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

Abstract

A new type of protein, PCaP1 (plasma membrane associated cation-binding protein-1) that is rich in glutamate and lysine resides, was found in Arabidopsis thaliana. PCaP1 has no transmembrane domain and localizes in the plasma membrane via N-myristoylation. The protein 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.
We investigated intracellular localization and its dynamics of PCaP1 by expressing a PCaP1-GFP construct with its own promoter. Fluorescence of PCaP1-GFP was clearly detected in the plasma membrane at even intensity in all tissues except for root tips and pollens. Stresses, such as excess Cu²⁺ and pathogens, did not alter the plasma membrane localization of PCaP1. In guard cells, however, PCaP1 was detected in outside half part of the plasma membrane. The stomata of pcap1 lines did not close completely under the dark. We are testing sensitivity of pcap1 lines to drought stress. From the present and previous results, we estimate that PCaP1 is stably associated with plasma membrane and involved in signal transduction through interaction with PtdInsPs and CaM/Ca²⁺.