The incorporation of iron into human cells involves the binding of diferric transferrin to a specific cell surface receptor. We studied the process of endocytosis in K562, a human erythroid cell line, by using tetramethylrhodamine isothiocyanate-labeled transferrin (TRITC-transferrin) and fluorescein isothiocyanate-labeled Fab fragments of goat antireceptor IgG preparation (FITC-Fab-antitransferrin receptor antibody). Because the antireceptor antibody and transferrin bind to different sites on the transferrin receptor molecule it was possible to simultaneously and independently follow ligand and receptor. At 4 degrees C, the binding of TRITC-transferrin or FITC-Fab antitransferrin receptor antibody exhibited diffuse membrane fluorescence. At 20 degrees C, the binding of TRITC-transferrin was followed by the rapid formation of aggregates. However, the FITC-Fab antitransferrin receptor did not show similar aggregation at 20 degrees C unless transferrin was present. In the presence of transferrin, the FITC-Fab antitransferrin receptor antibody formed aggregates at the same sites and within the same time period as TRITC transferrin, indicating co-migration. Although the diffuse surface staining of either label was removed by proteolysis, the larger aggregates were not susceptible to enzyme degradation, indicating that they were intracellular. The internal location of the aggregates was also demonstrated using permeabilized cells that had been preincubated with transferrin and fixed with 4% paraformaldehyde. These cells showed aggregated receptor in the interior of the cell when reacted with fluorescein-labeled antibody to the receptor. This indicated that the transferrin and the transferrin receptor co-internalize and migrate to the same structures within the cell.

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