NEW INSIGHTS INTO THE PROTEIN IMPORT MACHINERY OF THE CHLOROPLASTS OUTER ENVELOPE

A large number of plastid localized proteins are post-translationally imported as precursor proteins from the cytosol into the organelle. Re cognition and translocation is accomplished by a subset of chloroplast envelope proteins, which were identified by different but complementa ry methods. The outer envelope proteins OEP 86, OEP 75, OEP 70 (a heat shock cognate 70 homologue) and OEP 34 are clearly involved in the im port event and can be isolated as one functionally active translocatio n unit. For three of these proteins cDNA clones have been very recentl y obtained, namely OEP 86, OEP 75 and OEP 34. OEP 86 seems to be a pre cursor protein receptor which could be regulated by GTP binding and AT P-dependent phosphorylation-dephosphorylation. OEP 75 is part of the t ranslocation pore traversing the membrane in multiple beta-sheets. OEP 34 is tightly associated with OEP 75. It represents a new type of GTP -binding protein which possesses endogenous GTPase activity. Multiple GTP binding and hydrolysis cycles as well as protein phosphorylation-d ephosphorylation events might, therefore, regulate the interaction of a precursor protein with the translocation machinery of the outer enve lope, making it very distinct hom the mitochondrial outer membrane sys tem. Further proteins of the inner envelope membrane, namely IEP 97 an d IEP 36, have been implied to function in the translocation event. Th ese recent data allow not only identification of the players in the ga me but also speculation about mechanisms and regulation of translocati on.