Conservation of Lipid Functions in Cytochrome bc Complexes

Lipid binding sites and properties are compared in two sub-families of hetero-oligomeric membrane protein complexes known to have similar functions in order to gain further understanding of the role of lipid in the function, dynamics, and assembly of these complexes. Using the crystal structure information for both complexes, we compared the lipid binding properties of the cytochrome b6f and bc1 complexes that function in photosynthetic and respiratory membrane energy transduction. Comparison of lipid and detergent binding sites in the b6f complex with those in bc1 shows significant conservation of lipid positions. Seven lipid binding sites in the cyanobacterial b6f complex overlap three natural sites in the Chlamydomonas reinhardtii algal complex and four sites in the yeast mitochondrial bc1 complex. The specific identity of lipids is different in b6f and bc1 complexes: b6f contains sulfoquinovosyldiacylglycerol, phosphatidylglycerol, phosphatidylcholine, monogalactosyldiacylglycerol, and digalactosyldiacylglycerol, whereas cardiolipin, phosphatidylethanolamine, and phosphatidic acid are present in the yeast bc1 complex. The lipidic chlorophyll a and β-carotene (β-car) in cyanobacterial b6f, as well as eicosane in C. reinhardtii, are unique to the b6f complex. Inferences of lipid binding sites and functions were supported by sequence, interatomic distance, and B-factor information on interacting lipid groups and coordinating amino acid residues. The lipid functions inferred in the b6f complex are as follows: (i) substitution of a transmembrane helix by a lipid and chlorin ring, (ii) lipid and β-car connection of peripheral and core domains, (iii) stabilization of the iron–sulfur protein transmembrane helix, (iv) n-side charge and polarity compensation, and (v) β-car-mediated super-complex with the photosystem I complex.