Conditional Barley Mutant Chlorina-¹⁰⁴ Lacks A HSP70 Homologue of the Chloroplast Protein Import Complex Under Restrictive Growth Conditions

Abstract

Four proteins constitute the translocon complex of the outer envelope membrane of chloroplasts. Toc34, Toc86 and Toc75 mediate the initial binding of preproteins and subsequent translocation across the outer envelope in an ATP/GTP-dependent manner (1, 2, 3, 4). Subsequently, a translocon complex transports the preproteins across the inner envelope membrane driven by ATP hydrolysis in the stroma (5, 6). Both complexes associate at contact sites between the outer and inner envelope to facilitate direct translocation from the cytoplasma to the stroma. Molecular chaperones associated with or embedded in the outer envelope, Com70 and Hsc70, respectively, and at the stromal side of the inner envelope, C1pC and Cpn60, participate in driving the transport and may serve to induce or stabilize unfolded import-competent conformation of the preprotein, and/or folding of proteins in the stroma after import (7, 8, 9, 10).