The distribution of cell surface receptors for transferrin–iron and epidermal growth factor (EGF) on the surface of cultured epitheloid (A431) cells has been identified by immunocytochemical electron microscopy. The patterns of movement displayed by these two receptor populations as they transfer to their sites of internalization on the cell surface are different. The movement of recycling transferrin receptors over the surface is ligand-independent whereas EGF receptors are more stable residents and remain monodisperse until they bind ligand.
Prior to uptake transferrin receptors cluster, predominantly within existing clathrin-coated pits while the aggregates formed by EGF ligand–receptor complexes induce new membrane invaginations. These results are discussed in relation to receptor populations concerned with constitutive, high capacity uptake processes and receptors involved in signal transduction.