A COMPONENT OF THE CHLOROPLASTIC PROTEIN IMPORT APPARATUS IS TARGETEDTO THE OUTER ENVELOPE MEMBRANE VIA A NOVEL PATHWAY

A chloroplastic outer envelope membrane protein of 75 kDa (OEP75) was identified previously as a component of the protein import machinery. Here we provide additional evidence that OEP75 is a component of prote in import, present the isolation of a cDNA clone encoding this protein , briefly describe its developmental expression and tissue specificity , and characterize its insertion into the outer envelope membrane, OEP 75 was synthesized as a higher molecular weight precursor (prOEP75) wh ich bound to isolated chloroplasts in an in vitro import assay and sub sequently was processed to the mature form (mOEP75). During this impor t assay, two proteins intermediate in size between prOEP75 and mOEP75 were detected, One of these intermediates was also detected in chlorop last envelopes isolated from young pea leaves, Binding and processing of prOEP75 required ATP and one or more surface-exposed proteinaceous components, and was competed by prSSU, a stromal-targeted protein. We propose that the N-terminus of the prOEP75 transit peptide acts as a s tromal-targeting domain and a central, hydrophobic region of this tran sit peptide acts as a stop-transfer domain. A complex route of inserti on and processing of prOEP75 may exist to ensure high fidelity targeti ng of this import component.