Figure 7.

Two independent channels at the Tic translocon. Hypothetical model of Tic20 and Tic110 channels in the inner envelope (IE) of chloroplasts. After passing the Toc complex in the outer envelope (OE), preproteins are imported either via Tic110 (red) or Tic20 (blue) through the IE. Hypothetical precursors that might use both channel proteins are depicted in yellow. Tic110 is thought to form a homodimer with a total of eight amphipathic transmembrane helices forming the translocation channel and four hydrophobic α-helices involved in the insertion into the membrane (according to [8] and [11]). The proposed Tic20 channel is depicted as a homo-oligomer (only three molecules are shown, however, it is > 700 kDa) and, based on the low overall abundance, might be responsible for the import of a subset of preproteins, indicated by a smaller number of depicted precursors waiting at the entrance of the channel.