Red bell pepper chromoplasts exhibit in vitro import competency and membrane targeting of passenger proteins from the thylakoidal Sec and Delta pH pathways but not the chloroplast signal recognition particle pathway

Chloroplast to chromoplast development involves new synthesis and plastid l ocalization of nuclear-encoded proteins, as well as changes in the organiza tion of internal plastid membrane compartments. We have demonstrated that i solated red bell pepper (Capsicum annuum) chromoplasts contain the 75-kD co mponent of the chloroplast outer envelope translocon (Toc75) and are capabl e of importing chloroplast precursors in an ATP-dependent fashion, indicati ng a functional general import apparatus. The isolated chromoplasts were ab le to further localize the 33- and 17-kD subunits of the photosystem II O-2 -evolution complex (OE33 and OE17, respectively), lumen-targeted precursors that utilize the thylakoidal Sec and Delta pH pathways, respectively, to t he lumen of an internal membrane compartment. Chromoplasts contained the th ylakoid Sec component protein, cpSecA, at levels comparable to chloroplasts . Routing of OE17 to the lumen was abolished by ionophores, suggesting that routing is dependent on a transmembrane Delta pH. The chloroplast signal r ecognition particle pathway precursor major photosystem II light-harvesting chlorophyll a/b protein failed to associate with chromoplast membranes and instead accumulated in the stroma following import. The Pftf (plastid fusi on/translocation factor), a chromoplast protein, integrated into the intern al membranes of chromoplasts during in vitro assays, and immunoblot analysi s indicated that endogenous plastid fusion/translocation factor was also an integral membrane protein of chromoplasts. These data demonstrate that the internal membranes of chromoplasts are functional with respect to protein translocation on the thylakoid Sec and Delta pH pathways.