Characterization of γS-crystallin isoforms from a catfish: Evolutionary comparison of various γ-, γS-, and β-crystallins

γS-Crystallin from catfish eye lenses, formerly designated βS-crystallin in mammalian lenses, is structurally characterized in this study by cDNA cloning and sequencing. To facilitate sequence characterization of γS-crystallin with structural properties lying between β- and γ-crystallins, a cDNA mixture was constructed from the poly(A)⁺ mRNA isolated from catfish eye lenses, and amplification by polymerase chain reaction (PCR) was carried out to obtain nucleotide segments encoding multiple γS-crystallin isoforms. Sequencing several positive clones revealed that at least two distinct isoforms exist in the γS-crystallin class of this teleostean fish, similar to the authentic γ-crystallin family characterized previously in species of the piscine class. Comparison of protein sequences encoded by two representative catfish γS1 and γS2 cDNAs with the published sequences of β-, γ-, and γS-crystallins from shark, carp, bullfrog, bovine, and human lenses indicates that there is about 20-50% sequence homology between catfish γS-crystallins and various members of the related β/γ-crystallin superfamily from different evolutionary classes, with a higher sequence similarity being found between catfish γS- and mammalian γ-crystallins than between catfish γS- and bovine or carp γS-crystallins. Phylogenetic trees constructed on the basis of the nucleotide and protein sequence divergence among various β-, γ-, and γS-crystallins corroborate the closer relatedness of catfish γS- to authentic γ-crystallin than to bovine and carp γS-crystallins. The results suggest that evolution of catfish γS-crystallins follows a different path from that of bovine and carp γS-crystallins and may represent a more ancient offshoot from the ancestral γ/γS coding gene than carp and bovine γS-crystallins.