G-Protein Coupled Receptors

1. An Introduction to Receptors
2. Structure of GPCRs and G-Proteins
3. Signal Transduction
4. Signal Amplification
5. Clinically Relevant Examples
6. Quiz

• Cell signalling can be summarised by the following pathway: Ligand-receptor binding (a ligand is a signalling molecule that binds to a receptor) => Signal transduction => Effector protein(s) (proteins that carry out the next step of the response) => Cellular response.
• G-protein coupled receptors (GPCRs) are cell surface receptors with 7 transmembrane domains (stretches of the receptor that cross the cell membrane); they are coupled to G-proteins, which are peripheral proteins.
• G-proteins have an alpha subunit and a beta-gamma subunit (a subunit is one of the separate protein chains that make up the whole G-protein). The alpha subunit comes in several families. The main ones covered here are G_s (stimulatory), G_i (inhibitory) and G_q (which usually raises calcium inside the cell).
• G-proteins modulate enzymes that make second messengers (small molecules inside the cell that pass the signal on), which in turn control intracellular (inside the cell) signalling pathways.
• Just a few ligand-receptor binding events can result in a large cellular response due to the phenomenon of signal amplification.
• The autonomic nervous system uses GPCRs to exert its effects on target tissues.

An Introduction to Receptors

A receptor is a protein molecule which binds a chemical signalling molecule, termed a ligand. When an agonist (a ligand that activates the receptor) binds, the receptor undergoes a conformational change (a change in shape) that initiates signalling and a cellular response. Antagonists can bind without activating the receptor and can block agonist responses. Activation of a receptor is often followed by the production of a second messenger (a small molecule inside the cell that passes the signal on). The second messenger may activate intracellular signalling cascades (chains of steps in which each molecule activates the next). The whole sequence, from receptor activation to the change it causes inside the cell, is known as signal transduction.

There are several classes of receptor:

• G-protein coupled receptors
• Enzyme-linked receptors (receptors with enzyme activity, or linked to an enzyme), for example tyrosine kinase receptors (receptors that attach phosphate groups to the amino acid tyrosine in other proteins)
• Ligand-gated ion channels (channels in the membrane that open when a ligand binds)
• Intracellular receptors

This article will focus on G-protein coupled receptors and their respective signalling pathways.

Structure of GPCRs and G-Proteins

G-protein coupled receptors, also known as 7-TM (seven-transmembrane) receptors, are cell surface receptors (i.e. they are present on the plasma membrane). Each GPCR chain is a single polypeptide. The start of the chain, the extracellular N-terminus, lies outside the cell. The end of the chain, the intracellular C-terminus, lies inside the cell. Between them are 7 transmembrane domains (coiled stretches, called helices, that each cross the membrane). Receptors may function as monomers (single units) or dimers (pairs). The ligand-binding site depends on the receptor. It may lie in an extracellular domain or in a pocket within the bundle of transmembrane helices, rather than being confined to the N-terminus.

Image - Structure of a G-protein coupled receptor within a membrane, showing the 7 transmembrane domains

SimpleMed original by Dr. Thomas Burnell

G-proteins, also known as guanine nucleotide-binding proteins, are peripheral membrane proteins (attached to the surface of the membrane rather than crossing it). They consist of 3 distinct subunits (separate protein chains): an alpha subunit, a beta subunit and a gamma subunit. The beta and gamma subunits stay tightly associated as a stable beta-gamma complex, so in signalling the G-protein behaves as two units: the alpha subunit and the beta-gamma complex.

Signal Transduction

When a ligand (an agonist in this case) binds to its GPCR, the GPCR undergoes a conformational change and is said to become activated. This can be summarised by the equation R => R* (R is the resting receptor and R* is the activated receptor). The activated GPCR then interacts with its associated G-protein by causing GDP to exchange for GTP (guanosine diphosphate for guanosine triphosphate) on the G-protein’s alpha subunit. This exchange results in separation of the alpha and beta-gamma subunits. These subunits can then go on to activate various effector proteins (proteins that carry out the next step of the response). These include enzymes that make second messengers, and ion channels.

Diagram - Activation and deactivation of a GPCR

SimpleMed original by Dr. Joshua Bray

Different types of G-protein activate different signalling pathways. In general, the G-alpha subunit has a stimulatory or inhibitory effect on an enzyme. That enzyme generates a second messenger molecule, which then goes on to activate various signalling cascades. The beta-gamma subunit may also be involved in signalling, but in this article we will focus on the alpha subunit.

There are 3 main subtypes of the G-alpha protein (a fourth family, G_12/13, is not covered here):

G_s: The G-alpha_s protein activates the enzyme adenylyl cyclase, which generates the second messenger, cyclic AMP (cAMP), from ATP. cAMP activates protein kinase A (PKA) by binding to its regulatory subunits (the parts that keep the enzyme switched off), which releases the active catalytic subunits (the parts that carry out the enzyme's work). PKA is a kinase enzyme which phosphorylates (adds a phosphate group to) various proteins, resulting in activation or inhibition of the target protein. This phosphorylation may be one step in a larger signalling cascade.

G_i: The G-alpha_i protein inhibits adenylyl cyclase, resulting in reduced cAMP generation and therefore reduced PKA activity.

G_q: The G-alpha_q protein activates the enzyme phospholipase C, which converts PIP₂ (phosphatidylinositol 4,5-bisphosphate, a phospholipid) to IP₃ (inositol trisphosphate) and DAG (diacylglycerol), which are both second messengers. A rise in IP₃ is generally followed by an increase in intracellular calcium, because IP₃ stimulates calcium release from the endoplasmic reticulum, the cell’s internal calcium store. In muscle cells this store is called the sarcoplasmic reticulum (see the article on membrane transport and calcium signalling).

Diagram - G-protein coupled receptor common downstream activation pathways. Lines ending in an arrow show an increase in activity, for example Gs acting on adenylyl cyclase. Lines ending in a 'T' shape show a decrease in activity, for example Gi acting on adenylyl cyclase. 

SimpleMed original by Dr. Thomas Burnell

To terminate signalling, the G-alpha subunit has an intrinsic GTPase domain (a built-in enzyme region that breaks down GTP), which slowly hydrolyses (splits using water) the bound GTP to GDP. Once hydrolysis is complete, the alpha subunit, now carrying GDP, re-associates with the beta-gamma complex. The G-protein is inactive again and signalling stops.

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