Structural considerations of folded protein import through the chloroplast TOC/TIC translocons

Abstract

Protein import into chloroplasts is carried out by the protein translocons at the outer and inner envelope membranes (TOC and TIC). Detailed structures for these translocons are lacking, with only a low‐resolution TOC complex structure available. Recently, we showed that the TOC/TIC translocons can import folded proteins, a rather unique feat for a coupled double membrane system. We also determined the maximum functional TOC/TIC pore size to be 30–35 Å. Here, we discuss how such large pores could form and compare the structural dynamics of the pore‐forming Toc75 subunit to its bacterial/mitochondrial Omp85 family homologs. We put forward structural models that can be empirically tested and also briefly review the pore dynamics of other protein translocons with known structures.

The Plant Mitochondrial TAT Pathway Is Essential for Complex III Biogenesis

“…This could suggest that hydrophobicity is not the only constraint. However, comparisons between mitochondria and chloroplast import should be treated carefully, as has been shown in a recent work on the size of the TOC complex [40,41], indicating that mitochondria and chloroplasts may not be placed under the same constraints regarding the proteins they can import. The most obvious and simplest reason for changing from a TAT pathway to a BCS1 pathway is that BCS1 can be nuclear encoded and therefore is under nuclear control.…”

Protein import into chloroplasts and its regulation by the ubiquitin-proteasome system

“…The Toc75 channel has been found to reach a maximum diameter of 30 Å [54]. The stoichiometry of the Toc33 : Toc75 : Toc159 core complex has been determined to be in the range of 4 : 4 : 1 [55] to 3 : 3 : 1 [56], and so the channel may be formed by multiple copies of the Toc75 protein [57].…”

“…It is expected that the translocon channel is formed by Tic20, which has four membrane-spanning α-helical domains [71]. Three copies of Tic20 could theoretically exist within the 1 MDa complex [70], so a pore size of up to 30 Å has been predicted [57]. Tic21 has a similar structure to Tic20 and may function in a complementary way [72].…”

“…The situation was further complicated by the recent identification of a 2 MDa Ycf2-FtsHi protein complex that was proposed to act as an import motor associated with the 1 MDa TIC complex [86] (Figure 1). In view of the evidence supporting 30 Å pore diameters for the import channels [57], it was suggested that such a powerful ATPase might be specifically recruited to handle the import of recalcitrant or tightly-folded proteins [87]. It seems plausible that cpHsp70 is the general import motor [77][78][79], with the energetically more expensive Ycf2-FtsHi complex being utilized only under specific conditions.…”

“…Like SP1, SP2 physically interacts with TOC proteins, and it is hypothesized to form the retrotranslocon channel [114] (Figure 1). The entry of substrates into the channel may occur by lateral gating in the membrane, as with other Omp85 proteins [57].…”