PCaPs, possible regulators of PtdInsP signals on plasma membrane

In plants, Ca 2+ , phosphatidylinositol phosphates (PtdInsPs) and inositol phosphates are major components of intracellular signaling. Several kinds of proteins and enzymes, such as calmodulin (CaM), protein kinase, protein phosphatase, and the Ca 2+ channel, mediate the signaling. Two new Ca 2+ -binding proteins were identified from Arabidopsis thaliana and named PCaP1 and PCaP2 [plasma membrane (PM)-associated Ca 2+ (cation)-binding protein 1 and 2]. PCaP1 has an intrinsically disordered region in the central and C-terminal parts. The PCaP1 gene is expressed in most tissues and the PCaP2 gene is expressed predominantly in root hairs and pollen tubes. We recently demonstrated that these proteins are N-myristoylated, stably anchored in the PM, and are bound with phosphatidylinositol phosphates, especially PtdInsP2s. Here we propose a model for the switching mechanism of Ca 2+ -signaling mediated by PtdInsPs. Ca 2+ forms a complex with CaM (Ca 2+ -CaM) when there is an increase in the cytosol free Ca 2+ . The binding of PCaPs with Ca 2+ -CaM causes PCaPs to release PtdInsPs. Until the release of PtdInsPs, the signaling is kept in the resting state.