Structure-Function of Hetero-oligomeric Integral Membrane Proteins

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DESCRIPTION (provided by applicant): The cytochrome b6f complex is one of a small number (presently, 17) of hetero-oligomeric integral membrane proteins whose crystal structure has been solved to a resolution better than 3.0 A. The structure is a dimer that encloses a large (10,000 A3) lipophilic inter-monomer 'quinone exchange cavity that exchanges quinone/quinol with the lipid bilayer, and connects the quinone reduction site on the electrochemically negative n-side of one monomer with the oxidation site at the [2Fe-2S] cluster on the positive p-side. The quinone pathway between n- and p-sides involves passage through anllAx!2A portal. We have solved four b6f structures from the thermophilic cyanobacterium, M. laminosus, (i) a native structure with resolution recently improved to 2.95 A in the presence of Cd2+ cations, and three structures of complexes with quinone- analogue inhibitors, p-side TDS and DBMIB, and n-side NQNO. These structures have provided markers for the route of quinone passage across the complex and diffusion within it. Further understanding of the structure and dynamics of quinone transfer is relevant to understand transfer of hydrophobic drugs and metabolites across membrane proteins, and will be pursued through higher resolution structures, site-directed mutagenesis and molecular/steered dynamics. The complex contains 8 subunits: 4 of these form a core of 'large' subunits that bind 8 prosthetic groups. Each b6f monomer contains 4 hemes, one [2Fe-2S] cluster, one plastoquinone, and 3 unique prosthetic groups: (i) a novel heme en with one covalent linkage to a Cys residue of cytochrome b, and no amino acid side chains as axial ligands; (ii) one chlorophyll a, for which two H2O have been resolved as its 5th ligand; (iii) one beta-carotene to which the Chi, can transfer excited triplet state energy in spite of their 14A separation. These prosthetic groups unique to b6f raise new questions about redox function in be complexes. We will focus on the unique heme en, whose functions are linked to the ability of the b6f complex to carry out ferredoxin-dependent cyclic electron transport and of the photosynthetic membrane to evolve O2. A unique coupling between heme en and the nearby (4 A) heme bn was established, which implies an ability of hemes bn - en to serve as a 2 electron donor, thus providing a new mechanism to protect against formation of superoxide and reactive oxygen species (ROS). Studies on a second hetero- oligomeric integral membrane protein, Ndh-1 from the cyanobacterial membrane that also reduces quinone and generates ROS, have masses of 12 of the 14-15 subunits, and in which the goal is to obtain a crystal structure.