Characterization of a membrane-associated phosphoprotein (pp47) in rice (Oryza sativa L.) seedlings treated by gibberellin

Protein kinase activity was found in the leaf extracts of rice (Oryza sativa L.) grown in the presence of exogenous GA1. When proteins from leaves of rice grown in the presence of exogenous GA1 were separated by two-dimensional polyacrylamide gel electrophoresis, a protein with molecular weight of 47 kDa and isoelectric point of 5.1 (pp47) was significantly phosphorylated. An antibody raised against this pp47, referred to as anti-pp47 antibody, could detect the pp47 from 10 days to 2 months of plant development. Furthermore, pp47 was found to be phosphorylated in vitro in the membrane fraction of rice leaf extracts. The double membranes of nuclei, microbody-membranes, and outer and inner membranes of mitochondria in mesophyll cells of rice cross-reacted with the anti-pp47 antibody as determined by immunoelectron microscopy. The presence of this pp47 was unrelated to rice culm length when the rice culm length mutant cultivars were used. However, the phosphorylation of 47 kDa protein in the tall cultivar was significant and that in short cultivars was faint or significantly less. These results suggest GA signal transduction and amplification to be related to protein phosphorylation through putative Ca2+-dependent protein kinase(s).