Internalization of biotin-S-S-125I-transferrin (125I-BSST) into semiintact A431 cells were assessed by two different criteria which have allowed us to distinguish partial reactions in the complex overall process of receptor-mediated endocytosis. Early events resulting in the sequestration of ligand into deeply invaginated coated pits were measured by inaccessibility of 125I-BSST to exogenously added antibodies. Later events involving coated vesicle budding and membrane fission were measured by resistance of 125I-BSST to reduction by the membrane impermeant-reducing agent, MesNa. Acquisition of Ab inaccessibility occurred very efficiently in this cell-free system (∼50% of total cell-associated 125I-BSST became inaccessible) and could be inhibited by anti-clathrin mAbs and by antibodies directed against the cytoplasmic domain of the transferrin-receptor. In contrast, acquisition of MesNa resistance occurred less efficiently (∼10-20% of total cell-associated 125I-BSST) and showed differential sensitivity to inhibition by anti-clathrin and anti-transferrin receptor mAbs. Both partial reactions were stimulated by ATP and cytosol; indicating at least two ATP-requiring events in receptor-mediated endocytosis. The temperature dependence of both reactions was similar to that for 125I-BSST internalization in intact cells with no activity being observed below 10deg;C. Morphological studies using gold-labeled ligands confirmed that internalization of transferrin receptors into semiintact A431 cell occurred via coated pits and coated vesicles and resulted in delivery of ligand to endosomal structures.
ASJC Scopus subject areas
- Cell Biology