GPCRs/7-transmembrane receptors (7TM receptors)
G-protein-coupled receptors (GPCRs) are the largest and most diverse group of membrane receptors in eukaryotes.
- single polypeptide chain comprising of seven transmembrane α-helices
- extracellular N-terminal domain of varying length,
- intracellular C-terminal domain.
- length of the extracellular N terminus and the location of the agonist binding domain determines family.
- The long, third cytoplasmic loop couples to the G-protein
- Usually particular receptor subtypes couple selectively with particular G-proteins
- For small molecules, such as noradrenaline, the ligand-binding domain of class A receptors is buried in the cleft between the α-helical segments within the membrane. Peptide ligands bind more superficially to the extracellular loops
G protein system
GPCRs interact with G proteins in the plasma membrane when an external signaling molecule binds to a GPCR, causes a conformational change in the GPCR.
G-proteins comprise a family of membrane-resident proteins
whose function is to recognise activated GPCRs and
pass on the message to the effector systems that generate
a cellular response.
- G proteins are specialized proteins with the ability to bind the nucleotides guanosine triphosphate (GTP) and guanosine diphosphate (GDP).
- The G proteins that associate with GPCRs are heterotrimeric, (alpha beta and gamma subunits)
- alpha and gamma are attached to the plasma membrane by lipid anchors
- Trimer in resting state
- activated alpha monomer and beta/gamma dimer
Guanine nucleotides bind to the α subunit, which has enzymic activity, catalysing the conversion of GTP to GDP. The β and γ subunits remain together as a βγ complex. All three subunits are anchored to the membrane through a fatty acid chain, coupled to the G-protein through a reaction known as prenylation.
- G-proteins are freely diffusible so a single pool of G-protein in a cell can interact with several different receptors and effectors
- When GPCR is activated by an agonist, a conformational change causes it to acquire high affinity for αβγ (G protein)
- bound GDP dissociates and is replaced with GTP, which in turn causes dissociation of the G-protein trimer, releasing α-GTP and βγ subunits - the ‘active’ forms of the G-protein
- which diffuse in the membrane and can associate with various enzymes and ion channels
- Signalling is terminated on hydrolysis of GTP to GDP through the GTPase activity of the α subunit.
- resulting α–GDP dissociates from the effector, and reunites with βγ
- Attachment of the α subunit to an effector molecule increases its GTPase activity
- GTP hydrolysis is termination –> activation of the effector tends to be self-limiting
Second messenger targets for G proteins
- Adenylyl cyclase (responsible for cAMP formation)
- Phospholipase C (inositol phosphate and diacylglycerol (DAG) formation)
- Ion channels, particularly calcium and potassium channels
- Rho A/Rho kinase (system controlling the activity of many signalling pathways for cell growth and proliferation, smooth muscle contraction, etc.)
- Mitogen-activated protein kinase (MAP kinase) system controlling cell functions eg division.
(notes on these coming soon)