Mammalian RGS Proteins: Barbarians at the Gate*

Hundreds or thousands of chemical and physical stimuli regulate the functions of eukaryotic cells by controlling the activities of a surprisingly small number of core signaling units that have been duplicated and adapted to achieve the necessary diversity. The most prevalent of these units, at least in animal cells, are three-protein modules consisting of signal recognition elements (receptors) and signal generators (effectors) whose activities are linked and coordinated by heterotrimeric guanine nucleotide-binding proteins or G proteins. Collectively, mammalian cells contain hundreds of G proteincoupled receptors and dozens of effectors. It is difficult to count functionally distinct G proteins because we do not understand the significance of the heterogeneity offered by the possible combination of 16 a, 5 b, and at least 12 g subunits (for reviews, see Refs. 1–5). GDP-bound G protein a subunits have high affinity for a tight complex of b and g subunits. This interaction of a with bg occludes the sites of interaction of both of these signaling molecules with downstream effectors, and the inactive state is maintained by an extremely slow rate of dissociation of GDP from the oligomer (k ; 0.01/min). An agonist-bound receptor (typically a 35–60-kDa protein with seven plasma membranespanning helices) activates an appropriate G protein by poorly understood interactions that promote dissociation of GDP. High intracellular concentrations of GTP ensure a transient existence of the nucleotide-free G protein, and binding of GTP causes conformational changes in a that result in dissociation of GTP-a from bg. Both of these complexes can then activate or inhibit signaling pathways by engaging in interactions with effectors such as adenylyl cyclases, phospholipases, cyclic nucleotide phosphodiesterases, and ion channels. Termination of signaling is dependent on the GTPase activity of a. Typically slow (kcat ; 4/min) hydrolysis of GTP to GDP (which remains protein bound) promotes dissociation of a from effectors and reassociation with bg. The slow intrinsic rate of GTP hydrolysis by Ga proteins is regulated by interactions with so-called GTPase-activating proteins or GAPs. GAPs were first recognized as regulators of protein synthesis factors and low molecular weight GTPases such as Ras. It is now appreciated that certain effectors in G protein-regulated pathways act as GAPs on cognate Ga proteins (6, 7) and that there exists a large, newly discovered family of GAPs for Ga proteins known as regulators of G protein signaling or RGS proteins. Although one critical biochemical property of this novel RGS protein family has been defined, knowledge of the requisite regulation of these regulators is negligible. There are hints, however, that these proteins may be poised at centers of signaling to intercept activated G proteins, acting, from a G protein’s point of view, as “barbarians at the gate” of cellular signaling.