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 other organellar outer membrane proteins it is synthesized as a precursor polypeptide (preOEP86) in the cytosol and is post-translationally targeted to the organelles. PreOEP86 is targeted to and productively inserted into the chloroplastic outer envelope mediated by a bipartite signal consisting of the presequence and the COOH terminus of the precursor 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 mature form of the small subunit of ribulose bisphosphate carboxylase to intact chloroplasts. The presequence seems to be required to maintain preOEP86 in an integration competent state, whereas interaction of preOEP86 with chloroplasts is accomplished by a short sequence of amino acids in the COOH-terminal portion of the mature protein. The COOH-terminal portion of preOEP86 contains enough information to also direct mature OEP86 into the outer envelope membrane of pea chloroplasts. However, mature OEP86 enters the productive folding pathway much less efficiently than preOEP86. The COOH terminus of preOEP86 not only serves as a membrane anchor but seems to be required for a productive translocation through an interaction with other outer envelope proteins. Although the binding was ATP-dependent, productive folding was not. PreOEP86 seems to follow a unique road into the chloroplastic outer envelope.