Protein import into plastids: general versus specialized translocation

Abstract

An overwhelming part of higher-plant plastid polypeptides are nuclear-encoded and must be imported. In the past decade, protein precursor import and processing have received considerable attention and an envelope-based transport apparatus (the translocon) was identified. The current model for protein import involves the transient assembly of various components. First, the protein precursor interacts with plastid envelope lipids and with TOC34 and TOC159 of the translocon at the outer membrane of chloroplasts (Toc). GTP and ATP are hydrolysed before channeling through the TOC75 pore, and then the Toc complex participates in the formation of contact sites between inner and outer envelope membranes. The precursor is subsequently translocated through the inner membrane complex (Tic) and released in the plastid stroma, where it is eventually processed to its mature size. The complete translocation is assisted by molecular chaperones.