Rubisco activase chaperone activity is regulated by a post-translational mechanism in maize leaves

Rubisco activase (RCA) is a molecular chaperone present in maize as 43 kDa and 41 kDa polypeptides. They are encoded by two different genes comprising an identical ORF that corresponds to the 43 kDa RCA polypeptide, and their transcripts do not show putative splicing sites. To determine the origin of the 41 kDa polypeptide, leaf poly A(+) mRNA was in vitro translated. Results demonstrated de novo synthesis only for the 43 kDa RCA. Antibodies developed against peptides from either the carboxy- or the amino-terminal end of 43 kDa RCA showed by western blot that the 43 kDa polypeptide amino-terminal region is missing in the 41 kDa polypeptide, whereas both RCA polypeptides shared the carboxy-end region. Regulation of RCA polypeptide ratios was determined in plant leaves at different developmental stages and under stressing environmental conditions. Increased levels of 43/41 kDa RCA ratio were found in leaves under low light exposure, whereas this ratio declined under water stress. Measurements of chaperone activity either on each RCA polypeptide alone or in a mixture showed the functional relevance of different 43/41 kDa RCA polypeptide ratios. Greater chaperone activity was found for the 41 kDa than for the 43 kDa polypeptide. Taken together, these results indicate that 41 kDa RCA polypeptide formation is regulated by limited proteolysis of the 43 kDa RCA at its amino-terminal region. This pathway is sensitive to developmental and environmental signals, and seems to play a relevant function during plant stress.