Structural design principles that underlie the multi-specific interactions of Gαq with dissimilar partners.

Gαq is a ubiquitous molecular switch that activates the effectors phospholipase-C-β3 (PLC-β3) and Rho guanine-nucleotide exchange factors. Gαq is inactivated by regulators of G protein signaling proteins, as well as by PLC-β3. Gαq further interacts with G protein-coupled receptor kinase 2 (GRK2), although the functional role of this interaction is debated. While X-ray structures of Gαq bound to representatives of these partners have revealed details of their interactions, the mechanistic basis for differential Gαq interactions with multiple partners (i.e., Gαq multi-specificity) has not been elucidated at the individual residue resolution. Here, we map the structural determinants of Gαq multi-specificity using structure-based energy calculations. We delineate regions that specifically interact with GTPase Activating Proteins (GAPs) and residues that exclusively contribute to effector interactions, showing that only the Gαq "Switch II" region interacts with all partners. Our analysis further suggests that Gαq-GRK2 interactions are consistent with GRK2 functioning as an effector, rather than a GAP. Our multi-specificity analysis pinpoints Gαq residues that uniquely contribute to interactions with particular partners, enabling precise manipulation of these cascades. As such, we dissect the molecular basis of Gαq function as a central signaling hub, which can be used to target Gαq-mediated signaling in therapeutic interventions.