A Force-Generating Machine in the Plant’s Powerhouse: A Pulling AAA-ATPase Motor Drives Protein Translocation into Chloroplasts

Reply: The Revised Model for Chloroplast Protein Import

“…Nevertheless, as discussed by Li et al (2020), it is true that there are distinct interpretations of data presented in the literature for their functions. Thus, it will be important to reexamine the roles of these stromal chaperones in chloroplast protein import, as highlighted previously by Herrmann (2018). Similarly, the stereotypical view of Tic110/Tic40 as central to chloroplast import has tended to preclude reconsideration of their direct roles in chloroplast biogenesis rather than in protein import.…”

“…This idea is not unprecedented, because a similar non-photosynthetictype TOC is well known and involves Toc75 as a core, but it contains a set of peripheral receptor components that is distinct from that of the "photosynthetictype" TOC (Nakai, 2015a(Nakai, , 2015b(Nakai, , 2018. Alternatively, it might be possible that, during evolution, grasses somehow gained an energetically more efficient protein import system involving mechanically coupled Hsp70-type molecular chaperones just like an extant mitochondrial protein import system (Herrmann, 2018); it remains as an intriguing open question.…”

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

“…From these resting states, expanded pores actively importing stably folded proteins could potentially arise in many configurations involving Toc159 and/or additional Toc75 subunits as illustrated. The expanded pores are depicted with the maximum pore diameter of 35 A to clearly illustrate pore expansion. Again, it should be cautioned that we do not expect the Toc75 barrel to readily open to this fully expanded state and the maximum pore size may only be 30…”

“…Other than the rather trivial fact that the TIC subunits are composed of a-helical transmembrane proteins, there is no structural information regarding the TIC complex pore. However, given the maximum pore size measured for TOC/TIC (30)(31)(32)(33)(34)(35) A), the average a-helix diameter (12 A), and assuming the pore-forming transmembrane helices (TMHs) are arranged in a regular polygonal shape in the membrane plane, we can at least estimate the number of helices required to produce such a large pore. Under these assumptions, there is a linear relationship between pore size and poreforming TMHs (Fig.…”

“…3, blue line). It would require at least 11-12 TMHs to generate a 30- 35 A pore. As a proof of concept, the pore diameters of relatively large a-helical pores of known structure are also plotted as a function of pore-forming helix number (Fig.…”