Figure 2. The origin of plastid membranes and the translocons that allow proteins to cross them.

(A) Schematic outline of the acquisition and evolution of a complex plastid (using the apicoplast as an example). An algal cell (blue) carrying a chloroplast (red) is taken up into the endomembrane system (gray) of a protist host (pink). Co-adaptation lead to the establishment of complex protein import systems across each membrane allowing for a stable endosymbiotic relationship driven by massive gene transfer. P; Plastid, N; nucleus, Nm; nucleomorph. (B) Schematic outline of the protein import pathways of complex plastids of the red lineage (color coding for origin matches panel A, note that his only partially reflects systems in the green lineages). Question marks highlight elements that have not been experimentally validated or where the direct molecular function of a given protein remains to be established. *Note that Der1 is only one of several proteins that are considered candidates for the actual pore of the translocon, we only show one hypothesis for simplicity. Cargo proteins are shown as grey lines and proteins destined for degradation as dashed lines. PPC; the periplastid compartment.