title: Three-dimensional electron cryo-microscopy study of the extrinsic domains of the oxygen-evolving complex of spinach: assignment of the PsbO protein.
abstract: Three independent three-dimensional reconstructions of the spinach photosystem II-light-harvesting complex supercomplex were derived from single particle analyses of non-stained, vitrified samples imaged by electron microscopy. Each reconstruction was found to differ significantly in the composition of the lumenal oxygen-evolving complex extrinsic proteins. From difference mapping, aided by electron microscopy of negatively stained selectively washed samples, regions of density were assigned to the PsbO and PsbP/PsbQ proteins. Interpretation of the density assigned to the PsbO protein was explored using computer-aided structural predictions. PsbO is calculated to be mainly a beta-protein (38% beta) composed of two domains within an overall elongated shape (Pazos, F., Heredia, P., Valencia, A., and De Las Rivas, J. (2001) Proteins Struct. Funct. Genet. 45, 372-381). The positioning and fitting of the proposed structural model for the PsbO protein within the three-dimensional map indicated that there is a single copy per reaction center. Moreover, the structural model derived for PsbO, together with difference mapping, indicates that this protein stretches across the surface of the reaction center with its N- and C-terminal domains located toward the CP47 and CP43 side, respectively. This structural assignment is discussed in terms of the recent x-ray-derived cyanobacterial model of PSII (Zouni, A., Witt, H.-T., Kern, J., Fromme, P., Krauss, N., Saenger, W., and Orth, P. (2001) Nature 409, 739-743).
authors.affiliation: Wolfson Laboratories, Department of Biological Sciences, Imperial College of Science, Technology and Medicine, London SW7 2AY, United Kingdom.
authors.forename: Jon
authors.initials: J
authors.lastname: Nield
authors.forename: Monica
authors.initials: M
authors.lastname: Balsera
authors.forename: Javier
authors.initials: J
authors.lastname: De Las Rivas
authors.forename: J
authors.initials: J
authors.lastname: Barber
mesh_terms: D020285:Cryoelectron Microscopy; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D045332:Photosystem II Protein Complex; D011487:Protein Conformation; D018724:Spinacia oleracea / Q000737:chemistry* / Q000378:metabolism
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; C064388:oxygen-evolving complex, 33 kDa protein; D010100:Oxygen