Alpha subunit of G proteins (guanine nucleotide binding)
The alpha subunit of G proteins contains the guanine nucleotide binding site. The heterotrimeric GNP-binding proteins are signal transducers that communicate signals from many hormones, neurotransmitters, chemokines, and autocrine and paracrine factors. Extracellular signals are received by receptors, which activate the G proteins, which in turn route the signals to several distinct intracellular signaling pathways. The alpha subunit of G proteins is a weak GTPase. In the resting state, heterotrimeric G proteins are associated at the cytosolic face of the plasma membrane and the alpha subunit binds to GDP. Upon activation by a receptor GDP is replaced with GTP, and the G-alpha/GTP complex dissociates from the beta and gamma subunits. This results in activation of downstream signaling pathways, such as cAMP synthesis by adenylyl cyclase, which is terminated when GTP is hydrolized and the heterotrimers reconstitute.