RGS protein and G protein interactions: a little help from their friends.

G protein signaling pathways are essential for all aspects of cell and organ physiology, and the involved proteins have long served as primary drug targets. At the most basic level, these proteins include a signal-receiving G protein-coupled receptor (GPCR), a transducing heterotrimeric G protein (G subunits), and a signal-generating downstream target effector. These proteins work together to transmit signals across the plasma membrane. A neurotransmitter or hormone activated GPCR stimulates the exchange of GDP for GTP on G to initiate heterotrimer dissociation and activation of effector proteins that, in turn, initiate a cascade of cellular signaling events. The regulators of G protein signaling (RGS proteins) participate in this process by binding directly to activated G -GTP to serve as GTPase-activating proteins (GAPs), which limit the lifetime of G -GTP and terminate signaling event(s). RGS proteins are relatively new actors on this stage. All family members contain a signature RGS domain responsible for GAP activity. So far, more than 30 mammalian family members have been identified and classified into seven subfamilies based on sequence identity and functional similarities (De Vries et al., 2000; Ross and Wilkie, 2000; Hollinger and Hepler, 2002). Although many RGS proteins are relatively simple, others are more complex and contain multiple domains for binding various signaling proteins, and accumulated evidence now suggests that RGS proteins act as tightly regulated modulators and integrators of G protein signaling. Much has been learned in recent years about the biochemical mechanisms whereby RGS proteins stimulate the GTPase activity of G (Ross and Wilkie, 2000). However, much less is known about how RGS function(s) are regulated in living cells (Hollinger and Hepler, 2002). After RGS proteins were first shown to act as G GAPs, questions immediately emerged about which RGS proteins talked to which G subunits and how selectivity for these interactions is determined in a cellular context. Given that there are more than 20 G subunits and more than 30 RGS proteins, early speculation predicted that G and RGS proteins form discrete functional pairs. Surprisingly, this has not turned out to be true in most cases. Although certain RGS protein subfamilies do selectively bind and regulate the activity of a specific class of G (for example, p115RhoGEF and G 12/13), this is an exception. Most RGS proteins are perplexingly promiscuous regarding which G they can bind. In reconstitution assays using purified proteins, most can regulate the activity of many members of the G i subfamily or Gq (De Vries et al., 2000). So the question remains: exactly how do RGS proteins and G subunits decide to pair up in living cells? In this issue of Molecular Pharmacology, Roy et al. (2003) provide evidence that G may receive critical help from their linked receptors to recruit a preferred RGS protein. The authors show that two simple RGS proteins, RGS2 and RGS4, are recruited to the plasma membrane by expressing either G subunits (Gi , Gq , or Gs ) or linked GPCRs (M2-muscarinic cholinergic, AT1a-angiotensin, or 2-adrenergic, respectively). Not surprisingly, expression of G proteins initiates RGS protein membrane recruitment, whereas expression of RGS-insensitive G-protein mutants does not. However, among the remarkable observations the authors report is that RGS protein recruitment to membranes also occurs with receptors alone, is specific for receptors functionally linked to the target G protein, and is independent of the activation state of either receptor or G protein. Furthermore, RGS protein membrane recruitment mirrors RGS regulation of G protein function. Together, these findings suggest that GPCRs, either alone or in coordinated effort with their linked G proteins, can selectively recruit certain RGS proteins to the plasma membrane to determine their signaling functions. This work was supported by National Institutes of Health grants R01NS37112-05 and R01-GM61847-02.