The translocon at the outer envelope membrane of chloroplasts (TOC) initiates the import of thousands of nuclear encoded preproteins required for chloroplast biogenesis and function. The multimeric TOC complex contains two GTP-regulated receptors, Toc34 and Toc159, which recognize the transit peptides of preproteins and initiate protein import through a β–barrel membrane channel, Toc75. Different isoforms of Toc34 and Toc159 assemble with Toc75 to form structurally and functionally diverse translocons, and the composition and levels of TOC translocons is required for the import of specific subsets of coordinately expressed proteins during plant growth and development. Consequently, the proper assembly of the TOC complexes is key to ensuring organelle homeostasis. This review will focus on our current knowledge of the targeting and assembly of TOC components to form functional translocons at the outer membrane. Our analyses reveal that the targeting of TOC components involves elements common to the targeting of other outer membrane proteins, but also include unique features that appear to have evolved to specifically facilitate assembly of the import apparatus.
Keywords: chloroplast, outer envelope membrane, protein targeting, translocon, TOC assembly, protein import
Citation: Richardson LGL, Paila YD, Siman SR, Chen Y, Smith MD and Schnell DJ (2014) Targeting and assembly of components of the TOC protein import complex at the chloroplast outer envelope membrane. Front. Plant Sci. 5:269. doi: 10.3389/fpls.2014.00269
Received: 31 March 2014; Accepted: 24 May 2014;
Published online: 11 June 2014.
Edited by:Kentaro Inoue, University of California at Davis, USA
Reviewed by:Inhwan Hwang, Pohang University of Science and Technology, South Korea
Copyright © 2014 Richardson, Paila, Siman, Chen, Smith and Schnell. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Danny J. Schnell, Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst, Life Sciences Laboratories, Room N431, 240 Thatcher Way, Amherst, MA 01003-9364, USA e-mail: firstname.lastname@example.org
†These authors have contributed equally to this work.