Inhibits signal transduction by increasing the GTPase activity of G protein alpha subunits thereby driving them into their inactive GDP-bound form. Binds to GNAT1. Involved in phototransduction; key element in the recovery phase of visual transduction. Heterodimer with GNB5. Interacts with RGS7BP, leading to regulate the subcellular location of the heterodimer formed with GNB5 (PubMed:15632198, PubMed:15897264). Component of the RGS9-1-Gbeta5 complex composed of RGS9 (RGS9-1), Gbeta5 (GNB5) and RGS9BP (PubMed:12119397, PubMed:12499365, PubMed:14614075, PubMed:16908407). Interacts with PDE6G and GNAT1 (By similarity). Isoform 1 is targeted to the membrane via its interaction with RGS9BP. Isoform 1 is expressed in photoreceptor outer segments. Isoform 2 is expressed in brain striatum. Retinal isoform 1 is light-dependent phosphorylated at 'Ser-475'. Phosphorylation is decreased by light exposition. Interaction with RGS9BP is decreased when isoform 1 is phosphorylated at 'Ser-475'.