g protein coupled receptor

GPCRs/7-transmembrane receptors (7TM receptors)

G-protein-coupled receptors (GPCRs) are the largest and most diverse group of membrane receptors in eukaryotes. 

Structure

  • 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

Main targets:

  • 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)

THE DRESS
  • Romano: The dress is fucking gold & brown!
  • Feliciano: No Fratello! it's actually blue & bl-
  • Romano: NO FELI ITS FUCKING GOLD & BROWN
  • Feliciano: No Fratello I swear it's blue & black! Germany said so!
  • Romano: What did that Potato bastard say then huh!
  • Germany: *Talks about G protein–coupled receptor and colour spectrum* And that's why The dress is blue & black.
  • Romano: ...
  • Feliciano: ...
  • Romano: The dress is fucking gold & brown!
  • Spain: I think it's red & green like a Tomato!
  • Everybody: ...