Characterization of γ-Crystallins from a Hybrid Teleostean Fish: Multiplicity of Isoforms as Revealed by cDNA Sequence Analysis

Abstract

γ-Crystallins were isolated and characterized from the eye lenses of a hybrid species belonging to the teleostean fish. Isoelectric focusing of γ-crystallin fraction obtained from gel-permeation chromatography revealed that it consists of multiple charge isomers of a protein species with a molecular mass of about 20 kDa. To facilitate the cloning of γ-crystallin gene, cDNA was constructed from the poly(A) +mRNA isolated from fresh lenses, and amplification by polymerase chain reaction (PCR) was carried out to obtain cDNA encoding multiple γ-crystallins. Sequencing five of more than 10 positive clones revealed that a multiplicity of isoforms exists in the γ-crystallin class of teleostean lenses. Comparison of protein sequences encoded by these multiple cDNAs with those published sequences of γ-crystallins from bovine, mouse and carp lenses indicated that there is about 70-80 % sequence homology between different species of piscine species whereas only 50-60 % is found between mammals and fishes. Structural analysis of these γ-crystallins with high methionine contents (11-16 %) suggests that there are two major subclasses of piscine γ-crystallins, i.e. γM 1 and γM 2, existed long before the appearance of mammalian γ-crystallin with low methionines.