PCaPs, possible regulators of PtdInsP signals on plasma membrane

Abstract

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.

Keywords: calcium signal; calmodulin; inositol phosphate; intrinsically disordered protein; myristoylation; phosphatidylinositol phosphate; plasma membrane.

Figure 1

Morphological changes in mild overexpression lines of PCaP2. A PCaP2-GFP construct with its own promoter was introduced into the wild type of A. thaliana. Morphological changes were observed in root hairs of several mutant lines. Roots of 9-day-old seedlings were photographed. Branched root hairs, which are indicated by arrows, were observed in most mutant lines. Bar = 0.5 mm.

Figure 2

Schematic representation of the proposed role of PCaP1 and PCaP2 in the PM. During the resting state, PCaPs hold PtdlnsPs on the membrane surface. Increased cytosolic [Ca²⁺] leads formation of a Ca²⁺-CaM complex. Interaction with Ca²⁺CaM complex stimulates release of PtdlnsP₂ from PCaPs. the free PtdlnsP₂ interacts with particular ion channels and regulates its function (step 1). In another case, Ptdins(4,5)P₂ is hydrolyzed by PLC and a product InsP₃ enhances the release of ca²⁺ from ER and vacuole by activation of Ca²⁺ channels (InsP₃ receptors) (step 2). Remaining DAG activates PKC together with increased [Ca²⁺] (step 3). The activated PKC modifies target molecules (step 4).