• Fig. 3.8 GTP-Binding Proteins. GTP binding activates whereas hydrolysis of GTP to GDP inactivates GTP-binding proteins (A). All GTPase are controlled by regulatory proteins, including GTPase-activating proteins (GAP), which induce the hydrolysis of GTP to GDP, thus inactivating the GTPase, and guanine nucleotide exchange factors, (GEF) that which cause the GTPase to release GDP, which is rapidly replaced by GTP, thereby activating the GTPase (B). (Redrawn from Kantrowitx ER, Lipscomb WN. Escherichia coli aspartate transcarbamoylase: the molecular basis for a concerted allosteric transition. Trends Biochem
Sci. 1990;15:53-59.) proteins. Ras GTPases are involved in many signaling pathways that control cell division, proliferation and death. Arf GTPases, like Rab GTPases, regulate vesicular transport.
Heterotrimeric G proteins couple to more than 1000 different receptors and thereby mediate the cellular response to an incredibly diverse set of signaling molecules, including hormones, neurotransmitters, peptides, and odorants. Like monomeric G proteins, they can be membrane bound because of the addition of lipids posttranslationally. Heterotrimeric complexes are composed of three subunits: α, β, and γ. There exist 16 α subunits, 5 β subunits, and 11 γ subunits, which can assemble into hundreds of different combinations and thereby interact with a diverse number of receptors and effectors. The assembly of subunits and the association with receptors and effectors depend on the cell type.
An overview of heterotrimeric G protein activation is illustrated in
Fig. 3.9 . In the absence of ligand, these G proteins are inactive and form a heterotrimeric complex in which GDP binds to the α subunit. Binding of a signal molecule to an inactive GPCR induces a conformational change in the G protein that promotes the release of GDP and the subsequent binding of GTP to the α subunit. Binding of GTP to the α subunit stimulates dissociation of the α subunit from the heterotrimeric complex and results in release of the α subunit from the βγ dimer, each of which can interact with and regulate downstream effectors such as adenylyl cyclase and phospholipases (see
Fig. 3.9 ). Activation of downstream effectors by the α subunit and βγ dimer is terminated when the α subunit hydrolyzes the bound GTP to GDP and inorganic phosphate (Pi). The α subunit bound to GDP associates with the βγ dimer and terminates the activation of effectors.
Another way to attenuate or terminate signaling through a GPCR involves desensitization and endocytic removal of receptors from the plasma membrane. Binding of hormone to a GPCR increases the ability of GPCR kinases to phosphorylate the intracellular domain of GPCRs, which recruits proteins called β-arrestins to bind to the GPCR. β-Arrestins inactivate the receptor and promote endocytic