a, Representative Western Blot analysis of the HER2 (wild type, S310F or S310F mutant) and HER3 constructs co-transfected in the EXPI293 cells, and pulled-down via NRG1β immobilized on FLAG beads. TS – Twin Strep tag (present on both HER2 and HER3). Receptors were detected with the Strep-Tactin® HRP conjugate (anti-TS). For gel source data, see Supplementary Figure 1. b, Densitometry analysis of blots for data shown in panel a. Values are presented as mean values +/− SD of mean intensity ratios of HER2 over HER3 for each blot in n=3 independent biological replicates. The HER2/HER3 pulldown ratios are 0.09 +/− 0.04 for HER2-WT, 0.20 +/− 0.06 for HER2-S310F and 0.35 +/− 0.09 for HER2-S310Y complexes. Significance was determined via unpaired, two-tailed t-test via GraphPad Prism. WT vs S310F: p=0.0605, t=2.592, df=4. WT vs. S310Y: p=0.0107, t=4.516, df=4. c, Overlay of representative size exclusion chromatogram profiles from a Superose6 10/300 Increase column (GE Healthcare) of WT and oncogenic HER2-S310F heterocomplexes. Heterodimer peaks are denoted by asterisks. d, Cartoon representation of the cryo-EM structure of HER2-S310F/HER3/NRG1β complex overlayed on HER2/HER3/NRG1β. The HER2-S310F mutation is shown in red.