A novel, bipartite transit peptide targets OEP75 to the outer membrane of the chloroplastic envelope

OEP75 is an outer envelope membrane component of the chloroplastic protein import apparatus and is synthesized in the cytoplasm as a higher molecular weight precursor (prOEP75). During its own import, prOEP75 is processed first to an intermediate (iOEP75) and subsequently to the mature form (mOEP75). Experiments conducted with stromal extracts indicated that iOEP75 was generated from prOEP75 by the activity of the stromal processing peptidase. The specific processing site was determined and used to divide the prOEP75 transit peptide into N- and C-terminal domains. To determine the targeting functions of the two domains of the transit peptide and of the mature region of prOEP75, we created a deletion mutant construct from prOEP75 and chimeric constructs between domains of prOEP75 and the precursor to a small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase. Analysis of these constructs by in vitro chloroplastic protein import assays revealed that the transit peptide of prOEP75 is bipartite in that the N- and C-terminal portions contain chloroplastic and intraorganellar targeting information, respectively.