Detecting transient protein–protein interactions by X-ray absorption spectroscopy: The cytochrome c6-photosystem I complex

6 pages, 1 table, 3 fiures, 1 appendix.-- Edited by Peter Brzezinski.Reliable analysis of the functionality of metalloproteins demands a highly accurate description of both the redox state and geometry of the metal centre, not only in the isolated metalloprotein but also in the transient complex with its target. Here, we demonstrate that the transient interaction between soluble cytochrome c6 and membrane-embedded photosystem I involves subtle changes in the heme iron, as inferred by X-ray absorption spectroscopy (XAS). A slight shift to lower energies of the absorption edge of Fe2+ in cytochrome c6 is observed upon interaction with photosystem I. This work constitutes a novel application of XAS to the analysis of weak complexes in solution.This work was supported by the European Synchrotron Radiation Facility (ESRF SC- 1366), the Spanish Ministry of Education and Science (AP2000-2937, BMC2003- 00458 and MAT02-01221) and the Andalusian Government (PAI, CVI-0198).Peer reviewe