Gα–Gβγ dissociation may be due to retraction of a buried lysine and disruption of an aromatic cluster by a GTP‐sensing Arg–Trp pair

Abstract

The heterotrimeric G protein α subunit (Gα) functions as a molecular switch by cycling between inactive GDP‐bound and active GTP‐bound states. When bound to GDP, Gα interacts with high affinity to a complex of the β and γ subunits (Gβγ), but when bound to GTP, Gα dissociates from this complex to activate downstream signaling pathways. Gα's state is communicated to other cellular components via conformational changes within its switch I and II regions. To identify key determinants of Gα's function as a signaling pathway molecular switch, a Bayesian approach was used to infer the selective constraints that most distinguish Gα and closely related Arf family GTPases from distantly related translational and metabolic GTPases. The strongest of these constraints are imposed on seven residues within or near the switch II region. Likewise, constraints imposed on Gα but not on other, closely related molecular switches correspond to four nearby residues. These constraints are explained by a proposed mechanism for GTP‐induced dissociation of Gα from Gβγ where an Arg–Trp pair senses the presence of bound GTP leading to conformational retraction of a nearby lysine and to disruption of an aromatic cluster. Within a complex of Giα, Giβγ, and GDP, this lysine establishes greater surface contact with Giβ than does any other residue in Giα, whereas the aromatic cluster packs against a highly conserved tryptophan in Giβ that establishes greater surface contact with Giα than does any other residue in Giβ. Other structural features associated with Gα functional divergence further support the proposed mechanism.