title: Targeting and assembly of components of the TOC protein import complex at the chloroplast outer envelope membrane.
abstract: 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.
authors: Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst MA, USA.
authors: Lynn G L
authors: LG
authors: Richardson
authors: Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst MA, USA.
authors: Yamuna D
authors: YD
authors: Paila
authors: Department of Biology, Wilfrid Laurier University, Waterloo ON, Canada.
authors: Steven R
authors: SR
authors: Siman
authors: Department of Biology, Wilfrid Laurier University, Waterloo ON, Canada.
authors: Yi
authors: Y
authors: Chen
authors: Department of Biology, Wilfrid Laurier University, Waterloo ON, Canada.
authors: Matthew D
authors: MD
authors: Smith
authors: Department of Biochemistry and Molecular Biology, University of Massachusetts, Amherst MA, USA.
authors: Danny J
authors: DJ
authors: Schnell
keywords: TOC assembly; chloroplast; outer envelope membrane; protein import; protein targeting; translocon