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

Abstract

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 A(2) (PLA(2)) and lipase, and examined their effects on PSII structure and function. PLA(2)-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 PLA(2)- 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 PLA(2)- 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 PLA(2)- 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 PLA(2)- 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.