Protein translocation across the thylakoid membrane--a tale of two mechanisms

In vitro reconstitution assays have been used in recent years to probe the mechanisms by which a variety of cytosolically synthesised proteins are transported across the thylakoid membrane within higher plant chloroplasts. The emerging data suggest that two distinct mechanisms operate. Translocation of a subset of lumenal proteins, namely the 23 kDa and 16 kDa proteins of the oxygen-evolving complex, and of the CFo2 protein (an integral membrane protein), requires only the presence of the thylakoidal delta pH. In contrast, two other lumenal proteins, the 33 kDa oxygen-evolving complex protein and plastocyanin, require also the presence of ATP and at least one stromal factor for efficient transport into isolated thylakoids to take place.