title: Cryo-EM structure of a tetrameric cyanobacterial photosystem I complex reveals novel subunit interactions.
abstract: Photosystem I (PSI) of the thermophilic cyanobacterium Chroococcidiopsis sp. TS-821 (TS-821) forms tetramers Li et al. (2014). Two-dimensional maps obtained by single particle electron microscopy (EM) clearly show that the tetramer lacks four-fold symmetry and is actually composed of a dimer of dimers with C2 symmetry. The resolution of these negative stain 2D maps did not permit the placement of most of the small PSI subunits, except for PsaL. Therefore cryo-EM was used for 3D reconstruction of the PSI tetramer complex. A 3D model at ~11.5Å resolution was obtained and a 2D map within the membrane plane of ~6.1Å. This data was used to build a model that was compared with the high-resolution structure of the PSI of Thermosynechococcus elongatus (T. elongatus) at 2.5Å. This comparison reveals key differences in which subunits are involved in the two different interfaces, interface type 1 within a dimer and interface type 2 between dimers. The type 1 interface in TS-821 is similar to the monomer interface in the trimeric PSI from T. elongatus, with interactions between subunits PsaA, -B, -I, -L and M. In type 2 the interaction is only between PsaA, -B and -L. Unlike the trimeric PSI, the central cavity of the complex is not filled with the PsaL-derived helical bundle, but instead seems filled with lipids. The physiological or evolutionary advantage of the tetramer is unknown. However, the presence of both dimers and tetramers in the thylakoid membrane suggest a dynamic equilibrium that shifts towards the tetramers in high light.
authors.affiliation: Department of Electron Microscopy, Groningen Biomolecular Sciences & Biotechnology Institute, University of Groningen, 9747 AG Groningen, The Netherlands.
authors.forename: Dmitry A
authors.initials: DA
authors.lastname: Semchonok
authors.affiliation: Department of Biochemistry, Cellular, and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA; Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, TN 37996, USA.
authors.forename: Meng
authors.initials: M
authors.lastname: Li
authors.affiliation: Department of Biochemistry, Cellular, and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA; Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, TN 37996, USA; Department of Microbiology, University of Tennessee, Knoxville, TN 37996, USA; Sustainable Energy and Education Research Center, University of Tennessee, Knoxville, TN 37996, USA. Electronic address: bbruce@utk.edu.
authors.forename: Barry D
authors.initials: BD
authors.lastname: Bruce
authors.affiliation: Department of Electron Microscopy, Groningen Biomolecular Sciences & Biotechnology Institute, University of Groningen, 9747 AG Groningen, The Netherlands.
authors.forename: Gert T
authors.initials: GT
authors.lastname: Oostergetel
authors.affiliation: Department of Electron Microscopy, Groningen Biomolecular Sciences & Biotechnology Institute, University of Groningen, 9747 AG Groningen, The Netherlands. Electronic address: e.j.boekema@rug.nl.
authors.forename: Egbert J
authors.initials: EJ
authors.lastname: Boekema
mesh_terms: D020285:Cryoelectron Microscopy; D000458:Cyanobacteria / Q000737:chemistry*; D045331:Photosystem I Protein Complex / Q000737:chemistry / Q000648:ultrastructure*; D055503:Protein Multimerization; D021122:Protein Subunits
chemical_list: D045331:Photosystem I Protein Complex; D021122:Protein Subunits
keywords: Chroococcidiopsis; Cyanobacteria; Electron microscopy; Photosystem I; Structure