TARGETING OF PROTEINS TO THE OUTER ENVELOPE MEMBRANE USES A DIFFERENT PATHWAY THAN TRANSPORT INTO CHLOROPLASTS

The chloroplastic envelope is composed of two membranes, inner and outer, each with a distinct set of polypeptides. Like proteins in other chloroplastic compartments, most envelope proteins are synthesized in the cytosol and post-translationally imported into chloroplasts. Considerable knowledge has been obtained concerning protein import into most chloroplastic compartments. However, very little is known about the biogenesis of envelope membrane proteins. We isolated a cDNA clone from pea that encodes a 14-kilodalton outer envelope membrane protein. The precursor form of this protein does not possess a cleavable transit peptide and its import into isolated chloroplasts does not require either ATP or a thermolysin-sensitive component on the chloroplastic surface. These findings, together with similar observations made with a spinach chloroplastic outer membrane protein, led us to propose that proteins destined for the outer membrane of the chloroplastic envelope follow an import pathway distinct from that followed by proteins destined for other chloroplastic compartments.