The role of GTP regulation in chloroplast import
Plastids (e.g. chloroplasts) are a diverse group of interrelated organelles that have evolved to perform numerous and diverse metabolic processes within all plant cells, including essential roles in photosynthesis, amino acid and lipid synthesis, and cell signaling. The biogenesis of these organelles and their differentiation into specialized forms in different tissues relies on the import of thousands of nucleus-encoded proteins from the cytoplasm. Protein import is mediated by a group of multimeric membrane protein complexes, designated translocons, which reside within the outer and inner envelope membranes of the organelles. The major pathway for protein import is mediated by interactions between the transit peptide of newly synthesized proteins and receptor components of the translocon at the outer envelope of chloroplasts (Toc). Two Toc GTPases, Toc159 and Toc34, form an integrated transit peptide receptor system. They associate with Toc75, a major component of the Toc membrane channel. The GTPase activity of the two receptors regulates the import process, but the mechanism of this control point is not understood. I have investigated the role of the Toc159 GTPase in chloroplast import by examining the effects of Toc159 and Toc34 GTPase mutants in vivo and in vitro. My studies identify a new import intermediate and indicate that GTP hydrolyses at the receptors do not appear to be essential for protein import, but likely plays regulatory role at the early stages of import.