TARGETING DETERMINANTS AND PROPOSED EVOLUTIONARY BASIS FOR THE SEC AND THE DELTA-PH PROTEIN-TRANSPORT SYSTEMS IN CHLOROPLAST THYLAKOID MEMBRANES

Transport of proteins to the thylakoid lumen is accomplished by two pr ecursor-specific pathways, the Sec and the unique Delta pH transport s ystems, Pathway selection is specified by transient lumen-targeting do mains (LTDs) on precursor proteins. Here, chimeric and mutant LTDs wer e used to identify elements responsible for targeting specificity, The results showed that: (n) minimal signal peptide motifs consisting of charged N, hydrophobic H, and cleavage C domains were both necessary a nd sufficient for pathway-specific targeting; (b) exclusive targeting to the Delta pH pathway requires a twin arginine in the N domain and a n H domain that is incompatible with the Sec pathway; (c) exclusive ta rgeting to the Sec pathway is achieved by an N domain that lacks the t win arginine, although the twin arginine was completely compatible wit h the Sec system, A dual-targeting signal peptide, constructed by comb ining Delta pH and Sec domains, was used to simultaneously compare the transport capability of both pathways when confronted with different passenger proteins. Whereas Sec passengers were efficiently transporte d by both pathways, Delta pH passengers were arrested in translocation on the Sec pathway, This finding suggests that the Delta pH mechanism evolved to accommodate transport of proteins incompatible with the th ylakoid Sec machinery.