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.
Highlights
The plastids constitute a diverse array of organelles, which play central roles in plant growth, development, and defense by providing a remarkable range of metabolic and physiological capabilities in different cell and tissue types (Lopez-Juez and Pyke, 2005; Rolland et al, 2012; Jarvis and Lopez-Juez, 2013)
Our analyses reveal that the targeting of the outer envelope membrane of chloroplasts (TOC) components involves elements common to the targeting of other outer membrane proteins, and include unique features that appear to have evolved to facilitate assembly of the import apparatus
All Toc159 family members have a unique tripartite structure, consisting of an N-terminal acidic domain (A-domain) and a central GTPase domain (G-domain), both of which are exposed to the cytosol; and a C-terminal membrane anchor domain (Mdomain) that is protected from proteolysis and associates with the chloroplast outer envelope membrane through an unknown mechanism (Hirsch et al, 1994; Bauer et al, 2000; Ivanova et al, 2004; Lung and Chuong, 2012)
Summary
The plastids constitute a diverse array of organelles, which play central roles in plant growth, development, and defense by providing a remarkable range of metabolic and physiological capabilities in different cell and tissue types (Lopez-Juez and Pyke, 2005; Rolland et al, 2012; Jarvis and Lopez-Juez, 2013). TOC components bind the N-terminal transit peptides of newly synthesized preproteins and function in coordination with a second complex at the inner envelope membrane, referred to as TIC (translocon at the inner membrane of chloroplasts), to provide direct transport of preproteins from the cytoplasm to the stroma (Li and Chiu, 2010; Jarvis and Lopez-Juez, 2013).
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