INDUCTION OF NONBILAYER LIPID PHASE SEPARATIONS IN CHLOROPLAST THYLAKOID MEMBRANES BY COMPATIBLE CO-SOLUTES AND ITS RELATION TO THE THERMAL-STABILITY OF PHOTOSYSTEM-II

High concentrations of compatible co-solutes such as sugars, sugar alcohols and polyols induce the phase separation of non-bilayer forming lipids in thylakoid membrane preparations. Of the co-solutes tested, sucrose, trehalose, betaine and sorbitol all led to extensive phase separation at room temperature. Glucose and glycerol were much less effective. Measurements of the temperature dependence of the fluorescence yield of chlorophyll a associated with Photosystem II (PS II), and the relative oxygen evolution efficiency of chloroplasts incubated at elevated temperatures, indicated that the presence of compatible co-solutes led to substantial increases in the threshold temperature for inhibition of PS II-mediated electron transport. The protective effect of the different co-solutes, in order of efficiency, was sucrose > trehalose > sorbitol > betaine > glucose > glycerol. In general, the ability of the different co-solutes to stabilise PS II appeared to parallel their ability to induce non-bilayer lipid phase separation. Comparison of the effects of the co-solutes on isolated chloroplasts with that of heat-stress in unprotected chloroplasts suggest that there is no direct causal link between the phase-separation of non-bilayer lipids from the chloroplast membranes and the increased stability of PS II. It is concluded that the two phenomena are separate reflections of the effects of co-solutes on ordered water at lipid and protein interfaces, respectively.