Characterization and Crystallization of A Highly Active Cytochrome b 6 f Complex from the Thermophilic Cyanobacterium, Mastigocladus Laminosus

The cytochrome b6f complex, one of four integral protein complexes in the oxygenic photosynthetic membrane, mediates electron transport, H+ translocation, and generation of the electrochemical proton gradient (1–3), and may also participate in other non-bioenergetic functions, such as activation of a redox-controlled protein kinase (4). The polypeptide content of the b6f complex, purified from spinach chloroplasts (5) or the green alga, C. reinhardtii (6), includes 4 proteins of Mr > 17,000, all of which contain redox prosthetic groups (cytochromesf, b6, Rieske protein) and/or participate in quinone binding: the petA gene product, cytochrome f (MW = 32,038 in spinach chloroplasts); petB, cytochrome b6 (MW = 24,166); the petC Rieske [2Fe-2S] protein (MW = 19,116); and subunit IV (MW = 17,445). The complex also contains 3 small chloroplast-encoded hydrophobic subunits, petG (7), petM (8), and petL (9) consisting of 37, 39, and 32 residues. The molecular weight of the monomeric complex of 7 polypeptide components with unity stoichiometry, together with prosthetic groups, is approximately 110 kDa. The number of trans-membrane a-helices in the monomeric complex, inferred from nucleotide sequences and biochemical topography and binding studies, is 12 (4 in PetB, 3 in PetD, 1 each in PetA, C, G, L, and M). A dimeric b6f complex (10), would include 24 trans-membrane α-helices and have a molecular weight ≈ 220 kDa.