Sequence-Analysis of Frog ρ-Crystallin by cDNA Cloning and Sequencing: A Member of the Aldo-Keto Reductase Family

Abstract

ρ-Crystallin is a major enzyme crystallin present in the lenses of amphibian species with a blocked amino terminus. In order to facilitate the determination of the primary sequence of this taxon-specific crystallin, cDNA mixture was synthesized from the poly(A) +mRNA of bullfrog eye lenses. cDNAs encoding ρ-crystallin were then amplified by polymerase chain reaction (PCR) using a new protocol of Rapid Amplification of cDNA Ends (RACE). PCR-amplified product corresponding to ρ-crystallin was obtained, which was then subcloned into pUC18 vector and then transformed into E. coli strain JM109. Plasmids purified from the positive clones were prepared for nucleotide sequencing by the automatic fluorescence-based dideoxynucleotide chain-termination method. Sequencing more than 15 clones containing DNA inserts coding for ρ-crystallin constructed only one unique and complete full-length reading frame of 975 base pairs covering a deduced protein sequence of 324 amino acids including the universal initiating methionine. It shows 96, 59, 46 and 37 percent sequence similarity to another ρ-crystallin from European common frog, bovine prostaglandin-F synthase, human aldose reductase and human aldehyde reductase, respectively, revealing the close relationship between ρ-crystallins from related amphibian species and its possible evolutionary relatedness with various aldo-keto reductases. In this study a phylogenetic tree for ρ-crystallin and related enzymes is constructed based on multiple-sequence alignment program using a combination of distance matrix and approximate parsimony methods. We have thus established the remote phylogenetic relationship between ρ-crystallin and some aldehyde/aldose reductases, which may provide a possible link for the recruitment of this crystallin from detoxification-related enzymes and its physiological role in maintaining a transparent and clear lens.