MULTIPLE PATHWAYS FOR PROTEIN-TRANSPORT INTO OR ACROSS THE THYLAKOID MEMBRANE

Many thylakoid proteins are cytosolically synthesized and have to cros s the two chloroplast envelope membranes as well as the thylakoid memb rane en route to their functional locations. In order to investigate t he localization pathways of these proteins, we over-expressed precurso r proteins in Escherichia coli and used them in competition studies. C ompetition was conducted for import into the chloroplast and for trans port into or across isolated thylakoids. We also developed a novel in organello method whereby competition for thylakoid transport occurred within intact chloroplasts. Import of all precursors into chloroplasts was similarly inhibited by saturating concentrations of the precursor to the OE23 protein. In contrast, competition for thylakoid transport revealed three distinct precursor specificity groups. Lumen-resident proteins OE23 and OE17 constitute one group, lumenal proteins plastocy anin and OE33 a second, and the membrane protein LHCP a third. The spe cificity determined by competition correlates with previously determin ed protein-specific energy requirements for thylakoid transport. Taken together, these results suggest that thylakoid precursor proteins are imported into chloroplasts on a common import apparatus, whereupon th ey enter one of several precursor-specific thylakoid transport pathway s.