Heterotrimeric guanine nucleotide-binding proteins (G proteins) function as transducers downstream of G protein-coupled receptors (GPCRs) in numerous signaling cascades. The alpha chain contains the guanine nucleotide binding site and alternates between an active, GTP-bound state and an inactive, GDP-bound state. Signaling by an activated GPCR promotes GDP release and GTP binding. The alpha subunit has a low GTPase activity that converts bound GTP to GDP, thereby terminating the signal. Both GDP release and GTP hydrolysis are modulated by numerous regulatory proteins. Signaling is mediated via effector proteins, such as adenylate cyclase. Inhibits adenylate cyclase activity, leading to decreased intracellular cAMP levels. Stimulates the activity of receptor-regulated K(+) channels. May play a role in cell division. Heterotrimeric G proteins are composed of 3 units; alpha, beta and gamma. The alpha subunit contains the guanine nucleotide binding site. GTP binding causes dissociation of the heterotrimer, liberating the individual subunits so that they can interact with downstream effector proteins (By similarity). Localizes in the centrosomes of interphase and mitotic cells. Detected at the cleavage furrow and/or the midbody. Belongs to the G-alpha family. G(i/o/t/z) subfamily.