A G-protein signaling network mediated by an RGS protein.

A wide variety of extracellular stimuli induce signal transduction through receptors coupled to heterotrimeric G proteins, which consist of a, b, and g subunits (Gilman 1987). The Ga subunit has guanine nucleotide binding and GTP hydrolysis activities. Based on function and amino acid sequence homology, the Ga subunits can be classified into four families, including Gas, Gai/o, Gaq, and Ga12 (Simon et al. 1991; Hepler and Gilman 1992). As exemplified by the responsiveness of our five senses to environmental stimuli, signaling mediated by trimeric G proteins is often extremely rapid and transient. A key step in achieving such a rapid response is the ability of the Ga subunit to switch between its GDPand GTP-bound forms. The nucleotide binding state of Ga is regulated at both the GDP dissociation and GTP hydrolysis steps. Stimulation of receptors by agonists leads to a conformational change in the receptors which can function as a guanine nucleotide exchange factor to stimulate a rapid dissociation of GDP from the inactive Ga. The nucleotide-free Ga is then available to bind GTP, leading to the dissociation of Ga from the Gbg heterodimer. Both the Ga and Gbg subunits can interact with and regulate downstream effectors that include adenylyl cyclase, phospholipase C, and ion channels (Gilman 1987; Birnbaumer 1992).

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