CHARACTERIZATION OF GAMMA-CRYSTALLIN FROM A CATFISH - STRUCTURAL CHARACTERIZATION OF ONE MAJOR ISOFORM WITH HIGH METHIONINE BY CDNA SEQUENCING

gamma-Crystallin is the major and most abundant lens protein present i n the eye lens of most teleostean fishes. To facilitate structural cha racterization of gamma-crystallins isolated from the lens of the catfi shes (Clarias fuscus), a cDNA mixture was synthesized from the poly(A) (+)mRNA isolated from fresh eye lenses, and amplification by polymeras e chain reaction (PCR) was adopted to obtain cDNAs encoding various ga mma-crystallins. Plasmids of transformed E. coil strain JM109 containi ng amplified gamma-crystallin cDNAs were purified and prepared for nuc leotide sequencing by the dideoxynucleotide chain-termination method. Sequencing more than five clones containing DNA inserts of 0.52 kb rev ealed the presence of one major isoform with a complete reading frame of 534 base pairs, covering a gamma-crystallin (gamma M(1)) with a ded uced protein sequence of 177 amino acids excluding the initiating meth ionine. It was of interest to find that this crystallin of pI 9.1 cont ains a high-methionine content of 15.3 % in contrast to those gamma-cr ystallins of low-methionine content from most mammalian lenses. Sequen ce comparisons of catfish gamma M(1)-crystallin with those published s equences of gamma-crystallins from carp, bovine and mouse lenses indic ate that there is approx. an 82 % sequence homology between the catfis h 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 gamma-crystallins, i.e . one with a high-methionine content from teleostean fishes and the ot her with a low-methionine content from mammalian species, reflect a di stinct variance in the polarity distributions of surface amino acids i n these crystallins. The extensive molecular characterization of vario us gamma-crystallins from different species of the evolutionarily lowe r vertebrates such as the catfish may provide some insight into the me chanism underlying the evolution of the multigene gamma-crystallin fam ily.