G proteins play a vital role in fundamental signal transduction events, from vision to smell, to neurotransmitter and hormonal activities. 1 In their inactive state (State 1 in Fig. 1), they exist as heterotrimers, formed by the so-called a-subunit (Ga) in complex with GDP and a dimer formed by the tightly bound2 b-and g-subunits (Gb and Gg). Mammals feature a number of Ga subunits3, 4 divided in classes (Gai/o, Gas, Gaq, and Ga12/13) 5–9 with size ranging within 39–52 kDa, pairwise sequence identity (SI) between 35% and 95% and similar architecture. Ga consists of a six helices bundle domain (helical domain, HD) connected by two flexible linkers (Linkers 1 and 2) to the catalytic domain [CD, Fig. 2 (A)], and their interface hosts the GDP cofactor. Gb is made by the N-terminal a-helix followed by a b-propeller domain, formed by seven ‘‘WD’’repeat motifs, each made by $43-amino acids. Gg consists of two helices connected by a loop.

Virtually all of the residues at the interface between the CD of Ga and Gb are conserved or conservatively mutated. 12 Mutations of one of them (Q204, numbering as in Gai1) are associated with a variety of diseases: Q204X (X 5 L, R, K, H) mutations across the families are present in tumors, McCune-Albright syndrome and adenoma. 13–25 The inactive GDP bound state binds to G protein-coupled receptors (GPCRs), when these are activated by ligand binding (State 2 in Fig. 1) or by light exposure (in vision). This causes a structural rearrangement: bioluminescence resonance energy transfer (BRET) experiments indicate that structural changes of Gai1 involve the entire HD, 26 and, as shown by EPR spectroscopy, a movement of the a5 helix. 27 These rearrangements lead to