FIGURE 1.

Summary: structure aspects of the cytochrome b₆f complex and Stt7 kinase. A, the cytochrome b₆f complex from the filamentous cyanobacterium Nostoc PCC 7120 (PDB code 4OGQ), consisting of eight distinct subunits (shown as ribbons). Cyt b₆ (white) and subunit IV (yellow) are polytopic subunits containing four and three trans-membrane helices with seven prosthetic groups per monomer. Cyt f (cyan) and the Rieske ISP (red) contain a single trans-membrane helix and large p side extrinsic domains. Four single-helix trans-membrane subunits, PetG (blue), PetL (green), PetM (orange), and PetN (light brown), form the peripheral boundary of each monomer. B, electron transfer within the trans-membrane domain involves heme b_p, b_n (blue/red sticks) and heme c_n (black/red sticks) and, on the p side, a [2Fe-2S] cluster (brown/yellow sticks, spheres) linked to the ISP subunit and covalently linked heme f (black/red sticks). Two pigments, Chl-a (green/red/blue sticks) and a β-carotene (β-Car, yellow sticks), are present in single copies in the trans-membrane domain. C, the p side Q_p site is marked by a quinol analog (PDB code 2E76), shown as thin black/red sticks. On the p side, the Q_p site is covered by the [2Fe-2S] cluster in the extrinsic domain of the ISP subunit (red). F and G helices of subunit IV (pale yellow) are located at the periphery. The phytyl tail of the Chl-a (green/blue/red sticks) wrapped around the F helix gates quinone residence at the Q_p site (24). A lipid molecule (brown/blue sticks) is bound within the niche formed by F and G helices adjacent to the chlorin ring of Chl-a. Other protein subunits are represented in surface mode. D, schematic of the domain architecture of Stt7 kinase from the unicellular green alga C. reinhardtii, for which a crystal structure is not available. The Stt7 polypeptide consists of an N-terminal 41-residue signal peptide sequence (red) that targets the polypeptide to the chloroplast and is followed by a p side regulatory domain (blue, residues 42–96), containing two conserved Cys residues at positions 68 and 73 (yellow circles) implicated in the activation/deactivation of Stt7 (15, 18, 19). The single hydrophobic domain (brown, 97–122) is proposed (9) to span the membrane, connecting the regulatory and C-terminal kinase (yellow, residues 123–754) domains, the latter located on the n (stroma) side of the membrane. In this study, kinase activity was found to be restricted to residues 124–549 (defined by the dashed line), with residues numbered according to UNIPROT entry Q84V18.