Taxon-specific zeta-crystallin in Japanese tree frog (Hyla japonica) lens

The present study demonstrated that the 38-kDa protein, instead of rho -cry stallin (36 kDa), is expressed taxon specifically in the lens of Japanese t ree frog (Hyla japonica). The 38-kDa protein was distinguished from rho -cr ystallin expressed in the lenses of bullfrog (Rana catesbeiana) and Europea n common frog (Rana temporaria) immunochemically. Although the N terminus o f the 38-kDa protein was blocked, the analyses of partial amino acid sequen ces showed that the protein was zeta -crystallin. Analysis of cDNA sequence encoding zeta -crystallin of the tree frog lens demonstrated that the dedu ced protein consisted of 329 amino acids including initial methionine and h aving 62.2 and 62.9% identity with zeta -crystallin of camel and guinea pig lenses, respectively. The molecular mass of the deduced structure was calc ulated to be 35,564 Da. zeta -Crystallin of the tree frog lens exhibited th e intrinsic enzymatic activity of quinone reductase (EC 1.6.99.2, NADPH:qui none oxidoreductase). The crystallin specifically catalyzed the reduction o f 9,10-phenanthrenequinone (K-m, 42 mum) using NADPH (K-m, 60 mum) as a cof actor. The enzymatic activity was inhibited by dicumarol, anti-coagulant dr ug, with IC50 of 4 mum. On gel filtration chromatography, the crystallin wa s recovered as 150-kDa molecular mass complex, indicating that the crystall in was homotetramer consisting of 38-kDa subunits. The crystallin gene was expressed specifically in the lens. These results show that taxon-specific crystallins such as zeta- and rho -crystallins maybe available for the bioc hemical discrimination of Hyla- and Rana groups among frogs.