RGS14 is a bifunctional regulator of Gαi/o activity that exists in multiple populations in brain

Members of the regulators of G protein signaling (RGS) family modulate Ga-directed signals as a result of the GTPase-activating protein (GAP) activity of their conserved RGS domain. In addition to its RGS domain, RGS14 contains a Rap binding domain (RBD) and a GoLoco motif. To define the cellular and biochemical properties of RGS14 we utilized two different affinity purified antisera that specifically recognize recombinant and native RGS14. In brain, we observed two RGS14-like immunoreactive bands of distinct size (60 kDa and 55 kDa). Both forms are present in brain cytosol and in two, biochemically distinct, membrane subpopulations: one detergent-extractable and the other detergent-insensitive. Recombinant RGS14 binds specifically to activated G alpha (i/o), but not G alpha (q/11), G alpha (12/13), or G alpha (s) in brain membranes. In reconstitution studies, we found that RGS14 is a non-selective GAP for G alpha (i), and G alpha (o) and that full-length RGS14 is an approximately 10-fold more potent stimulator of G alpha GTPase activity than the RGS domain alone. In contrast, neither full-length RGS14 nor the isolated RBD domain is a GAP for Rapt. RGS14 is also a highly selective guanine nucleotide dissociation inhibitor (GDI) for G alpha (i) but not G alpha (o), and this activity is restricted to the C-terminus containing the GoLoco domain. These findings highlight previously unknown biochemical properties of RGS14 in brain, and provide one of the first examples of an RGS protein that is a bifunctional regulator of G alpha actions.