Model for protein trafficking to the apicoplast. In the proposed model, vesicles bound for the apicoplast (green) bud from the ER or nuclear envelope (brown) and are distinct from vesicles targeted to the Golgi (violet). Vesicles derived from the latter move to other secretory destinations. The boxed enlargement shows some potential features of the apicoplast-destined vesicles, cargo, and apicoplast. Molecules involved in ER exit (orange) are proposed here to be a protein coat (C), a GTPase (G), and the putative transit peptide receptor (Tr). Molecules mediating fusion to the apicoplast (blue) are suggested to include an apicoplast-specific Rab (R) and v-SNAREs (v-Sn) and t-SNAREs (t-Sn) as well as others not shown. Apicoplast-destined proteins (turquoise) are proposed to interact directly with the coat or indirectly via the transit peptide receptor. Once the vesicles reach the apicoplast, they fuse with the outer membrane, releasing their contents to the space between the outer two membranes. Within the apicoplast, three models are shown to explain the translocation across the apicoplast membranes. The mediators in these models include Toc (orange hexagon) and Tic complexes (yellow ovals), ER-derived or mitochondrially derived translocators (blue rectangle), and vesicles between the periplastid and chloroplast outer membranes (green circles). Once proteins achieve the lumen of the apicoplast, the transit peptide domain is cleaved by the processing protease. Parsons M, Karnataki A, Feagin JE, DeRocher A. Protein trafficking to the apicoplast: deciphering the apicomplexan solution to secondary endosymbiosis. Eukaryot Cell. 2007 PMID: 17513565