Lipid functions in cytochrome bc complexes: an odd evolutionary transition in a membrane protein structure

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Abstract
Lipid-binding sites and properties were compared in the hetero-oligomeric cytochrome (cyt) b 6 f and the yeast bc 1 complexes that function, respectively, in photosynthetic and respiratory electron transport. Seven lipid-binding sites in the monomeric unit of the dimeric cyanobacterial b 6 f complex overlap four sites in the Chlamydomonas reinhardtii algal b 6 f complex and four in the yeast bc 1 complex. The proposed lipid functions include: (i) interfacial–interhelix mediation between (a) the two 8-subunit monomers of the dimeric complex, (b) between the core domain (cyt b , subunit IV) and the six trans membrane helices of the peripheral domain (cyt f , iron–sulphur protein (ISP), and four small subunits in the boundary ‘picket fence’); (ii) stabilization of the ISP domain-swapped trans-membrane helix; (iii) neutralization of basic residues in the single helix of cyt f and of the ISP; (iv) a ‘latch’ to photosystem I provided by the β-carotene chain protruding through the ‘picket fence’; (v) presence of a lipid and chlorophyll a chlorin ring in b 6 f in place of the eighth helix in the bc 1 cyt b polypeptide. The question is posed of the function of the lipid substitution in relation to the evolutionary change between the eight and seven helix structures of the cyt b polypeptide. On the basis of the known n-side activation of light harvesting complex II (LHCII) kinase by the p-side level of plastoquinol, one possibility is that the change was directed by the selective advantage of p- to n-side trans membrane signalling functions in b 6 f , with the lipid either mediating this function or substituting for the trans membrane helix of a signalling protein lost in crystallization.

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