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Figure 2


[import apparatus]


Figure 2: The Chloroplast Protein Import Apparatus

Schematic illustration of the components implicated in the translocation of preproteins across the chloroplast envelope membranes. The outer envelope membrane (OM) components form the TOC complex, and the inner envelope membrane (IM) components form the TIC complex. Individual components are identified by their predicted molecular weights (black text), and some key functional domains are indicated (white text). The core TOC complex is formed by Toc159, Toc34 and Toc75. The former two proteins control preprotein recognition, while Toc75 forms the outer envelope channel. Cytosolic 14-3-3, Hsp70 and Hsp90 proteins are proposed to form ‘guidance complexes’ that direct preproteins to the TOC apparatus; these may dock at either Toc34 or the peripheral component, Toc64/OEP64, as indicated. Alternatively, protein import may be initiated in the absence of such a guidance complex. It was suggested that Toc12, Hsp70 and Tic22 act to facilitate the passage of preproteins across the intermembrane space, but recent evidence suggests that Toc12 and Hsp70 do not exist in this compartment. The inner envelope translocation channel may be formed by Tic110, Tic20 and/or Tic21. The Tic110 protein is also believed to coordinate late events in the process by recruiting stromal molecular chaperones to import sites; in particular, Tic110 has been proposed to collaborate with Tic40 and Hsp93 in a putative stromal import motor complex. Upon arrival in the interior, the transit peptide is cleaved by SPP, and other chaperones (e.g., Cpn60 or Hsp70) may assist in the folding or onward transport of the mature domain. Finally, the Tic62, Tic55 and Tic32 components may (in conjunction with FNR or calmodulin [CaM]) enable the regulation of import in response to redox signals; these components might only be recruited to import sites under certain conditions, or during the passage certain preproteins. Taken from New Phytol., 2008, 179: 257-285.

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 Last updated: Jan 2014
 Paul Jarvis