Streptavidin clusters bTfnR at the plasma membrane. (A) Immunoblot of lysates from bTfnR-expressing cells treated with either A1 or A4 for 10 min and probed with αSA-HRP to identify SA-bound bTfnR. (B) Intensity profiles of the immunoblot in A showing multiple higher molecular weight species (arrows) in A4-treated cells. (C) Electron micrographs of the ventral surface of un-roofed bTfnR-expressing cells treated with either A1 (topl) or A4 (bottom). TfnRs were labeled with D65 immunogold particles. Small circles indicate pairs of gold particles bound to TfnR dimers; large circles indicate clusters of gold particles, exclusively found in A4-treated cells. Insets: enlarged areas highlighted by the dotted squares. (D) Thin-section electron micrographs of bTfnR-expressing cells treated with either A1 (top two panels) or A4 (bottom two panels). Representative micrographs of shallow CCPs show more gold particles in CCPs in cells treated with A4, as summarized in the table.