Characterization of Chloroplast Inner Envelope Membrane Protein,CIA5(Chloroplast-Import-Apparatus)

Abstract

Most chloroplast proteins are encoded by the nucleus, synthesized in the cytosol and imported into chloroplasts. These nucleus-encoded proteins are synthesized as precousors with N-terminal transit peptides, and are transported into chloroplasts by a set of translocon complex located in the chloroplast outer and inner envelope membranes. Translocon components are termed Toc (translocons at the outer envelope membrane of chloroplasts) and Tic (translocons at the inner envelope membrane of chloroplasts) proteins. We have isolated an Arabidopsis mutant, cia5 (chloroplast import apparatus 5), which is defective in chloroplast protein import. CIA5 is predicted to be a membrane premease-like protein, and is extremely conserved in many plant species. Here I show that CIA5 is a chloroplast inner envelope membrane protein containing four transmembrane domains and both its N and C termini face the stroma. I also found that loss of CIA5 resulted in defects in chloroplast protein import, and in reduced amount of inner membrane translocon proteins. Coimmunoprecipiptation studies suggest that CIA5 is a new Tic component.