The protein sequence homology of γ-crystallins among major vertebrate classes and their DNA sequence homology to heat-shock protein genes

Abstract

A systematic characterization of lens crystallins from five major classes of vertebrates was carried out by exclusion gel filtration, cation-exchange chromatography and N-terminal sequence determination. All crystallin fractions except that of γ-crystallin were found to be N-terminally blocked. γ-Crystallin is present in major classes of vertebrates except the bird, showing none, or decreased amounts, of this protein in chicken and duck lenses, respectively. N-Terminal sequence analysis of the purified γ-crystallin polypeptides showed extensive homology between different classes of vertebrates, supporting the close relatedness of this family of crystallin even from the evolutionarily distant species. Comparison of nucleotide sequences and their predicted amino acid sequences between γ-crystallins of carp and rat lenses and heat-shock proteins demonstrated partial sequence homology of the encoded polypeptides and striking homology at the gene level. The unexpected strong homology of complementary DNA (cDNA) lies in the regions coding for 40 N-terminal residues of carp γ-II, rat γ2-1, and the middle segments of 23,000- and 70,000-M _r heat-shock proteins. The optimal alignment of DNA sequences along these two segments shows about 50% homology. The percentage of protein sequence identity for the corresponding aligned segments is only 20%. The weak sequence homology at the protein level is also found between the invertebrate squid crystallin and rat γ-crystallin polypeptides. These results pointed to the possibility of unifying three major classes of vertebrate crystallins into one α/β/γ superfamily and corroborated the previous supposition that the existing crystallins in the animal kingdom are probably mutually interrelated, sharing a common ancestry.