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

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 transmembrane 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 b-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.

Transmembrane signaling and assembly of the cytochrome b6f-lipidic charge transfer complex

“…2B ). The finding that the hydrophobic space occupied by the eighth transmembrane helix of the respiratory and anoxic photosynthetic cytochrome bc 1 complex is occupied by a lipid and a chlorophyll in the b 6 f complex raises the question of the pressure in evolution that led to this change [12, 57]. 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 [57].…”

Trans-membrane Signaling in Photosynthetic State Transitions

“…It has been suggested previously that the Stt7 kinase, with its singlepass trans-membrane domain, may be located proximal to the F and G helices of subunit IV of the cyt b 6 f complex (Fig. 1C) (39,(43)(44)(45). Such a complex between cyt b 6 f and Stt7 may place the p side extrinsic domain of Stt7 in proximity to the Q p site of cyt b 6 f (Fig.…”

“…It is noted that isomerization of proline has been documented previously to be crucial to neurodegenerative disorders, cellular signaling, gating of ion channels, and control of access to enzyme active sites (53)(54)(55)(56)(57)(58). It has been suggested previously that the Stt7 trans-membrane domain may be bound between the F and G trans-membrane helices of cyt b 6 f (43)(44)(45).…”

Preface

“…Studies of the LHCI-PSI complex are much further advanced following the determination of its structure by X-ray crystallography, and the Mazor et al [3] now consider how the structure found in plants and green algae has evolved, notably by looking at the reservoir of PSI genes found in viruses. Similarly, the availability of high-resolution structures of the cytochrome b 6 f complex and its relatives is allowing discussion of novel functional aspects such as the role of bound lipids, as described by Hasan et al [4]. On the other hand, Semchonok et al [5] show how electron microscopy and single particle analysis can unravel the structural organization of larger macro-complexes-in this case, the reaction-centreantenna complexes of photosynthetic bacteria.…”