Fig. 3. Analysis of ErbB2 co-clustering with EGFR and ErbB3 in SKBR3 cells. (A-D) Membrane sheets were prepared from serum-starved cells (A,C) or from cells that were serum starved and then stimulated with either 20 nM EGF for 2 minutes (B) or 3.2 nM heregulin for 5 minutes (D). Sheets in A and B were double labeled from the inside with antibodies to ErbB2 (10-nm gold) and EGFR (5-nm gold); Sheets in C and D were double labeled from the inside for ErbB2 (5-nm gold) and ErB3 (10-nm gold). Circles in A-C, as well as the bracket in D, indicate co-clusters of ErbB2 with either EGFR or ErbB3. Labeled arrows point to clusters containing only a single species of ErbB receptor. Bars, 0.1 µm.