22.2: Activation and Inactivation of G Proteins
Heterotrimeric G proteins are guanine nucleotide-binding proteins. As the name suggests, heterotrimeric G proteins are composed of three subunits: alpha, beta, and gamma. They remain GDP-bound or GTP-bound inside the cells and switch between inactive/active states. The Gα subunit possesses the nucleotide-binding pocket that binds guanine nucleotides and switches between GDP or GTP-bound states. In contrast, the Gꞵ and Gγ subunits are always bound together with high affinity and are together referred to as Gꞵγ subunits.
Heterotrimeric G proteins regulate signaling downstream of G protein-coupled receptors (GPCRs) and remain anchored to the membrane by lipid modifications. Myristoylation or palmitoylation at the N-terminus of the Gα subunit, and prenylation at the C-terminus of the Gγ subunit anchor these subunits to the membrane and stabilize the heterotrimeric complex.
Upon ligand binding, GPCRs bind the Gα subunit of heterotrimeric G proteins with high affinity. The receptor-G protein interaction leads to the opening of the nucleotide-binding site of the Gα subunit, releasing the GDP. GPCRs function as guanine nucleotide exchange factors (GEFs) that facilitate Gα-GTP binding.
The activated GTP-Gα subunit undergoes a conformational change and detaches from the receptor and Gꞵγ subunit. GTP-Gα and Gꞵγ subunits now individually participate in signal transduction pathways and activate effectors like adenylyl cyclase (AC), phospholipase Cꞵ (PLCꞵ) and Na+, K+, and Ca2+-specific ion channels, thereby triggering the production of different second messengers.
The human genome encodes 21 different Gα subunits that can be classified into subfamilies such as Gαs, Gαi, Gαq, Gtα, Golf, G12α, and G13α. Each of these Gα subunits performs a specific function upon binding their effector. For example, the binding of Gαs to adenylyl cyclase activates adenylyl cyclase to produce cAMP, an important second messenger. cAMP regulates muscle contraction and the metabolism of fats or sugars. In contrast, the binding of Gαi inhibits adenylyl cyclase activity and cAMP synthesis.
Other subfamilies of Gα subunits perform various other cellular processes. The Gαq family activates PLCꞵ, which produces second messengers like inositol trisphosphate (IP3) and diacylglycerol (DAG). Both IP3 and DAG affect various cellular pathways, including growth and differentiation. The transducin or Gtα family binds to rhodopsin to transmit information from visual stimuli by activating specific phosphodiesterases, which hydrolyze cGMP to GMP. Golf is the variant of the Gαs family and is used in odorant signaling pathways. The family of G12α and G13α proteins are involved in cytoskeleton regulation.
Gα subunits have intrinsic GTPase activity, and binding of GTP-Gα to the effector enhances their rate of GTP hydrolysis. The activated GTP-Gα form is thus short-lived and hydrolyzed to GDP-Gα within minutes, switching back to the inactive state. This feedback mechanism helps avoid the overstimulation of cells in response to a signal. G proteins are also regulated by another family of GAP, Regulators of G protein Signaling (RGS). The human genome encodes 25 RGS proteins, each interacting with a particular set of Gα subunits, and helps shut off G protein-mediated cellular responses.
