FIGURE 1.

Chaperone involvement in cytosolic targeting and recognition of preproteins at the outer envelope membrane of chloroplasts. Preproteins could be chaperoned by the guidance complex or by Hsp90 alone. The guidance complex is represented by Hsp70 that binds to both mature region and cTP of the preprotein and 14-3-3 proteins which bind to the phosphorylated cTP. Hsp70-chaperoned preproteins are recognized by the GTP-dependent receptor proteins Toc159 and Toc34, followed by delivery to the import channel Toc75, whereas precursor proteins bound to Hsp90 are docked to the third receptor Toc64 via its TPR domain and are then handed over to Toc34.