Activation and Inactivation of G Proteins

Heterotrimeric G proteins consist of an alpha subunit, a beta subunit, and a gamma subunit. The alpha subunit contains the nucleotide-binding pocket that binds guanine nucleotides. The beta and gamma subunits remain tightly bound together as a functional unit called Gβγ. All three subunits are anchored to the cell membrane through lipid modifications.

When a ligand binds to a GPCR, the receptor undergoes a conformational change and binds the Gα subunit with high affinity. This interaction opens the nucleotide-binding site, releasing bound GDP. The GPCR acts as a guanine nucleotide exchange factor, allowing GTP to bind the Gα subunit. GTP-bound Gα then dissociates from the receptor and Gβγ subunits to activate downstream effectors.

When GTP-bound Gα binds effectors like adenylyl cyclase or phospholipase C-beta, it activates them to produce second messengers. For example, adenylyl cyclase produces cAMP, which regulates muscle contraction and metabolism. Phospholipase C-beta generates IP3 and DAG, affecting cellular growth and differentiation. Both GTP-Gα and free Gβγ subunits can independently activate their respective effectors.

G-alpha subunits possess intrinsic GTPase activity that hydrolyzes GTP to GDP within minutes. Binding of GTP-Gα to effectors enhances this hydrolysis rate. Once GTP is hydrolyzed to GDP, Gα can no longer bind effectors and reassociates with Gβγ subunits, returning the complex to its inactive resting state. This feedback mechanism prevents overstimulation of cells.

The human genome encodes 21 Gα subunits classified into subfamilies: Gαs activates adenylyl cyclase to produce cAMP; Gαi inhibits adenylyl cyclase; Gαq activates phospholipase C-beta; Gtα transmits visual signals; Golf mediates odorant signaling; and G12α and G13α regulate cytoskeleton. Each subfamily performs specific cellular functions based on their effector interactions.

Regulators of G protein Signaling (RGS) proteins enhance the intrinsic GTPase activity of Gα subunits, accelerating GTP hydrolysis and terminating signaling. The human genome encodes 25 RGS proteins, each interacting with specific Gα subunits. This regulatory mechanism helps shut off G protein-mediated cellular responses and prevents excessive signal amplification.

G proteins are membrane-anchored through lipid modifications on their subunits. Myristoylation or palmitoylation at the N-terminus of the Gα subunit anchors it to the membrane. Prenylation at the C-terminus of the Gγ subunit provides additional membrane anchoring. These lipid modifications stabilize the heterotrimeric complex and keep G proteins positioned near GPCRs for efficient signal relay.