Regulation of the Subcellular Localization of the G-protein Subunit Regulator GPSM3 through Direct Association with 14-3-3 Protein

G-protein signaling modulator-3 (GPSM3), also known as G18 or AGS4, is a member of the G alpha(i/o)-Loco (GoLoco) motif containing proteins. GPSM3 acts through its two GoLoco motifs to exert GDP dissociation inhibitor activity over G alpha(i) subunits; recently revealed is the existence of an additional regulatory site within GPSM3 directed toward monomeric G beta subunits during their biosynthesis. Here, using in silico and proteomic approaches, we have found that GPSM3 also interacts directly with numerous members of the 14-3-3 protein family. This interaction is dependent on GPSM3 phosphorylation, creating a mode II consensus 14-3-3 binding site. 14-3-3 binding to the N-terminal disordered region of GPSM3 confers stabilization from protein degradation. The complex of GPSM3 and 14-3-3 is exclusively cytoplasmic, and both moieties mutually control their exclusion from the nucleus. Phosphorylation of GPSM3 by a proline-directed serine/threonine kinase and the resultant association of 14-3-3 is the first description of post-translational regulation of GPSM3 subcellular localization, a process that likely regulates important spatio-temporal aspects of G-protein-coupled receptor signaling modulation by GPSM3.