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 G_α subunit possesses the nucleotide-binding pocket that binds guanine nucleotides and switches between GDP or GTP-bound states. In contrast, the G_ꞵ and G_γ subunits are always bound together with high affinity and are together referred to as G_ꞵγ 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 G_ꞵγ  subunit.  GTP-Gα and  G_ꞵγ 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 G_α subunits that can be classified into subfamilies such as G_αs, G_αi, G_αq, G_tα, G_olf, G_12α, and G_13α. Each of these G_α subunits performs a specific function upon binding their effector. For example, the binding of G_αs 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 G_αi inhibits adenylyl cyclase activity and cAMP synthesis.

Other subfamilies of G_α subunits perform various other cellular processes. The G_αq 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 G_tα family binds to rhodopsin to transmit information from  visual stimuli by activating specific phosphodiesterases, which hydrolyze cGMP to GMP.  G_olf is the variant of the G_αs family and is used  in odorant signaling pathways. The family of G_12α and G_13α  proteins are involved in cytoskeleton regulation.

G_α subunits have intrinsic GTPase activity, and binding of GTP-G_α to the effector enhances their rate of GTP hydrolysis. The activated GTP-G_α form is thus short-lived and hydrolyzed to GDP-G_α  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 G_α subunits, and helps shut off G protein-mediated cellular responses.