SEQUENCE AND EXPRESSION OF CHICKEN BETA-A2-CRYSTALLIN AND BETA-B3-CRYSTALLIN

Crystallins are a diverse group of proteins that contribute to the tra nsparency and refractive properties of the eye lens. Previously, the c hicken orthologs of four out of the six known bovine beta-crystallin g enes have been cloned and sequenced. In the present study, cDNAs corre sponding to the chicken orthologs of beta A2- and beta B3-crystallin, the two previously unidentified chicken beta-crystallins, have been is olated. In addition, sequence analysis of three independent chicken be ta B2-crystallin cDNAs yielded a deduced connecting peptide sequence w hich is considerably shorter than that reported previously. Thus, dire ct homologs of all of the known bovine beta-crystallins are expressed in the chicken lens. This demonstrates that the duplications giving ri se to the known vertebrate beta-crystallins occurred over 300 million years ago. beta B2- and beta B3/A1-crystallin are the most highly cons erved of the beta-crystallins suggesting that these genes may be impor tant for other functions besides their refractive role in the lens. By Northern blot hybridization analysis, both beta A2- and beta B3-cryst allin were shown to be lens-specific in the chicken embryo. The relati ve levels of beta A2-crystallin remained stable from five days of embr yogenesis until adulthood, while the relative amounts of beta B3-cryst allin 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 chicke n embryos; by contrast, beta B3-crystallin mRNA was detected preferent ially in the lens fibers. These data in combination with previous stud ies suggest that beta-crystallin genes are regulated independently fro m each other in the developing chicken lens. The elucidation of the pr imary structures for all seven chicken beta-crystallin polypeptides wi ll facilitate future studies on the structure/function relationships r esponsible for lens transparency and on the molecular basis for beta-c rystallin gene expression during development. (C) 1996 Academic Press Limited