A PROTEIN IMPORT RECEPTOR OF CHLOROPLASTS IS INSERTED INTO THE OUTER ENVELOPE MEMBRANE BY A NOVEL PATHWAY

The outer envelope protein OEP86 functions as a receptor for precursor proteins in the chloroplastic import machinery, In contrast to most o ther organellar outer membrane proteins it is synthesized as a precurs or polypeptide (preOEP86) in the cytosol and is posttranslationally ta rgeted to the organelles. PreOEP86 is targeted to and productively ins erted into the chloroplastic outer envelope mediated by a bipartite si gnal consisting of the presequence and the COOH terminus of the precur sor protein. The cleavable presequence alone does not seem to contain sufficient information to target preOEP86 without the COOH terminus or a hybrid protein consisting of the presequence of preOEP86 and the ma ture form of the small subunit of ribulose bisphosphate carboxylase to intact chloroplasts. The presequence seems to be required to maintain pre OEP86 in an integration competent state, whereas interaction of p reOEP86 with chloroplasts is accomplished by a short sequence of amino acids in the COOH-terminal portion of the mature protein. The COOH-te rminal portion of preOEP86 contains enough information to also direct mature OEP86 into the outer envelope membrane of pea chloroplasts. How ever, mature OEP86 enters the productive folding pathway much less eff iciently than preOEP86. The COOH terminus of preOEP86 not only serves as a membrane anchor but seems to be required for a productive translo cation through an interaction with other outer envelope proteins. Alth ough the binding was ATP-dependent, productive folding was not, PreOEP 86 seems to follow a unique road into the chloroplastic outer envelope .