AGS3 inhibits GDP dissociation from Gα subunits of the Gi family and rhodopsin-dependent activation of transducin

Abstract

A number of recently discovered proteins that interact with the α subunits of Gi-like G proteins contain homologous repeated sequences named G protein regulatory (GPR) motifs. Activator of G protein signaling 3 (AGS3), identified as an activator of the yeast pheromone pathway in the absence of the pheromone receptor, has a domain with four such repeats. To elucidate the potential mechanisms of regulation of G protein signaling by proteins containing GPR motifs, we examined the effects of the AGS3 GPR domain on the kinetics of guanine nucleotide exchange and GTP hydrolysis by Giα1 and transducin-α (Gtα). The AGS3 GPR domain markedly inhibited the rates of spontaneous guanosine 5′-O-(3-thiotriphosphate) (GTPγS) binding to Giα and rhodopsin-stimulated GTPγS binding to Gtα. The full-length AGS3 GPR domain, AGS3-(463-650), was ∼30-fold more potent than AGS3-(572-629), containing two AGS3 GPR motifs. The IC50 values for the AGS3-(463-650) inhibitory effects on Giα and transducin were 0.12 and 0.15 μM, respectively. Furthermore, AGS3-(463-650) and AGS3-(572-629) effectively blocked the GDP release from Giα and rhodopsin-induced dissociation of GDP from Gtα. The potencies of AGS3-(572-629) and AGS3-(463-650) to suppress the GDP dissociation rates correlated with their ability to inhibit the rates of GTPγS binding. Consistent with the inhibition of nucleotide exchange, the AGS3 GPR domain slowed the rate of steady-state GTP hydrolysis by Giα. The catalytic rate of Gtα GTP hydrolysis, measured under single turnover conditions, remained unchanged with the addition of AGS3-(463-650). Altogether, our results suggest that proteins containing GPR motifs, in addition to their potential role as G protein-coupled receptor-independent activators of Gβγ signaling pathways, act as GDP dissociation inhibitors and negatively regulate the activation of a G protein by a G protein-coupled receptor.