Freeze-fracture analysis of membrane appression and protein segregation in model membranes containing the chlorophyll-protein complexes from chloroplasts.

Cation-induced membrane appression and lateral segregation of chlorophyll-protein complexes have been investigated by freeze-fracture analysis of model membranes containing photosystem 1 and the light-harvesting complex of photosystem 2. In light-harvesting complex proteoliposomes, cations caused extensive membrane adhesion and a segregation of protein into appressed regions. A marked flattening of the appressed membranes, sometimes together with a co-alignment of the particles on the opposing membrane faces, strongly suggests a direct transmembrane attraction between the protein particles. Photosystem 1 membranes were not appressed by cations but some clustering of the particles occurred, together with their exclusion from some regions of the lipid. By incorporating dipalmitoylglycerophosphocholine into the membranes, it is shown that a similar exclusion of the particles can occur due to liquid-crystalline to gel state transitions of the lipids. Proteoliposomes containing both the light-harvesting complex and photosystem 1 displayed cation-induced membrane appression, but only between regions containing the light-harvesting complex. Photosystem 1 was largely confined to unappressed membranes. Destacking occurred at low salt concentrations irrespective of whether photosystem 1 was present, showing that in proteoliposomes, and probably in thylakoids, this process does not require the presence of highly charged complexes from unappressed regions.