tailieunhanh - Báo cáo khoa học: Protein transport in organelles: The Toc complex way of preprotein import

Most of the estimated 1000 or so chloroplast proteins are synthesized as cytosolic preproteins with N-terminal cleavable targeting sequences (transit peptide). Translocon complexes at the outer (Toc) and inner chloroplast envelope membrane (Tic) concertedly facilitate post-translational import of preproteins into the chloroplast. | MINIREVIEW Protein transport in organelles The Toc complex way of preprotein import Birgit Agne and Felix Kessler Laboratoire de Physiologie Végétale Universite de Neuchatel Switzerland Keywords chloroplast outer membrane preprotein translocon Correspondence F. Kessler Laboratoire de Physiologie Vẽgẽtale Universite de Neuchatel Rue Emile-Argand 11 CH-2009 Neuchatel Switzerland Fax 41 32 718 22 71 Tel 41 32 718 22 92 E-mail Received 22 July 2008 revised 5 December 2008 accepted 23 December 2008 Most of the estimated 1000 or so chloroplast proteins are synthesized as cytosolic preproteins with N-terminal cleavable targeting sequences transit peptide . Translocon complexes at the outer Toc and inner chloroplast envelope membrane Tic concertedly facilitate post-translational import of preproteins into the chloroplast. Three components the Toc34 and Toc159 GTPases together with the Toc75 channel form the core of the Toc complex. The two GTPases act as GTP-dependent receptors at the chloroplast surface and promote insertion of the preprotein across the Toc75 channel. Additional factors guide preproteins to the Toc complex or support their stable ATP-dependent binding to the chloroplast. This minireview describes the components of the Toc complex and their function during the initial steps of preprotein translocation across the chloroplast envelope. doi The key players The first plastid import studies were performed with isolated chloroplasts from pea Pisum sativum . Initially the energetics of preprotein translocation were addressed and three major steps were identified 1 2 a reversible binding to the surface of the outer chloroplast membrane in the absence of added nucleotides b stable binding of preproteins to the outer chloroplast membrane in the presence of 100 pM ATP subsequently an additional requirement for GTP was demonstrated and c translocation into the chloroplast stroma requiring the presence of at least