A 18 KDA CADMIUM INDUCIBLE PROTEIN COMPLEX - ITS ISOLATION AND CHARACTERIZATION FROM RICE (ORYZA-SATIVA L.) SEEDLINGS

The present study shows the pattern of cadmium absorption and distribu tion in growing seedlings of two Indian rice (Oryza sativa L.) cvs. Ra tna and Jaya. Cadmium was readily absorbed by roots of growing seedlin gs and its localisation was greater in roots than shoots. Low transloc ation of cadmium from roots to shoots was observed. Seedlings exposed for 20 days under 500 mu mol/L Cd2+ in the medium showed accumulation of Cd2+ to a level far beyond the ambient. Under 500 mu mol/L Cd2+ in the growth medium about 2.2 to 2.8 mu mols Cd2+ g dry wt. equivalent t o 245 to 311 ppm Cd2+ was observed in roots and 0.5 to 1.2 mu mol Cd2 g(-1) dry wt. equivalent to 56 to 134 ppm Cd2+ was noted in shoots in 20-day-grown seedlings. One Cd2+ binding protein complex with an appa rent molecular weight of 18 kDa was purified by DEAE-cellulose chromat ography, Sephadex G-200 gel filteration and fast protein liquid chroma tography (FPLC). This complex had 4 sulfhydryl groups per protein mole cule. The specific Cd2+ content of the complex was 3.7 mu mol mg(-1) p eptide. The results suggest that Cd2+ binds possibly with the help of 4-SH groups to the peptide in mercaptide bonds and that this complex, which has a comparatively higher molecular weight than conventional ph ytochelatins, helps in sequestration of excess Cd ions in rice plants.