Stabilization of Interdomain Interactions in G protein α i Subunits Determines Gα i Subtype Signaling Specificity.

Highly homologous members of the Gα i family, Gα i1-3 , have distinct tissue distributions and physiological functions, yet the functional properties of these proteins with respect to GDP/GTP binding and regulation of adenylate cyclase are very similar. We recently identified PDZ-RhoGEF (PRG) as a novel Gα i1 effector, however, it is poorly activated by Gα i2 . Here, in a proteomic proximity labeling screen we observed a strong preference for Gα i1 relative to Gα i2 with respect to engagement of a broad range of potential targets. We investigated the mechanistic basis for this selectivity using PRG as a representative target. Substitution of either the helical domain (HD) from Gα i1 into Gα i2 or substitution of a single amino acid, A230 in Gα i2 to the corresponding D in Gα i1 , largely rescues PRG activation and interactions with other Gα i targets. Molecular dynamics simulations combined with Bayesian network models revealed that in the GTP bound state, dynamic separation at the HD-Ras-like domain (RLD) interface is prevalent in Gα i2 relative to Gα i1 and that mutation of A230 s4h3.3 to D in Gα i2 stabilizes HD-RLD interactions through formation of an ionic interaction with R145 HD.11 in the HD. These interactions in turn modify the conformation of Switch III. These data support a model where D229 s4h3.3 in Gα i1 interacts with R144 HD.11 stabilizes a network of interactions between HD and RLD to promote protein target recognition. The corresponding A230 in Gα i2 is unable to form the "ionic lock" to stabilize this network leading to an overall lower efficacy with respect to target interactions. This study reveals distinct mechanistic properties that could underly differential biological and physiological consequences of activation of Gα i1 or Gα i2 by GPCRs.