Isolation of membrane protein subunits in their native state: evidence for selective binding of chlorophyll and carotenoid to the b(6) subunit of the cytochrome b(6)f complex.

Cytochrome (cyt) b-c complexes play a central role in electron transfer chains and are almost ubiquitous in nature. Although similar in their basic structure and function, the cyt b(6)f complex of photosynthetic membranes and its counterpart, the mitochondrial cyt bc(1) complex, show some characteristic differences which cannot be explained by the high resolution structure of the cyt bc(1) complex alone. Especially the presence of a chlorophyll molecule is a striking feature of all cyt b(6)f complex preparations described so far, imposing questions as to its structural and functional role. To allow a more detailed characterization, we here report the preparation of native subunits cyt b(6) and IV starting from a monomeric cyanobacterial cyt b(6)f complex. Spectroscopical and reversed-phase HPLC analyses of the purified cyt b(6) subunit showed that it contained not only two b-type hemes, but also one chlorophyll a molecule and a cyanobacterial carotenoid, echinenone. Evidence for selective binding of both pigments to this subunit is presented and their putative function is discussed.