GPCR 2: G proteins

7 transmembrane (TM) domain receptors

• 7 TM receptors - single polypeptide chain traverses plasma membrane 7 times

  • 3 intracellular & 3 extracellular loops

  • N terminus = extracellular

  • C terminus = intracellular

  • each TM hydrophobic alpha helix = 22-24 AA

• important differences: variety of ligands

  • growth factors, glucagon, epinephrine, prostaglandins, acetylcholine (muscarinic effects), opioid peptides, etc.

types of ligand-GPCR interactions

• binds to hydrophobic core of 7TM helices

  • low MW - acetylcholine, histamine, serotonin, prostaglandins

• short peptides bind core and external loops

  • peptides - glucagon, growth hormone, parathyroid hormone

• interact with N-terminus and external loops

  • thyroid and gonadotropin releasing hormone

• reorganization of extended N-terminal segments

  • GABA and glutamate

• cleaved receptor and newly exposed N-terminus acts as autoligand

  • proteinase-activated receptors

GPCRs

• GPCRs are the targets of 50% of marketed drugs

• involved in vision, olfaction, CNS, immune system, digestive system and many other processes

• many examples

  • beta adrenergic receptors

  • thyrotropin receptors

  • glucagon receptors

  • some types of dopamine and serotonin receptors

nucleotides

• CTP = important in phospholipid biosynthesis

• UTP = polysaccharide assembly

• ATP = metabolism and cell activity

• GTP = receptor signaling acting as cofactor

G-proteins

• guanine nucleotide-binding proteins

• intermediary proteins in signal transduction

• found alone inner surface of the plasma membrane

• associated with GPCRs

• other names = molecular switches and effectors

• switches between inactive form (bound to guanine diphosphate, GDP) to active form (bound to guanine triphosphate, GTP)

• heterotrimeric proteins (3 subunits)

  • alpha subunit has the nucleotide binding site

  • beta and gamma subunits behave as 1 entity

    • anchored to membrane by lipid attachments

G protein activation

1. begins as inactive heterotrimer: Gαβγ

2. agonist binding and receptor conformation change

3. recruit G protein to agonist-bound receptor

4. exchange GDP for GTP (on alpha subunit)

5. the GTP-Gα and Gβγ subunits can go on an activate different signaling cascades through exposed regions

• [GTP] > [GDP] = NOT rate limiting step once we have ligand bound to GPCR

G-protein inactivation

1. once Gα subunit comes into contact with effector enzyme, its innate GTPase will be activated

2. Gα subunits will hydrolyze GTP → GDP

3. reassociation of Gα with Gβγ

4. restore inactive form

• hydrolysis of GTP is slow as a result, the Gα can interact with number of effector molecules before it returns to GDP bound state → part of amplification of downstream mechanism

alpha subunits

• interactions

  • C-terminus interacts with receptors

  • N-terminus interacts with βγ subunit

• following binding of GTP and detachment of βγ dimer, a surface on α-subunit is revealed for interaction with effector molecule

• the diversity of heterotrimeric G proteins = due to alpha subunit

• different types of alpha subunits

  • universal expression (α5, αi, α11, αq)

  • sensory cells: taste (αt), olfactory (αolf)

  • neural crest and endocrine tissues (αo)

  • neurons (αz)

  • hematopoetic cells (α16)

beta and gamma subunits

• dimer subtypes

  • 5 beta subtypes with similar AA identity

  • 12 gamma subtypes with more diverse sequences

• identity of the βγ dimer contributes to the coupling of G proteins to particular receptors

• the C-terminus of the γ-subunit is modified by geranylgeranyl (20C) or farnesyl (15C) groups to tether the βγ subunit to plasma membrane

• functions

  • ensure localizaiton, coupling and deactivation of α subunit

  • reduce tendency of GDP to dissociate from α-subunits

  • regulate affinity of receptors for ligands

  • regulate receptor phosphorylation by specific kinases

  • βγ subunits can also act as signaling proteins

    • increase K+ channel activity

    • decrease Ca2+ channel activity