As mentioned already, G proteins were originally named because of their ability to bind the guanine nucleotides guanosine triphosphate and guanosine diphosphate. Receptors coupled to G proteins include those that respond to catecholamines, serotonin, ACh, various peptides, and even sensory signals such as light and odorants. Gs and Gi were among the first G proteins identified and received their names because of their ability to stimulate or inhibit adenylyl cyclase. Since that time, a multitude of G protein subunits have been identified by a combination of biochemical and molecular cloning techniques.
Indeed, genes for 16 G° subunits are known and give rise via alternative splicing to at least 20 mature subunits with differential tissue expression (Simonds 2003) (see Table 1-1). There are four homology-based subfamilies of G® subunits: the Gs subfamily, whose members stimulate adenylyl cyclase; the Gj subfamily, which includes Gji-3 and Gz, which inhibit adenylyl cyclase; the Gq subfamily, whose members activate PLC-" ; and the G12 subfamily, whose members interact with regulators of G protein signaling (RGS) domain-containing Rho exchange factors (see Table 1-1 and Figure 1-2) (Simonds 2003). Genes encoding 5 G' isoforms and 12 different GT subunits are known in humans; effectors of G' ' complexes include ion channels, isoforms of adenylyl cyclase, isoforms of PLC-1, and MAP kinase pathways (Simonds 2003) (see Table 1-1).
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