RGS14 is a bifunctional regulator of G alpha(i/o) activity that exists in multiple populations in brain

Members of the regulators of G protein signaling (RGS) family modulate Ga-d irected signals as a result of the GTPase-activating protein (GAP) activity of their conserved RGS domain. In addition to its RGS domain, RGS14 contai ns a Rap binding domain (RBD) and a GoLoco motif. To define the cellular an d biochemical properties of RGS14 we utilized two different affinity purifi ed antisera that specifically recognize recombinant and native RGS14. In br ain, we observed two RGS14-like immunoreactive bands of distinct size (60 k Da and 55 kDa). Both forms are present in brain cytosol and in two, biochem ically distinct, membrane subpopulations: one detergent-extractable and the other detergent-insensitive. Recombinant RGS14 binds specifically to activ ated 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 no n-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 activ ity than the RGS domain alone. In contrast, neither full-length RGS14 nor t he 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 a lpha (o), and this activity is restricted to the C-terminus containing the GoLoco domain. These findings highlight previously unknown biochemical prop erties of RGS14 in brain, and provide one of the first examples of an RGS p rotein that is a bifunctional regulator of G alpha actions.