Mechanism of action of GPCR
These lecture notes give you a brief idea on: How G protein–coupled receptors transduce signals from extracellular hormones to associated effector proteins.
Figure 1 to 7: Ligand-induced activation of effector proteins associated with G protein coupled receptors.
1) Resting state (Figure 1):
- When no ligand is bound to the receptor, the Gα subunit is bound to GDP and complexed with Gβγ unit (exist in heterotrimeric form).
2) Binding of ligand to the receptor (Figure 2)
Mechanism of GPCR activation
- Inactive conformation of GPCR is stabilized by non covalent interactions between specific residues in TM helices.
- When ligand binds to the receptor, these interactions are disturbed.
- TM helices undergo many rotations and shifts. It will unlock the G protein- binding site in the intracellular face of the receptors (change the conformation of cytoplasmic loop) leading to G protein activation.
- This will increase affinity of receptor for G proteins in the cytoplasmic side.
- It results in the formation of an active receptor – G protein complex.
- (G proteins and arrestin compete for 3rd loop in the cytoplasmic side).
3) Activated receptor binds to Gα subunit. (Figure 3)
- When a ligand (agonist) binds to the GPCR it causes a conformational change in the GPCR, which allows it to act as a guanine nucleotide exchange factor (GEF).
- G-proteins composed of three different subunits: alpha, beta, and gamma (stay together here).
- The subunits are tethered at the membrane surface by covalently attached lipid molecules.
4) Dissociation of α subunit from βϒ subunit (Figure 4)
- When a ligand binds, the receptor activates the attached G-protein by causing the exchange of GTP for GDP.
- The activated G-protein then dissociates into an alpha (G-alpha) and a beta-gamma
- Once the exchange of nucleotides has occurred, the Gα – GTP complex dissociates from the Gβϒ subunit, but both remain anchored in the membrane.
5) Activation of effector protein (Figure 5)
- 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.
- Hormone then dissociates from receptor.
- The interaction of the agonist-bound GPCR with the G protein is transient; following activation of one G protein, the receptor is freed to interact with other G proteins.
There are three principal signal transduction pathways followed by the G protein-coupled receptors:
1) Adenylyl cyclase (effector protein): cAMP pathway
Example: Transduction of the epinephrine signal: the β adrenergic pathway.
2) Phospholipase C (effector protein): IP3-DAG pathway
Example: Peptide and protein hormones like TSH, Vasopressin etc bind to G protein-coupled receptors (GPCRs) that activate the intracellular enzyme phospholipase C (PLC).
3) Channel regulation
Example: Many neurotransmitter receptors are ligand-gated ion channels. These include some types of glutamate and serotonin receptors, as well as the nicotinic acetylcholine receptor found at nerve-muscle synapses
6) Synthesis and Release of second messengers by effector proteins (Figure 6)
The most common examples of second messengers are: cAMP, cGMP, IP3, DAG and calcium ions (Ca 2+)
The second messenger, in turn, initiates a series of intracellular events such as:
- Phosphorylation and activation of enzymes
- Release of Ca2+into the cytosol from stores within the endoplasmic reticulum
The cell begins to produce the appropriate gene products in response to the signal it had received at its surface.
In addition to their roles in affecting gene expression, GPCRs regulate many immediate effects within the cell that do not involve gene expression.
Figure 8: Downstream signaling happens when different second messengers are released
one example: In the case of cAMP, these enzymatic changes activate the transcription factor CREB (cAMP response element binding protein) which then bound to its response element (5’TGACGTA3′) in the promoters of genes that are able to respond to the ligand. Activated CREB turns on gene transcription.
6) GTP hydrolysis and inactivation of G protein (Figure 7)
- Inactivation occurs because G-alpha has intrinsic GTPase activity.
- After GTP hydrolysis, G-alpha bound to GDP will re associate 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.