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Thylakoidal proteins of plant chloroplasts are transported to thylakoids via several different pathways, including the ⌬pH-dependent and the Sec-dependent pathways. In this study, we asked if these two pathways utilize a common translocation pore. A fusion protein consisting of a 23-kDa subunit of the oxygen evolving complex and Escherichia coli biotin carboxyl carrier protein was biotinylated in E. coli cells and purified. When incubated with isolated pea thylakoids in the absence of avidin, the purified fusion protein was imported into the thylakoids via the ⌬pH-dependent pathway. However in the presence of avidin, the fusion protein became lodged in the thylakoid membranes, with its N terminus reaching the thylakoidal lumen, while its C-terminal segment complexed with avidin exposed on the thylakoidal surface. The translocation intermediate of the fusion protein inhibited the import of authentic 23-kDa subunit, suggesting that it occupies a putative translocation pore for the ⌬pH-dependent pathway. However the intermediate did not block import of the 33-kDa subunit of the oxygen evolving complex, which is a substrate for the Sec-dependent pathway. These results provide evidence against the possibility of a common translocation pore shared by the Sec-dependent pathway and the ⌬pH-dependent pathway.Nuclear-encoded thylakoidal proteins are synthesized in the cytosol, are imported into the chloroplast stroma, and are subsequently inserted into or translocated across the thylakoid membranes. Recent evidence has shown that thylakoidal proteins are transported to the thylakoids via several different pathways, including the Sec-dependent and the ⌬pH-dependent pathways (1, 2). In bacterial cells, secretory proteins are translocated across the cytoplasmic membrane through a channel consisting of SecY/E/G proteins with the aid of a translocation ATPase, SecA. In chloroplasts, homologues of SecA (cpSecA), SecY (cpSecY) and SecE (cpSecE) have been identified and shown to mediate transport of a class of thylakoid lumenal proteins in vitro and in vivo (3-12). Another class of thylakoid lumenal proteins is translocated across the thylakoid membranes via a transport pathway that requires ⌬pH across the thylakoid membranes as a sole energy source and Hcf106 protein in the thylakoid membranes (13-15). This ⌬pH pathway appears to have ability to translocate folded proteins that are not accepted by the Sec-dependent pathway (16 -18). Recent studies have revealed that a protein transport pathway similar to the thylakoidal ⌬pH-dependent pathway is present in bacterial cells (19 -21).Although the Sec-dependent and the ⌬pH-dependent pathways appear to utilize some pathway-specific components, they could share common translocation pore in the thylakoid membranes. In vitro competition experiments have shown that the import of a certain substrate protein into isolated thylakoids is effectively competed with saturating amounts of another competitor protein which utilizes the same thylakoidal transport pathway as the substrate protein but...

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