title: Parallel subfunctionalisation of PsbO protein isoforms in angiosperms revealed by phylogenetic analysis and mapping of sequence variability onto protein structure.
abstract: BACKGROUND
  PsbO, the manganese-stabilising protein, is an indispensable extrinsic subunit of photosystem II. It plays a crucial role in the stabilisation of the water-splitting Mn4CaO5 cluster, which catalyses the oxidation of water to molecular oxygen by using light energy. PsbO was also demonstrated to have a weak GTPase activity that could be involved in regulation of D1 protein turnover. Our analysis of psbO sequences showed that many angiosperm species express two psbO paralogs, but the pairs of isoforms in one species were not orthologous to pairs of isoforms in distant species.
  
  
  RESULTS
  Phylogenetic analysis of 91 psbO sequences from 49 land plant species revealed that psbO duplication occurred many times independently, generally at the roots of modern angiosperm families. In spite of this, the level of isoform divergence was similar in different species. Moreover, mapping of the differences on the protein tertiary structure showed that the isoforms in individual species differ from each other on similar positions, mostly on the luminally exposed end of the β-barrel structure. Comparison of these differences with the location of differences between PsbOs from diverse angiosperm families indicated various selection pressures in PsbO evolution and potential interaction surfaces on the PsbO structure.
  
  
  CONCLUSIONS
  The analyses suggest that similar subfunctionalisation of PsbO isoforms occurred parallelly in various lineages. We speculate that the presence of two PsbO isoforms helps the plants to finely adjust the photosynthetic apparatus in response to variable conditions. This might be mediated by diverse GTPase activity, since the isoform differences predominate near the predicted GTP-binding site.
authors.affiliation: Department of Experimental Plant Biology, Faculty of Science, Charles University in Prague, Viničná 5,, 128 44 Praha 2, Czech Republic. duchmil@gmail.com.
authors.forename: Miloš
authors.initials: M
authors.lastname: Duchoslav
authors.affiliation: Department of Experimental Plant Biology, Faculty of Science, Charles University in Prague, Viničná 5,, 128 44 Praha 2, Czech Republic. lukasf@natur.cuni.cz.
authors.forename: Lukáš
authors.initials: L
authors.lastname: Fischer
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000596:Amino Acids / Q000378:metabolism; D017343:Genes, Plant; D019684:Magnoliopsida / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D016366:Open Reading Frames / Q000235:genetics; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D010802:Phylogeny; D020033:Protein Isoforms / Q000737:chemistry / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013045:Species Specificity
chemical_list: D000596:Amino Acids; D045332:Photosystem II Protein Complex; D020033:Protein Isoforms; C054266:photosystem II manganese-stabilizing protein