Plastid protein import through the TOC-TIC translocons

Almost all plastid proteins are made in the cytosol with an N-terminal transit peptide and enter the organelle through two translocons in series. At the outer envelope, the GTPase receptors Toc34 and Toc159 recognise the transit peptide and pass the preprotein to the Toc75 beta-barrel channel; at the inner envelope Tic236 tethers the TOC complex to the TIC complex, whose Tic20 channel and Tic110 receive it, and a stromal ATP-driven motor of the Hsp100 ClpC and the stromal Hsp70 (cpHsc70), with Hsp90C, draws it into the stroma, where the transit peptide is removed. This module is the land-plant core of that pathway, written so that it can be evaluated in a plant genome.

Embryophyta scope, with Arabidopsis representatives. The process-level PAINT nodes, for the Toc159-family receptors (PTN000856052) and Toc75 (PTN000981426), are placed at Embryophyta; only the Toc75 location node (PTN002292219) is at Viridiplantae. Green algae are excluded because Chlamydomonas TOC34 (A8HYJ1) falls in the Toc159-family subfamily SF135, so the two receptor steps would assign it wrongly. Read as a satisfiability rule, a genome needs a Toc34-type receptor, a Toc159-family receptor, a Toc75 channel, the Tic236 linker, the Tic20 and Tic110 inner-envelope subunits, the ClpC and cpHsc70 stromal motor and the stromal processing peptidase. Optional: the other 1-MDa TIC subunits (Tic214, Tic56, Tic100), Tic40, Hsp90C, FtsHi, and the Tic21, Tic22, Tic62, Tic32 and Tic55 proteins. Three selectors rest on PANTHER's own classification files rather than the local entries table (cpHsc70, Hsp90C, FtsHi), and two disagree with them (Tic100, Tic32), as does the existing ClpC selector. Selectors are subfamilies, because each parent family (IMAP GTPases, Omp85, Clp ATPases) is far wider than plastid import. The Toc75, ClpC and Tic214 selectors list members from several land plants (pea, rice, tomato and others); the Tic20, Tic110 and Tic40 selectors list Arabidopsis and pea only, and the stromal processing peptidase selector Arabidopsis, pea and rice. The receptor family PTHR10903 lists far fewer: its only land-plant members locally are Arabidopsis, pea TOC34 and four Physcomitrium Toc159-family proteins. The required Toc34 step (SF149, SF172) is therefore met only by Arabidopsis and pea, and Physcomitrium and rice fail the module at that step for want of a selector rather than for want of the protein; TOC159 (SF120) and TOC90 (SF68) are listed for Arabidopsis only, but the Toc159-family step is met by SF135 in Physcomitrium. Computed from the local entries tables over the required steps other than cpHsc70 (whose family table is capped, so it cannot be checked this way), Arabidopsis is the only genome that meets every one: pea fails the Toc159-family and Tic236 steps; rice fails both receptor steps, Tic20 and Tic110; Physcomitrium fails every step except the Toc159-family one, since its only local entries in these families are its four SF135 receptors; and tomato, listed only for ClpC, fails the rest. Not modelled: Ycf2, the plastome-encoded partner of FtsHi, and the remaining FtsHi paralogues; red algae, glaucophytes and plastids of secondary origin, which use related but differently organised import routes, are out of scope.