Selective externalization of the transferrin receptor by sheep reticulocytes in vitro. Response to ligands and inhibitors of endocytosis.

The transferrin receptor of sheep reticulocytes is released in vesicular form during in vitro incubation of the reticulocytes. A polyclonal antibody against the transferrin receptor slows down the release of the vesicles bearing the receptor, whereas transferrin and calf serum accelerate vesicle release. Vesicle formation and receptor release are inhibited at low temperatures and by the presence of inhibitors of ATP formation. In addition, lysosomotropic agents or transglutaminase inhibitors block receptor externalization. The externalized receptor has the same molecular size and peptide map as the receptor isolated from the membrane, suggesting that an intact receptor is removed and released from the cell. An unidentified peptide of 70 kDa is externalized with the transferrin receptor. Peptide maps show that the 70-kDa species is not a degradation product of the receptor. No function has yet been assigned to the 70-kDa peptide.