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.
Duncan
M K
MK
Laboratory of Molecular and Developmental Biology, National Eye Institute, National Institutes of Health, Bethesda, MD 20892-2730, USA.
Banerjee-Basu
S
S
McDermott
J B
JB
Piatigorsky
J
J
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D015152:Blotting, Northern; D015139:Blotting, Southern; D002642:Chick Embryo; D002645:Chickens / Q000235:genetics*; D003459:Crystallins / Q000737:chemistry / Q000235:genetics*; D018076:DNA, Complementary / Q000235:genetics; D019143:Evolution, Molecular; D015870:Gene Expression; D007908:Lens, Crystalline / Q000196:embryology / Q000254:growth & development; D008969:Molecular Sequence Data
D003459:Crystallins; D018076:DNA, Complementary