The role of Toc receptor interactions in controlling protein import into chloroplasts
Nuclear-encoded chloroplast proteins are synthesized as precursors in the cytosol with N-terminal cleavable transit peptides. The post translational import of proteins into chloroplasts occurs first through the outer membrane via the Toc complex (Translocon of Outer membrane of Chloroplast). The high fidelity of the protein import process is maintained by the specific recognition of the transit peptide of nucleus-encoded proteins by the coordinate activities of two homologous GTPase Toc receptors, Toc34 and Toc159. Structural and biochemical studies suggest that dimerization of the Toc receptors functions as a component of the mechanism to control access of preproteins to the membrane translocation channel of the translocon chloroplast envelope. I show that specific mutations that disrupt receptor dimerization in vitro reduce the rate of protein import in transgenic Arabidopsis compared to the wild type receptor. The mutations do not affect the GTPase activities of the receptors. Interestingly, these mutations do not disrupt initial preprotein binding at the receptors, but they reduce the efficiency of the transition from preprotein binding to membrane translocation. These data indicate that dimerization of receptors has a direct role in protein import, and support a hypothesis in which conformational changes that initiate membrane translocation of chloroplast preproteins is part of the molecular mechanism of GTP-regulated protein import.