title: Synthesis and assembly of thylakoid protein complexes: multiple assembly steps of photosystem II.
abstract: To study the synthesis and assembly of multisubunit thylakoid protein complexes, we performed [35S]Met pulse and chase experiments with isolated chloroplasts and intact leaves of spinach (Spinacia oleracea L.), followed by Blue Native gel separation of the (sub)complexes and subsequent identification of the newly synthesized and assembled protein subunits. PSII (photosystem II) core subunits were the most intensively synthesized proteins, particularly in vitro and at high light intensities in vivo, and could be sequestered in several distinct PSII subassemblies. Newly synthesized D1 was first found in the reaction centre complex that also contained labelled D2 and two labelled low-molecular-mass proteins. The next biggest PSII subassembly contained CP47 also. Then PsbH was assembled together with at least two other labelled chloroplast-encoded low-molecular-mass subunits, PsbM and PsbTc, and a nuclear-encoded PsbR. Subsequently, CP43 was inserted into the PSII complex concomitantly with PsbK. These assembly steps seemed to be essential for the dimerization of PSII core monomers. Intact PSII core monomer was the smallest subcomplex harbouring the newly synthesized 33 kDa oxygen-evolving complex protein PsbO. Nuclear-encoded PsbW was synthesized only at low light intensities concomitantly with Lhcb polypeptides and was distinctively present in PSII-LHCII (where LHC stands for light-harvesting complex) supercomplexes. The PsbH protein, on the contrary, was vigorously synthesized and incorporated into PSII core monomers together with the D1 protein, suggesting an intrinsic role for PsbH in the photoinhibition-repair cycle of PSII.
authors.affiliation: Department of Biology, Plant Physiology and Molecular Biology, University of Turku, Turku FI-20014, Finland.
authors.forename: Anne
authors.initials: A
authors.lastname: Rokka
authors.forename: Marjaana
authors.initials: M
authors.lastname: Suorsa
authors.forename: Ammar
authors.initials: A
authors.lastname: Saleem
authors.forename: Natalia
authors.initials: N
authors.lastname: Battchikova
authors.forename: Eva-Mari
authors.initials: EM
authors.lastname: Aro
mesh_terms: D019281:Dimerization; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000096:biosynthesis* / Q000737:chemistry; D045332:Photosystem II Protein Complex / Q000096:biosynthesis* / Q000737:chemistry; D018515:Plant Leaves / Q000378:metabolism; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism; D021122:Protein Subunits; D018724:Spinacia oleracea / Q000378:metabolism*; D020524:Thylakoids / Q000737:chemistry / Q000378:metabolism*
chemical_list: D045342:Light-Harvesting Protein Complexes; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D021122:Protein Subunits