Fig 1.
Models for apicoplast evolutionary history and lumenal protein import.
(A) Model for apicoplast evolutionary history. Red algae arose following primary endosymbiosis, during which a eukaryotic cell engulfed a photosynthetic cyanobacterium that underwent evolutionary reduction to become a chloroplast. The ancestors of the Apicomplexa emerged following secondary endosymbiosis, during which another eukaryotic cell engulfed a red alga, which then underwent evolutionary reduction to become a four-membraned, photosynthetic secondary plastid. During evolution of the Apicomplexa, the secondary plastid lost its photosynthetic machinery but retained components of key metabolic pathways to become what we now know as the apicoplast. Note that this model is simplified and that the precise evolutionary events that gave rise to the apicoplast (e.g., vertical plastid inheritance from a common chromalveolate ancestor versus acquisition by tertiary endosymbiosis) are not yet resolved. (B) Model for import of lumenal apicoplast proteins via the secretory system and retooled ERAD machinery. Most apicoplast proteins contain a bipartite N-terminal targeting signal consisting of a eukaryotic SP followed by a plant-like TP. The SP mediates cotranslational import into the ER via the SEC61 complex and is cleaved by the signal peptidase complex to reveal the TP. The TP then mediates sorting and trafficking to the apicoplast and import across its membranes. The machinery involved in recognizing apicoplast proteins in the endomembrane system is unknown. Apicoplast proteins are presumed to traffic from the ER to the outermost apicoplast membrane via a vesicular trafficking pathway. After crossing the outermost apicoplast membrane, apicoplast proteins cross the periplastid membrane using retooled ERAD machinery. Finally, lumenal apicoplast cargo crosses the innermost apicoplast membranes via complexes related to the TOC and TIC machinery of primary plastids. The apicoplast outer membrane contains PI(3)P and ATG8, which are associated with the endomembrane system in model systems. ATG8, autophagy-related 8; CDC48, cell division cycle 48; DER1, degradation in the ER 1; DUB, deubiquitinase; ER, endoplasmic reticulum; ERAD, ER-associated degradation; P, phosphate; PE, phosphatidylethanolamine; PI(3)P, phosphatidylinositol 3-phosphate; PUBL, plastid ubiquitin-like protein; SNARE, soluble N-ethylmaleimide–sensitive factor attachment protein receptor; SP, signal peptide; SPP, stromal processing peptidase; TIC, translocon of the inner chloroplast membrane; TOC, translocon of the outer chloroplast membrane; TP, transit peptide; UFD1, ubiquitin fusion protein degradation 1.