Visualization and characterization of receptor clusters by transmission electron microscopy.

Multivalency is a feature common to many natural glycoconjugates, including glucosaminoglycans, mucins, and lipopolysaccharides. Multivalent ligands often bind avidly to their target receptors. In addition, they can simultaneously bind multiple receptors and cause them to become clustered (Fig. 1). Receptor clustering by multivalent ligands modulates the cell adhesion and signaling functions of many carbohydratebinding proteins, including galectins and selectins. Not all clusters, however, are equivalent in signaling potency or adhesion properties. Clusters that contain multiple copies of a receptor often elicit greater responses; therefore, the stoichiometry of receptor–ligand clusters can be an important determinant of activity. The relationship between stoichiometry and activity is evident in the systems that govern cellular proliferation,