A common feature of a wide range of membrane complexes is that they exist in oligomeric states in vivo despite the fact that when isolated as monomers they can carry out their basic enzymic activities. From 2D cristal studies it is known that the photosystem II complex is dimeric. Redox controlled reversible phosphorylation occurs in higher plants at the N-terminus of the D1, D2, CP43 of PSII (12). A non-chlorophyll 10 kDa protein located in the core and encoded by the psbH gene also undergoes reversible phosphorylation (12).

The function of the phosphorylation of D1, D2, CP43 and PsbH proteins in higher plant chloroplasts is not clear. Evidence is emerging, however, that the phosphorylation of these components plays a role in the regulation of D1 protein turnover. The turnover of this reaction center protein seems to be closely linked to the damage and repair of PSII which occurs as a consequence of its activity as a water oxidase and which increases under conditions of exposure to excess light. In our studies we were able to show that, whether phosphorylated or not, the dimeric form of the PSII complex was more stable than the monomeric form. However, when treated with photoinhibitory light the isolated dimers converted to monomers in their non-phosphorylated state but not when phosphorylated. Phosphorylation, however, did not prevent photoinhibition as judged by the loss of oxygen evolving activity. In a new model we propose a role of PSII phosphorylation in controlling the conversion of dimeric PSII to its monomeric form and in this way regulate the rate of degradation of D1 protein during the photoinhibitory repair cycle

The functional role of the dimeric organization is as yet not understood nor is its implication for conformational changes which must occur as a consequence of the light induced turnover of the D1 reaction center protein . The dynamic process of D1 protein exchange is reported to involve the monomerization of the PSII core dimer followed by the release of CP43. It is suggested that in this way the damaged D1 protein is exposed, allowing its exchange for a new, fully functional D1 subunit. Little or nothing is known about the factors controlling these structural changes and nor do we have any knowledge of the role of lipids in the thylakoid membrane in stabilizing the different conformational states of PSII. The thylakoid membrane in which PSII is embedded consists of phosphatidyl glycerol (PG), monogalactosyl diacyl glycerol (MGDG), digalactosyl diacyl glycerol (DGDG), sulphoquinovosyl diacyl glycerol (SQDG) and traces of phosphatidyl choline (PC). The distribution of these lipids within the thylakoid membrane has been studied in considerable detail. In particular, PG with its unusual trans-D3-hexadecanoic fatty acid (C16:1-D 3tr) at the sn-2 position is reported to have a structural and functional role in PSII (24-26) and is tightly bound to the D1 protein of PSII in cyanobacteria . Furthermore, it is reported to be involved in the trimerization of LHCII and the dimerization of LHCI. In htis project we are focused on the possible involvement of specific PG molecules in PSII dimer-monomer inter-conversion and the release of CP43 from the D1/D2/CP47 subcomplex. The molecular dynamics of PSII dimer dissociation is under investigation in the light of recently determined structural information obtained for the PSII core dimer and the monomeric CP47-RC complex by cryo-electron crystallography and with respect to D1 turnover studies.