Lateral distribution of gangliosides in bilayer membranes: lipid and ionic effects

Native Ricinus communis agglutinin (RCA 60) has been used as a visual marker to localize the ganglioside GM1 under a variety of conditions in model membranes. By employing the technique of freeze-etch electron microscopy, it was possible to resolve membrane structural details down to some 3 nm. The most striking feature was frequent existence of the marker lectin in clusters--presumably reflecting the underlying presence of clustered receptor. This feature persisted at glycolipid concentrations from 0.5 to 7 mol %. It was visible in membranes of single pure phospholipids above and below their phase transition temperatures, in membranes of mixed phospholipids, and in membranes containing cholesterol. The presence of cations, including Ca2+, was not seen to alter the pattern of lectin binding at a resolution of 3 nm. In pure dipalmitoyl phosphatidylcholine, a lining-up of glycolipid in the "troughs" between ripples in rigid lipid was apparent, in agreement with a similar phenomenon reported by Tillack et al for a neutral glycosphingolipid in pure dimyristoyl phosphatidylcholine [1982: Biochim Biophys Acta 691:261-273]; but this feature was not evident in host matrices composed of several different lipids.