Characterization of γ-crystallin from a catfish: Structural characterization of one major isoform with high methionine by cDNA sequencing

γ-Crystallin is the major and most abundant lens protein present in the eye lens of most teleostean fishes. To facilitate structural characterization of γ-crystallins isolated from the lens of the catfishes (Clarias fuscus), a cDNA mixture was synthesized from the poly(A)⁺mRNA isolated from fresh eye lenses, and amplification by polymerase chain reaction (PCR) was adopted to obtain cDNAs encoding various γ-crystallins. Plasmids of transformed E. coli strain JM109 containing amplified γ-crystallin cDNAs were purified and prepared for nucleotide sequencing by the dideoxynucleotide chain-termination method. Sequencing more than five clones containing DNA inserts of 0.52 kb revealed the presence of one major isoform with a complete reading frame of 534 base pairs, covering a γ-crystallin (γM₁) with a deduced protein sequence of 177 amino acids excluding the initiating methionine. It was of interest to find that this crystallin of pI 9.1 contains a high-methionine content of 15.3% in contrast to those γ-crystallins of low-methionine content from most mammalian lenses. Sequence comparisons of catfish γM₁-crystallin with those published sequences of γ-crystallins from carp, bovine and mouse lenses indicate that there is approx. an 82% sequence homology between the catfish and the carp species of piscine class whereas only 51-58% homology is found between mammals and the catfish. Moreover the differences in the hydropathy profiles for these two groups of γ-crystallins, i.e. one with a high-methionine content from teleostean fishes and the other with a low-methionine content from mammalian species, reflect a distinct variance in the polarity distributions of surface amino acids in these crystallins. The extensive molecular characterization of various γ-crystallins from different species of the evolutionarily lower vertebrates such as the catfish may provide some insight into the mechanism underlying the evolution of the multigene γ-crystallin family.