to the chicken orthologs of beta A2- and beta B3-crystallin, the two previously unidentified chicken beta-crystallins, have been isolated. In addition, sequence analysis of three independent chicken beta B2-crystallin cDNAs yielded a deduced connecting peptide sequence which is considerably shorter than that reported previously. Thus, direct homologs of all of the known bovine beta-crystallins are expressed in the chicken lens. This demonstrates that the duplications giving rise to the known vertebrate beta-crystallins occurred over 300 million years ago. beta B2- and beta B3/A1-crystallin are the most highly conserved of the beta-crystallins suggesting that these genes may be important for other functions besides their refractive role in the lens. By Northern blot hybridization analysis, both beta A2- and beta B3-crystallin were shown to be lens-specific in the chicken embryo. The relative levels of beta A2-crystallin remained stable from five days of embryogenesis until adulthood, while the relative amounts of beta B3-crystallin increased until hatching and were appreciably lower in the adult lens. Approximately equal relative amounts of beta A2-crystallin mRNA were found in the lens epithelia and fibers of 5 day embryonic chicken embryos; by contrast, beta B3-crystallin mRNA was detected preferentially in the lens fibers. These data in combination with previous studies suggest that beta-crystallin genes are regulated independently from each other in the developing chicken lens. The elucidation of the primary structures for all seven chicken beta-crystallin polypeptides will facilitate future studies on the structure/function relationships responsible for lens transparency and on the molecular basis for beta-crystallin gene expression during development.