FIGURE 22-36 Signal transduction from the olfactory GPCRs. Binding of an odorant to its cognate odorant receptor (OR) triggers activation of the trimeric G protein G_αolf · G_βγ, releasing the active G_αolf · GTP. Activated G_αolf · GTP in turn activates type III adenylyl cyclase (AC3), leading to the production of cyclic AMP (cAMP) from ATP. Molecules of cAMP bind to and open the cyclic nucleotide–gated (CNG) ion channel, leading to the influx of Na⁺ and Ca²⁺ and depolarizing the cell. cAMP also activates protein kinase A (PKA), which phosphorylates and thus regulates transcription factors and other intracellular proteins.