Freeze-etch study of an unmodified lectin interacting with its receptors in model membranes

Experiments are described in which liposomes bearing ganglioside or glycophorin as receptor were exposed to the native (unmodified) lectin, wheat germ agglutinin, and subsequently examined by freeze-etch electron microscopy. Visualized in this way in the absence of lectin, phosphatidylcholine bilayer membranes show no features attributable to the presence of small amounts of glycolipid. Similarly bilayers bearing glycophorin show no obvious etch face (outer surface) features attributable to that species, although they do have intramembranous particles associated with the hydrophobic interior. However, the otherwise smooth and featureless model membrane outer surface permitted ready visualization of bound lectin molecules, and thus the indirect localization of receptors. The lipid membrane itself in the vicinity of receptors was not visibly affected by lectin binding to glycolipid or glycoprotein. The only identifiable lectin-induced change was lateral redistribution of receptor glycoproteins. Ganglioside molecules showed some evidence of existing in small clusters; but their distribution was apparently unaffected by lectin binding. Clumps of lectin bound to glycophorin were found associated predominantly with fluid bilayer regions of phase separated membranes; and lectin distribution on the etch face correlated with intramembranous particle distribution in the fracture face. In contrast, ganglioside lateral distribution was not appreciably influenced by the host lipid matrix phase separation.