Cytochrome c6A is a funnel for thiol oxidation in the thylakoid lumen

Cytochrome c6A is a dithio-cytochrome recently discovered in land plants and green algae, and believed to be derived from the well-known cytochrome c6. The function of cytochrome c6A is unclear. We propose that it catalyses the formation of disulphide bridges in thylakoid lumen proteins in a single-step disulphide exchange reaction, with subsequent transfer of the reducing equivalents to plastocyanin. The haem group of cytochrome c6A acts as an electron sink, allowing rapid resolution of a radical intermediate formed during reoxidation of cytochrome c6A. Our model is consistent with previously published data on mutant plants, and the likely evolution of the protein.