Characterization of γS-Crystallin Isoforms from Lip Shark (Chiloscyllium colax): Evolutionary Comparison between γS and β/γ Crystallins

Abstract γS-Crystallin from shark eye lenses, formerly termed βs crystallin in mammalian lenses, is structurally characterized in this study by cDNA cloning and sequencing. To facilitate sequence characterization of γS-crystallin possessing intermediate structural properties between β- and γ-crystallins, cDNA mixture was constructed from the poly(A) + mRNA isolated from shark eye lenses, and amplification by polymerase chain reaction (PCR) was carried out to obtain nucleotide segments encoding multiple shark γS-crystallins. Sequencing several positive clones revealed that a multiplicity of isoforms exists in the γS-crystallin class of this cartilaginous fish, similar to authentic γ-crystallin family characterized from the same shark species. Comparison of protein sequences encoded by two representative shark γS1 and γS2 cDNAs with those published sequences of β-, γ-, and γS crystallins from bovine, human, bullfrog and carp lenses indicated that there is about 35–64% sequence homology between shark γS crystallins and structurally related crystallins from different evolutionary classes, with a higher sequence similarity between shark γS and mammalian γ-crystallins than that of shark γS and carp γS or bovine γS crystallins. A phylogenetic tree constructed on the basis of the sequence divergence among various β-, γ-, and γS crystallins corroborates the closer relatedness of shark γS to authentic γ-crystallin than to mammalian and teleostean γS crystallins. It further strengthens the supposition that ancestral precursors of γS-crystallins were present in the shark lens long before the appearance of present-day teleostean and mammalian γS-crystallins.