Receptor/ Ligand Trafficking

Abstract In Chapter 2, we explored models of receptor/ligand binding on the cell surface. For each of those models, we assumed that the number and location of receptors are constant, i.e., that no significant synthesis, degradation, internalization, or recycling of receptors occurs over the time frame for which the models apply. Under normal physiological conditions, however, these dynamic trafficking events take place concurrently with receptor/ligand binding. In particular, cell surface receptors and receptor/ligand complexes can be internalized in a process known as receptor-mediated endocytosis (RME). The basic steps of the endocytic cycle have been described in a number of reviews (Brown et al., 1983; Wileman et al., 1985b; Mellman et al., 1986; van Deurs et al., 1989; Schwartz, 1990) and are shown schematically in Figure 3-1. The selection and internalization of ligands via RME plays a role in nutrition (e.g., LDL, transferrin), clearance or retrieval of molecules (e.g., immune complexes, asialoglycoproteins, mannose-6-phosphate glycoproteins) and presumably the elicitation or attenuation of cellular responses (e.g., EGF, chemotactic peptides).