Fig. 3.

Ca²⁺ interacts with the amino terminal helix of G_i3. a Membrane-facing surfaces of G_s and G_i1 Ras domains, with β₂AR shown in gray. The membrane-facing surface of G_i1 was modeled by superimposing the Ras domain of G_i1 (PDB: 1GP2⁷⁰) onto the structure of G_s in complex with β₂AR (PDB: 3SN6⁶⁹). Red and blue signify negative and positive charge on G protein, respectively. Dashed lines highlight the region in αN where charge differs: The sequence is KDKQ in WT G_s vs. EDGE in WT G_i1 (and in G_i2, G_i3). b mB−β₂AR−G protein dose−response curves ± 10 mM CaCl₂. Data were generated with the G protein depicted below the curves: mutations were made in αN and corresponding electrostatic models are shown. Epinephrine was not included in experiments titrating G_s WT, G_i3-G_s Chimera, or G_s-neg. to enhance the effect of CaCl₂. Epinephrine (100 μM) was included in experiments titrating G_i3 WT and G_i3-pos. mB−β₂AR concentration is 250 nM. Data are mean ± s.e.m. of three independent experiments. Multiplicity adjusted P values were computed by two-way ANOVA followed by Sidak’s post hoc test between CaCl₂ conditions. Source data are provided in the Source Data File