THE MAIZE ABSCISIC ACID-RESPONSIVE PROTEIN RAB17 IS LOCATED IN THE NUCLEUS AND INTERACTS WITH NUCLEAR-LOCALIZATION SIGNALS

The maize abscisic acid (ABA)-responsive rab17 mRNA and Rab17 protein distribution in maize embryo tissues was investigated by in situ hybridization and immunocytochemistry. rab17 mRNA and Rab17 protein were found in all cells of embryo tissues. Synthesis of rab17 mRNA occurred initially in the embryo axis. As maturation progressed, rab17 mRNA was detectable in the scutellum and accumulated in axis cells and provascular tissues. However, the response to exogenous ABA differed in various embryo cell types. The Rab17 protein was located in the nucleus and in the cytoplasm, and qualitative differences in the phosphorylation states of the protein were found between the two subcellular compartments. Based on the similar domain arrangements of Rab17 and a nuclear localization signal (NLS) binding phosphoprotein, Nopp140, interaction of Rab17 with NLS peptides was studied. We found specific binding of Rab17 to the wild-type NLS of the SV40 T antigen but not to an import incompetent mutant peptide. Moreover, binding of the NLS peptide to Rab17 was found to be dependent upon phosphorylation. These results suggest that Rab17 may play a role in nuclear protein transport.