Fig. 3. (A–D) Dynamin-dependent sequestration of ß-adrenergic receptor in isoproterenol-treated Paramecium cells, visualized by post-embedding immunogold electron microscopy. Immunolocalization of ßAR (10 nm gold particles) and dynamin (5 nm gold particles), was performed as described in Materials and methods. Colocalization of ßAR and dynamin in the small endocytic vesicles is observed (A, arrowheads; B–D). No or only scarce ßAR on the surface membrane is observed (A, arrow). (E) In untreated cells ßAR and dynamin colocalized on the plasma membrane. (F–H) Immunolocalization of ßAR alone (F) and dynamin alone (G,H), detected by anti-ßAR and anti-dynamin antibodies, respectively. A significant presence of ßAR on the cell surface (F) occurs, whereas dynamin was localized both on the membrane (H) and in the coated pits (G). Bars, 100 nm.