PARENT SESSION
Posters P5A Type II reaction centres : Structure. Abstracts (289-312)

Sequential assembly of Photosystem II. Eva-Mari Aro^*,1, Anne Rokka¹, Marjaana Suorsa¹, Virpi Paakkarinen¹, Ammar Saleem¹, Natalia Battchikova¹, ¹ Department of Biology, Plant Physiology and Molecular Biology, University of Turku, Finland

ABSTRACT- Photosystem II (PSII) is a multisubunit pigment-protein complex of thylakoid membranes catalyzing water oxidation and reduction of plastoquinone. It is composed of more than 20 subunits, which are both nuclear- and plastid-encoded. To unravel the assembly steps of both the chloroplast- and nuclear-encoded proteins of PSII, we performed pulse-chase experiments with isolated chloroplasts and intact leaves, followed by Blue Native gel separation of the complexes and subsequent identification of the protein subunits. The smallest PSII assembly contained labeled D2, beta-subunit of Cyt b559 and another LMW protein, most probably PsbI. This complex functioned as a receptor for D1 to form the PSII-RC complex. Then associated CP47, followed by an assembly of PsbH together with at least two other labeled chloroplast-encoded LMW subunits (most likely PsbM and PsbT) and a nuclear-encoded PsbR. Subsequently CP43 inserted into the PSII complex concomitantly with PsbK, and these steps seemed to be essential for dimerization of the PSII core monomers. PSII monomer was the smallest subcomplex harboring the 33 kDa OEC protein. Nuclear-encoded PsbW protein was synthesized only at low light concomitantly with Lhcb polypeptides and accumulated mostly in the PSII-LHCII supercomplexes. The PsbH protein, on the other hand, was vigorously synthesized and incorporated into PSII core monomers together with the D1 protein suggesting an intrinsic role for PsbH in the turnover of the D1 protein.

KEY WORDS: assembly, Photosystem II, PsbH, low molecular weight proteins