Enhances GTPase-activating protein (GAP) activity of regulator of G protein signaling (RGS) proteins, hence involved in the termination of the signaling initiated by the G protein coupled receptors (GPCRs) by accelerating the GTP hydrolysis on the G-alpha subunits, thereby promoting their inactivation (Probable). Increases RGS9 GTPase-activating protein (GAP) activity, hence contributes to the deactivation of G protein signaling initiated by D(2) dopamine receptors (By similarity). May play an important role in neuronal signaling, including in the parasympathetic, but not sympathetic, control of heart rate (By similarity). Component of a complex composed of RGS9, GNB5 and RGS9BP; within this complex, the presence of GNB5 stabilizes both itself and RGS9 and increases RGS9 GTPase-activating protein (GAP) activity (By similarity). Interacts with RGS6 and RGS7 (By similarity). Belongs to the WD repeat G protein beta family.