The largest family of cell surface receptors are the G-protein
coupled receptors (GPCRs). There are hundreds of different
GPCR proteins, and nearly a third of all drugs target this type of
receptor. A diverse set of ligands bind to this type of
receptor, including peptide hormones, neurotransmitters,
and odor molecules. These receptors all have a
similar structure with seven transmembrane domains.
On the basis of their seven transmembrane domain structure, many
GPCRs have been identified in the human genome. Proteins
that were identified by sequence homology, but whose ligands are
not known, are termed orphan receptors.
GPCRs associate with heterotrimeric G-proteins (green),
that is, G-proteins composed of three different subunits: alpha,
beta, and gamma. The subunits are tethered at
the membrane surface by covalently attached lipid molecules.
When a ligand binds, the receptor activates the attached
G-protein by causing the exchange of GTP (yellow) for GDP
(red). The activated G-protein then dissociates into an alpha
(G-alpha) and a beta-gamma complex. G-alpha
bound to GTP is active, and can diffuse along the membrane surface
to activate (and sometimes inhibit) target proteins, often enzymes
that generate second messengers. Likewise, the
beta-gamma complex is also able to diffuse along the inner
membrane surface and affect protein activity.
Inactivation occurs because G-alpha has intrinsic GTPase activity. After GTP hydrolysis, G-alpha bound to GDP will reassociate with a beta-gamma complex to form an inactive G-protein that can again associate with a receptor.
The GTPase activity of the G-alpha can be made faster by other proteins--sometimes the target protein, sometimes a separate regulatory protein. Cholera toxin causes a chemical modification that prevents GTP hydrolysis and leads to unregulated signaling (more below).
There are several different classes of heterotrimeric G-proteins that are defined by their different G-alpha subunits. One type of G-alpha activates the enzyme adenylyl cyclase, which catalyzes the formation of the second messenger cyclic AMP (cAMP). Because an activated adenylyl cyclase can generate many molecules of cAMP, this is a means to amplify the signal. cAMP can have several effects, but a major effect is to bind to and activate protein kinase A (PKA; also known as cAMP-dependent kinase). PKA then phosphorylates target proteins in the cell. cAMP is rapidly broken down by phosphodiesterases, limiting the length of the signal.
A specific example of a receptor that couples to this type of
G-protein is the beta-1 adrenergic receptor found in the
heart (see figure below). Beta 1 receptors are the principal
type of adrenergic receptor found in the heart. The ligand
for this receptor is norepinephrine, the neurotransmitter
that is released by sympathetic postganglionic neurons.
(As well, the hormone epinephrine, released from the adrenal
medulla, is also a ligand for these receptors.)
Stimulation of beta-1 receptors causes increased cAMP and PKA
activation. PKA phosphorylates various target proteins in
cardiac cells to cause an increase in both the heart rate and the
strength of cardiac muscle contraction. Beta-1 receptors are
the targets of drugs (beta blockers) that are used to treat
heart failure and hypertension.
Another example involving GPCR signaling that stimulates adenylyl
cyclase is the regulation of secretion in the small intestine.
This regulation is disrupted by cholera toxin. The effect of
cholera toxin is to lead to persistent activation of adenylyl
cyclase because it destroys the GTPase activity of G-alpha. There
is over-production of cAMP, continuous activation of PKA, and
continuous phosphorylation of CFTR, causing excessive fluid
secretion. See the section on cholera
in the page "Clinical Examples: Epithelial Transport".
A different type of G-alpha activates the enzyme phospholipase C. This type of G-alpha couples to various GPCRs found on smooth muscle, such as the oxytocin receptor shown in the example below.
Phospholipase C is an enzyme that cuts PIP2, a membrane
phospholipid, to generate two second messengers, IP3
and diacylglycerol (DAG). IP3 is
water soluble, diffusing through the cytosol to bind to and open a
ligand-gated Ca++ channel in the endoplasmic
reticulum (or sarcoplasmic reticulum in muscle cells). Thus,
stimulation of a receptor linked to this G-alpha is a way to
increase Ca++ inside the cytosol. Ca++
in the cytosol exerts its effects by binding to Ca++-binding
proteins such as calmodulin. In the uterus, the
increase in intracellular Ca++ that results from
oxytocin signaling causes the smooth muscle to contract.
DAG is lipid soluble and stays in the membrane. It activates protein kinase C (PKC), which, like PKA phosphorylates particular target proteins.
In the continuing presence of ligand, many GPCRs show desensitization. The mechanism is shown in the figure. A protein known as a G-protein Receptor Kinase (GRK) phosphorylates the receptor on particular residues. This increases its affinity for a protein called beta-arrestin (red), that binds to the receptor. This reduces signaling by preventing the association with the G-protein.
There are several potential outcomes once beta-arrestin binds to the receptor. One possibility is that beta-arrestin targets the receptor for endocytosis, leading to receptor downregulation (a decreased number of receptors on the cell surface). Another possibility is the activation of beta-arrestin-dependent signaling pathways that are independent of G-protein signaling. Beta-arrestin can act as a scaffold that binds and brings together other intracellular signaling proteins. The physiological significance of beta-arrestin-dependent signaling is still being worked out.