AbstractWe have studied the process of transferrin endocytosis in human erythromyeloid cell line K562 using fluorescein (FL) and rhodamine (RD) labeled iron‐saturated transferrin (FeTF), and a fluorescein labeled monoclonal antibody to the transferrin receptor (FL‐mAB). Because the antireceptor antibody and FeTF bind to different sites on the TF receptor molecule, it is possible to simultaneously and independently follow receptor and ligand. We have measured the relative amounts of transferrin or antireceptor antibody bound in the presence or absence of proteolytic enzymes using a cell sorter. At 4°C almost all of the FL‐TF and the FL‐mAB is surface bound in a diffuse pattern. Within minutes of elevating the temperature to 37°C surface aggregates form and the FL‐TF is internalized. At this time about one sixth of the transferrin is still surface bound and accessible to papain digestion. The remainder localizes in a perinuclear cluster of vesicles. Monoclonal antibody binds to the cell surface transferrin receptor but is not internalized at 4°C or 37°C. When unlabeled diferric transferrin is added, it promotes the uptake of the FI‐mAB. The addition of goat anti‐mouse immunoglobulin also promotes FL‐mAB uptake. These studies support the concept that a specific trigger is required for transferrin r
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