A novel, hydrophilic cation-binding protein PCaP1 is stably associated with plasma membrane and involved in stomatal opening in Arabidopsis

Abstract

A novel protein PCaP1 (plasma membrane associated cation-binding protein-1) was found in Arabidopsis thaliana. PCaP1 is localized in the plasma membrane via N-myristoylation and binds Ca²⁺, Cu²⁺, PtdInsPs, and CaM/Ca²⁺ (Nagasaki et al. 2008a, b). A loss-of-function mutant pcap1 becomes sensitive to excessive Cu²⁺ and pathogens. In this study, we investigated intracellular localization and dynamics of PCaP1 by expressing a PCaP1-GFP construct. Fluorescence of PCaP1-GFP was clearly detected on the plasma membrane at even intensity in almost all tissues. Under stress conditions, such as excess Cu²⁺ and an elicitor, the fluorescence intensity of PCaP1-GFP increased. In guard cells, PCaP1-GFP was detected in outside half part of the plasma membrane. When we measured stomatal apertures of the wild type and pcap1 leaves, the stomatal apertures of pcap1 were higher than that of wild type plants. Thus, we compared stomatal conductance and growth of them under dark and high CO₂ concentrations. From these results, we estimate that PCaP1 is involved in signal transduction through interaction with PtdInsPs and CaM/Ca²⁺ in various physiological phenomena including stomatal closing.