Biochemical properties and physiological responses of an Arabidopsis Ca²⁺-binding protein PCaP2, which is associated with plasma membrane and interacts with calmodulin and phosphatidylinositol phosphates

Abstract

Recently, we found novel calcium-binding proteins in Arabidopsis thaliana. One of these proteins, AtPCaP2 consists of 168 amino acid residues and contains N-myristoylation signal. To estimate its physiological role we characterized molecular properties of AtPCaP2. When PCaP2::GFP protein was expressed in Arabidopsis cells, green fluorescence was clearly observed in the plasma membrane. But fluorescence of PCaP2^G2A::GFP whose Gly-2 was replaced with Ala was observed in the cytosol. The results indicate that PCaP2 binds to the plasma membrane via myristoylation at Gly-2. Promoter-GUS analysis showed the expression in root epidermal cells, root hairs and pollens. And we found PCaP2 has the binding capacity to Ca²⁺ and specific phosphatidylinositol phosphates (PtdInsPs). Furthermore, calmodulin was associated with PCaP2 in a Ca²⁺-dependent manner, and its association weakened the interaction of PCaP2 with PtdInsPs. Expression of PCaP2 was significantly enhanced by NaCl and KCl. We propose that PCaP2 may be a molecular switch or break of PtdInsPs-mediated intracellular signaling in root hairs and/or pollen tubes.