Effects of phospholipase and lipase treatments on photosystem II core dimer from a thermophilic cyanobacterium

Lipids are important components of transmembrane protein complexes. In order to study the roles of lipids in photosystem II (PSII), we treated the PSII core dimer complex from a thermophilic cyanobacterium Thermosynechococcus vulcanus with phospholipase A₂ (PLA₂) and lipase, and examined their effects on PSII structure and function. PLA₂-treatment decreased the content of phospholipid, phosphatidylglycerol (PG) by 59%, leading to a decrease of oxygen evolution by 40%. On the other hand, although treatment with lipase specifically decreased the content of monogalactosyldiacylglycerol (MGDG) by 52%, it decreased oxygen evolution only by 16%. This indicates that PG plays a more important role in PSII than MGDG. Both PLA₂- and lipase-treatments induced neither the dissociation of PSII dimer, nor any loss of polypeptides. The degradation of PG resulted in a damage to the Q_B-binding site as demonstrated from photoreduction activity of 2,6-dichlorophenolindophenol and chlorophyll fluorescence yields in the absence or presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and the dependencies of oxygen evolution on various electron acceptors before and after PLA₂- or lipase-treatments. However, there were approximately three and five molecules of PG and MGDG per PSII reaction center left in the PSII dimeric complex after the PLA₂- and lipase-treatments. These lipids are therefore bound to the interior of the protein matrix and resistant to the lipase treatments. The resistance of these lipids against PLA₂- and lipase-treatments may be a specific feature of PSII from the thermophilic cyanobacterium, suggesting a possible correlation between binding of lipids and thermostability of PSII.