ABSTRACT
Activator of G protein signaling 3 (AGS3) is a newly identified protein shown to act at the level of the G protein itself. AGS3 belongs to the GoLoco family of proteins, sharing the 19-aa GoLoco motif that is a Galpha(i/o) binding motif. AGS3 interacts only with members of the Galpha(i/o) subfamily. By surface plasmon resonance, we found that AGS3 binds exclusively to the GDP-bound form of Galpha(i3). In GTPgammaS binding assays, AGS3 behaves as a guanine dissociation inhibitor (GDI), inhibiting the rate of exchange of GDP for GTP by Galpha(i3). AGS3 interacts with both Galpha(i3) and Galpha(o) subunits, but has GDI activity only on Galpha(i3), not on Galpha(o). The fourth GoLoco motif of AGS3 is a major contributor to this activity. AGS3 stabilizes Galpha(i3) in its GDP-bound form, as it inhibits the increase in tryptophan fluorescence of the Galpha(i3)-GDP subunit stimulated by AlF(4)(-). AGS3 is widely expressed as it is detected by immunoblotting in brain, testis, liver, kidney, heart, pancreas, and in PC-12 cells. Several different sizes of the protein are detected. By Northern blotting, AGS3 shows 2.3-kb and 3.5-kb mRNAs in heart and brain, respectively, suggesting tissue-specific alternative splicing. Taken together, our results demonstrate that AGS3 is a GDI. To the best of our knowledge, no other GDI has been described for heterotrimeric G proteins. Inhibition of the Galpha subunit and stimulation of heterotrimeric G protein signaling, presumably by stimulating Gbetagamma, extend the possibilities for modulating signal transduction through heterotrimeric G proteins.
Subject(s)
GTP-Binding Protein alpha Subunits, Gi-Go/metabolism , GTP-Binding Proteins/metabolism , Guanine Nucleotide Dissociation Inhibitors/metabolism , Guanine/metabolism , Aluminum Compounds , Amino Acid Motifs , Amino Acid Sequence , Animals , Cell Fractionation , Fluorides , Guanosine Diphosphate/metabolism , Heterotrimeric GTP-Binding Proteins/metabolism , Molecular Sequence Data , Rats , Tissue DistributionABSTRACT
Studies of the desensitization of G protein-coupled signal transduction have led to the discovery of a family of guanosine triphosphatase-activating proteins (GAPs) for heterotrimeric G protein alpha subunits - the "regulator of G protein signaling" or RGS proteins. In considering both documented and potential functions of several RGS protein family members with demonstrable multidomain compositions (p115RhoGEF, PDZRhoGEF, Axin, Axil/Conductin, D-AKAP2, the G protein-coupled receptor kinases [GRKs], the DEP/GGL/RGS subfamily [RGS6, RGS7, RGS9, RGS11], and RGS12), this review explores the shift in our appreciation of the RGS proteins from unidimensional desensitizing agents to multifocal signal transduction regulators.
