Figure 5: Client-Specific Chloroplast Protein Import Pathways

The TOC receptors are encoded by small gene families in Arabidopsis and other species, and therefore exist in multiple isoforms. Evidence from genetic and biochemical studies indicates that the different isoforms have different client (preprotein) specificities, and associate selectively to form different client-specific TOC complexes. Specifically, atToc159 (the most abundant Toc159 isoform in Arabidopsis) associates preferentially with atToc33 (the most abundant Toc33 isoform in Arabidopsis), to form a TOC complex with specificity for highly-abundant photosynthetic preproteins; whereas atToc132 and/or atToc120 (the other Toc159 isoforms in Arabidopsis) associate preferentially with atToc34 (the other Toc33 isoform in Arabidopsis) to form a TOC complex with specificity for relatively low-abundance, housekeeping plastid preproteins. The existence of these client-specific import complexes may prevent photosynthetic preproteins from out-competing much less abundant preproteins during the (potentially rate-limiting) early stages of import, and/or may play a role in the regulating the plastid's proteome and developmental fate. Following outer envelope membrane translocation, the import pathways may converge at the TIC complex. Taken from Curr. Biol. 14: R1064-R1077.