Figure 2: The Chloroplast Protein Import Apparatus

The TOC-TIC machinery responsible for canonical, transit peptide-mediated protein import is depicted. Most nucleus-encoded chloroplast proteins are imported via the TOC and TIC multiprotein translocons located in the outer envelope membrane (OEM) and inner envelope membrane (IEM), respectively. The core TOC complex comprises two transit peptide receptors, Toc159 and Toc33/Toc34, and the channel-forming β-barrel protein Toc75. The receptors possess cytosolic GTPase domains, accounting for a GTP requirement during the early stages of import; Toc159 additionally possesses an N-terminal acidic (A) domain that may contribute to transit peptide recognition specificity. Different models exist for the TIC translocon. In one model, a 1-MDa TIC complex incorporating plastome-encoded Tic214 (or Ycf1) and five nucleus-encoded subunits (Tic100, Tic56, Tic21, Tic20, and Tic12) has a central role. In other models, Tic110 and Tic40 play important roles, but researchers generally agree that Tic20 has a central, channel-forming function. The Tic110 and Tic40 proteins are proposed to recruit and regulate a variety of stromal chaperones (cpHsp70, Hsp90C, and Hsp93/ClpC), which drive protein import and/or assist in protein folding at the expense of ATP hydrolysis. Alternatively, a 2-MDa motor complex, comprising plastome-encoded Ycf2, five FtsH-like ATPase subunits, and a stabilizing pdNAD-MDH subunit (indicated with an asterisk), is proposed to act as the import motor. This import motor may be required to deliver the import of recalcitrant or tightly-folded preproteins. Abbreviations: FtsH, filamentation temperature-sensitive H; FtsHi, FtsH-inactive; Hsp, heat shock protein; IMS, intermembrane space; pdNAD-MDH, plastidic NAD-dependent malate dehydrogenase subunit; SPP, stromal processing peptidase; Ycf2, hypothetical chloroplast open reading frame 2. Taken from Annu. Rev. Plant Biol. 74: 259-283.