The Binding of Protein-Ligands to Cell-Surface Receptors

The study of proteins and glycoproteins as biologically active ligands is an expanding area of research, stimulated in part by the identification of growth factors and cytokines and, in particular, those involved in the regulation of hemopoiesis and the inmiune response. The exquisite sensitivity and specificity of cellular responses to minute extracellular concentrations of these macromolecular ligands has demanded the design of equally sensitive assays in order to study their physiological properties. Additionally, abnormal responses owing to receptor or ligand mutation cannot be fully characterized without reliable analysis of the normal state. This chapter discusses some of the practical considerations for studying protein-protein binding. This can best be illustrated by describing the binding of serum-derived asialoglycoproteins to the asialoglycoprotein receptor, which is expressed almost exclusively on the sinusoidal plasma membrane of hepatocytes. This is a particularly definitive assay that identifies equimolar binding between the ligand and a single class of binding sites. However, it must be emphasized that this should not be regarded as a paradigm for ligandreceptor binding assays, but merely a guideline from which other assays appropriate for a particular ligand and its receptor may be fashioned.