The ABA-induced MA12 cDNA from maize, which encodes a set of highly phosphorylated embryo proteins, was used to isolate the corresponding genomic clone. This gene, called RAB-17 (responsive to ABA), encodes a basic, glycine-rich protein (mol. wt. 17,164) containing a cluster of 8 serine residues, seven of them contiguous. It is a homologue of the rice RAB-21 gene (Mundy J, Chua NH, EMBO J 7; 2279-2286, 1988). Phosphoamino acid analysis of the isolated protein indicates that only the serine residues are phosphorylated and a putative casein-type kinase phosphorylatable sequence was identified in the protein. The pattern of expression and in vivo phosphorylation of the RAB-17 protein was studied during maize embryo germination and in calli of both meristematic or embryonic origin. ABA treatment induced the synthesis of RAB-17 mRNA and protein in calli, however, the RAB-17 proteins were found to be highly phosphorylated only in embryos.

The maize abscisic acid (ABA)-responsive rabl7 mRNA and Rabl7 protein distribution in maize embryo tissues was investigated by in situ hybridization and immunocytochemistry. rabl7 mRNA and Rabl7 protein were found in all cells of embryo tissues. Synthesis of rabl7 mRNA occurred initially in the embryo axis. As maturation progressed, rabl7 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 Rabl7 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 Rabl7 and a nuclear localization signal (NLS) binding phosphoprotein, Noppl40, interaction of Rabl7 with NLS peptides was studied. We found specific binding of Rabl7 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 Rabl7 was found to be dependent upon phosphorylation. These results suggest that Rabl7 may play a role in nuclear protein transport.

The maize abscisic acid responsive protein Rab17 is a highly phosphorylated late embryogenesis abundant protein involved in plant responses to stress. In this study, we provide evidence of the importance of Rab17 phosphorylation by protein kinase CK2 in growth-related processes under stress conditions. We show the specific interaction of Rab17 with the CK2 regulatory subunits CK2␤-1 and CK2␤-3, and that these interactions do not depend on the phosphorylation state of Rab17. Live-cell fluorescence imaging of both CK2 and Rab17 indicates that the intracellular dynamics of Rab17 are regulated by CK2 phosphorylation. We found both CK2␤ subunits and Rab17 distributed over the cytoplasm and nucleus. By contrast, catalytic CK2␣ subunits and a Rab17 mutant protein (mRab17) that is not a substrate for CK2 phosphorylation remain accumulated in the nucleoli. A dual-color image shows that the CK2 holoenzyme accumulates mainly in the nucleus. The importance of Rab17 phosphorylation in vivo was assessed in transgenic plants. The overexpression of Rab17, but not mRab17, arrests the process of seed germination under osmotic stress conditions. Thus, the role of Rab17 in growth processes is mediated through its phosphorylation by protein kinase CK2. T he plant hormone abscisic acid (ABA) plays a major role in adaptation to osmotic stress and induces a number of genes that encode proteins generally assumed to be involved in protecting the cell and promoting recovery from stress. Proteins responsive to ABA accumulate during seed maturation; they naturally disappear during seed germination and can be induced to reappear by ABA treatment or osmotic stress in vegetative tissues (1, 2).Late embryogenesis abundant proteins (Lea) from group 2, responsive to ABA (Rab), or dehydrins are among the most common plant proteins involved in adaptation to water or osmotic stress. Several hypothetical roles have been proposed for Rab͞dehydrin proteins based on different experimental evidence, including binding to phosphate or sulfate ions (3), nuclear localization signal (NLS) peptides (4), calcium (5), and lipid vesicles containing acidic phospholipids (6), among others. All are aimed toward a protective role as chaperones to stabilize molecules or structures under stress conditions (4,7,8).Many proteins included in this Lea family contain the S domain (8), consisting of a tract of serines with several phosphorylation sites (9). Maize Rab17 protein is one of the most heavily phosphorylated proteins in mature embryos and is found both in nucleus and cytoplasm (4). The S domain of Rab17 is followed by a protein kinase CK2 phosphorylation consensus site (9). We previously established that Rab17 is phosphorylated by CK2 in serine residues of the S domain (10). Phosphorylation͞ dephosphorylation is an important mechanism that may regulate the function of Rab17 in the cell; however, the actual physiological function for Rab͞dehydrin proteins is still unknown, and a precise understanding of the function of Rab17 and the importance of its phosphorylation...