Structural and functional contributions of lipids to the stability and activity of the photosynthetic cytochrome b 6 f lipoprotein complex.

The photosynthetic cytochrome b 6 f complex, a homodimer containing eight distinct subunits and 26 transmembrane helices per monomer, catalyzes proton-coupled electron transfer across the thylakoid membrane. The 2.5-Å-resolution structure of the complex from the cyanobacterium Nostoc sp. revealed the presence of 23 lipid-binding sites per monomer. Although the crystal structure of the cytochrome b 6 f from a plant source has not yet been solved, the identities of the lipids present in a plant b 6 f complex have previously been determined, indicating that the predominant lipid species are monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), phosphatidylglycerol (PG), and sulfoquinovosyldiacylglycerol (SQDG). Despite the extensive structural analyses of b 6 f-lipid interactions, the basis of the stabilization by lipids remains poorly understood. In the present study, we report on the effect of individual lipids on the structural and functional integrity of the b 6 f complex, purified from Spinacea oleracea It was found that (i) galactolipids (MGDG, DGDG, and SQDG) and phospholipids dilinolenoyl-phosphatidylglycerol (DLPG), 1,2-dioleoylphosphatidylglycerol (DOPG), and 1,2-dioleoyl-sn-glycerol-3-phosphatidylcholine (DOPC) structurally stabilize the complex to varying degrees; (ii) SQDG has a major role in stabilizing the dimeric complex; (iii) the b 6 f complex is stabilized by incorporation into nanodiscs or bicelles; (iv) removal of bound phospholipid by phospholipase A2 inactivates the cytochrome complex; and (v) activity can be restored significantly by the addition of the anionic lipid PG, which is attributed to stabilization of the quinone portal and the hinge region of the iron-sulfur protein.