Heterotrimeric G Proteins

Core Concept

Heterotrimeric G proteins are the molecular switches that couple activated GPCRs to intracellular effectors. Each is a trimer of Gα, Gβ, and Gγ subunits. In the resting state, Gα is bound to GDP and associated with the Gβγ dimer. When a ligand-activated GPCR acts as a guanine-nucleotide exchange factor (GEF), it induces Gα to release GDP and bind GTP, whereupon Gα-GTP and free Gβγ separate and each regulates downstream effectors (adenylyl cyclase, phospholipase C, ion channels). Gα's intrinsic GTPase activity, accelerated by RGS proteins, hydrolyzes the GTP back to GDP, allowing the subunits to reassociate and turn the switch off.

Key Components

• Gα subunit: binds GDP/GTP; has intrinsic GTPase activity; carries lipid anchor (myristate/palmitate) for membrane attachment.
• Gβγ dimer: an obligate heterodimer (Gγ is prenylated); in the resting state it holds Gα-GDP near the receptor; when released, it is a second messenger-regulator in its own right.
• Gα subfamilies: Gαs (stimulates adenylyl cyclase → cAMP↑), Gαi (inhibits adenylyl cyclase → cAMP↓; also opens K⁺ channels via Gβγ), Gαq (activates phospholipase C-β → IP3/DAG/Ca²⁺), Gα12/13 (activates RhoGEFs → cytoskeletal regulation).
• Effectors: adenylyl cyclase, phospholipase C-β, ion channels (e.g., GIRK), phosphodiesterases.
• RGS proteins: GTPase-activating proteins (GAPs) that speed up Gα's GTP hydrolysis, turning the signal off faster.
• GRKs + arrestins: phosphorylate/bind the receptor to stop G-protein activation (desensitization).

Mechanism / How It Works

1. Resting: Gα-GDP is bound to Gβγ, and the trimer associates with an inactive GPCR.
2. Ligand binding activates the GPCR, which contacts Gα and acts as a GEF: it opens Gα's nucleotide pocket so GDP dissociates.
3. Because GTP is ~10× more abundant than GDP in the cytoplasm, GTP binds Gα, causing a conformational change.
4. GTP-bound Gα dissociates from Gβγ; both Gα-GTP and free Gβγ are now active and diffuse along the membrane to regulate their effectors.
5. For example, Gαs-GTP activates adenylyl cyclase → cAMP↑ → PKA; Gαq-GTP activates PLC-β → IP3 + DAG → Ca²⁺ release and PKC activation.
6. Gα's intrinsic GTPase hydrolyzes GTP → GDP (accelerated by RGS GAPs), returning Gα to its GDP state, which reassociates with Gβγ — the switch turns off.

Energy and Directionality

The G-protein cycle is a GTP-driven switch: the energy of the γ-phosphate bond (from GTP) is spent on Gα's hydrolysis, and this hydrolysis is what makes the "on" state transient and irreversible in direction (Gα-GDP cannot spontaneously activate effectors). The cycle is further made directional by the ~10:1 GTP:GDP ratio favoring activation, and by GEF (receptor) and GAP (RGS) activities that push the switch on and off, respectively.

Experimental Evidence / Technique

• GTPγS (non-hydrolyzable) assays: agonist + GTPγS causes persistent Gα activation (measured as radiolabeled GTPγS binding), proving GTP binding is the activation step.
• Cholera toxin: ADP-ribosylates Gαs, locking it in the GTP-bound (on) state → persistent cAMP↑ → intestinal fluid secretion (diarrhea). This mapped Gαs → adenylyl cyclase.
• Pertussis toxin: ADP-ribosylates Gαi, locking it in the GDP-bound (off) state (prevents receptor coupling), implicating Gαi in inhibitory pathways (whooping cough).
• AlF₄⁻ (aluminum fluoride): mimics the γ-phosphate and constitutively activates Gα, used to identify Gα subtypes and effectors.
• Gα knockout / dominant-negative mutants: deleting Gαq, Gαs, etc., ablates specific effector responses, assigning each subfamily to its pathway.

How it works

1. Resting: Gα-GDP is bound to Gβγ, and the trimer associates with an inactive GPCR.
2. Ligand binding activates the GPCR, which contacts Gα and acts as a GEF: it opens Gα's nucleotide pocket so GDP dissociates.
3. Because GTP is ~10× more abundant than GDP in the cytoplasm, GTP binds Gα, causing a conformational change.
4. GTP-bound Gα dissociates from Gβγ; both Gα-GTP and free Gβγ are now active and diffuse along the membrane to regulate their effectors.
5. For example, Gαs-GTP activates adenylyl cyclase → cAMP↑ → PKA; Gαq-GTP activates PLC-β → IP3 + DAG → Ca²⁺ release and PKC activation.
6. Gα's intrinsic GTPase hydrolyzes GTP → GDP (accelerated by RGS GAPs), returning Gα to its GDP state, which reassociates with Gβγ — the switch turns off.

Common confusions

• "The G protein is inside the GPCR." — They are separate proteins; the GPCR activates the G protein by contact (GEF action).
• "Gβγ is just a passive anchor." — Free Gβγ is an active signal (opens GIRK channels, activates kinases/PLCβ), not merely a brake.
• "GTP hydrolysis activates the G protein." — GTP binding activates; hydrolysis turns it off.
• "Cholera and pertussis toxins do the same thing." — Cholera locks Gαs on (cAMP↑); pertussis locks Gαi off (blocks its inhibitory signal) — opposite subunits and effects.
• "All Gα subunits activate adenylyl cyclase." — Only Gαs does; Gαi inhibits it, and Gαq/Gα12/13 act on entirely different effectors.

Quick review

• Gαβγ: Gα-GDP + Gβγ (resting) → GPCR (GEF) → GTP binds → Gα-GTP + Gβγ dissociate.
• Effectors by subtype: Gαs (AC↑), Gαi (AC↓), Gαq (PLCβ), Gα12/13 (Rho).
• Off: intrinsic GTPase + RGS GAPs → GDP → reassociation.
• Toxins: cholera (Gαs locked on), pertussis (Gαi locked off).
• Gβγ is an active signal; lipids anchor the trimer to the membrane.
• Evidence: GTPγS, cholera/pertussis toxins, AlF₄⁻, Gα knockouts.