A proton gradient is required for the transport of two lumenal oxygen-evolving proteins across the thylakoid membrane.

The 33- and 23-kDa proteins of the photosynthetic oxygen-evolving complex are synthesized in the cytosol as larger precursors and transported into the thylakoid lumen via stromal intermediate forms. We have investigated the energetics of protein transport across the thylakoid membrane using import assays that utilize either intact chloroplasts or isolated thylakoids. We have found that the light-driven import of the 23-kDa protein into isolated thylakoids is almost completely inhibited by electron transport inhibitors or by the ionophore nigericin but not by valinomycin. These compounds have similar effects in chloroplast import assays: precursors of both the 33- and 23-kDa proteins are imported and processed to intermediate forms in the stroma, but transport into the thylakoid lumen is blocked when electron transport is inhibited or nigericin is present. These results indicate that the transport of these proteins across the thylakoid membrane requires a protonmotive force and that the dominant component in this respect is the proton gradient and not the electrical potential.