We present comprehensive maps at single–amino acid resolution of the residues stabilizing the human Gα_i1 subunit in nucleotide- and receptor-bound states. We generated these maps by measuring the effects of alanine mutations on the stability of Gα_i1 and the rhodopsin–Gα_i1 complex. We identified stabilization clusters in the GTPase and helical domains responsible for structural integrity and the conformational changes associated with activation. In activation cluster I, helices α1 and α5 pack against strands β1–β3 to stabilize the nucleotide-bound states. In the receptor-bound state, these interactions are replaced by interactions between α5 and strands β4–β6. Key residues in this cluster are Y320, which is crucial for the stabilization of the receptor-bound state, and F336, which stabilizes nucleotide-bound states. Destabilization of helix α1, caused by rearrangement of this activation cluster, leads to the weakening of the interdomain interface and release of GDP.