SDS-dependent proteases induced by ABA and its relation to Rubisco and Rubisco activase contents in rice leaves

Protease activities and its relation to the contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and Rubisco activase were investigated in detached leaves of rice (Oryza sativa L.) floated on the solutions containing abscisic acid (ABA) or benzyladenine (BA). Rubisco and Rubisco activase contents were decreased during the time course and the decreases were enhanced by ABA and suppressed by BA. The decrease in Rubisco activase was faster than that in Rubisco. SDS-dependent protease activities at 50-70 kDa (rice SDS-dependent protease: RSP) analyzed by the gelatin containing PAGE were significantly enhanced by ABA. RSPs were also increased in attached leaves during senescence. RSPs had the pH optimum of 5.5, suggesting that RSPs are vacuolar protease. Both decrease in Rubisco and Rubisco activase contents and increase in RSPs activities were suppressed by cycloheximide. These findings indicate that the activities of RSPs are well correlated with the decrease in these protein contents. Immunoblotting analysis showed that Rubisco in the leaf extracts was completely degraded by 5h at pH 5.5 with SDS where it was optimal condition for RSPs. However, the degradation of Rubisco did not proceed at pH 7.5 without SDS where it is near physiological condition for stromal proteins. Rubisco activase was degraded at similar rate under both conditions. These results suggest that RSPs can functions in a senescence related degradation system of chloroplast protein in rice leaves. Rubisco activase would be more susceptible to proteolysis than Rubisco under physiological condition and this could affect the contents of these proteins in leaves.