An Arabidopsis Hydrophilic Ca2-Binding Protein with a PEVK-Rich Domain, PCaP2, is Associated with the Plasma Membrane and Interacts with Calmodulin and Phosphatidylinositol Phosphates

We found a new hydrophilic protein in Arabidopsis thaliana. Real-time PCR demonstrated that the protein was expressed in roots. Histochemical analysis of promoter-glucuronidase fusions demonstrated its extensive expression in root hairs. The protein is rich in proline, glutamate, valine and lysine residues (PEVK-rich domain), and bound Ca-2 even in the presence of Mg-2 and K when examined by the Ca-45 overlay assay. Treatment of seedlings with K, Mn-2, Zn-2, Na, ABA and gibberellic acid, and cold and drought stresses enhanced the transcription. Expression of the protein linked to green fluorescent protein in A. thaliana showed its plasma membrane localization and cell-specific expression in the epidermal cells including root hairs and the elongating pollen tubes. Therefore, we named the protein PCaP2 (plasma membrane-associated Ca-2-binding protein-2). The substitution of glycine at position 2 with alanine resulted in cytoplasmic localization of PCaP2. These results and the N-terminal characteristic motif suggest that PCaP2 is N-myristoylated at Gly2. We examined the capacity for binding to phosphatidylinositol phosphates (PtdInsPs), and found that PCaP2 interacts strongly with PtdIns(3,5)P-2, PtdIns(4,5)P-2 and PtdIns(3,4,5)P-3, and weakly with PtdIns(3,4)P-2. Furthermore, calmodulin was associated with PCaP2 in a Ca-2-dependent manner, and its association weakened the interaction of PCaP2 with PtdInsPs. These results indicate that PCaP2 is involved in intracellular signaling through interaction with PtdInsPs and calmodulin in growing root hairs. PCaP2 was previously reported as microtubule-associated protein-18. We discuss the physiological roles of PCaP2 in relation to microtubules in cells.