9 - GPCR Flashcards
What is the difference between Ionotropic and Metabotropic receptors?
Ionotropic = ligand gates ion channel
Metabotropic = G-protein coupled (7TM) receptor (use second messengers)
What is the structure of GPCR’s?
- Small membrane proteins
- Structurally conserved
- 7 trans-membrane alpha-helices
- Extracellular loops: ligand binding
- Intracellular loops: G-protein binding
- Maintain similar alpha-helix configuration following activation
What is an orphaned receptor?
GPCR whose endogenous agonist is unknown
(most are olfactory)
What are G-proteins?
Guanine nucleotide (GDP, GTP) binding proteins
G-proteins have intrinsic ____ activity
G-proteins have intrinsic GTPase activity
How do G-proteins allow signal transduction?
By switching between GTP- or GDP- binding to regulate cell function
What are the two types of G-proteins?
- Heterotrimeric (alpha, beta-gamma)
-alpha subunit - most diversity -BINDS GTP OR GDP - Beta-gamma subunits - function as a unit - Monomeric (Ras superfamily)
- Homologous to alpha subunit of heterotrimeric
What anchors G-proteins to the membrane? Why?
Lipid anchors – allows movement close to the membrane
What are the four main classes of heterotrimeric G-proteins?
- Galphas - stimulates adenylate cyclase
- Galphai
- includes:
1. G-alphao - inhibit adenylate cyclase
2. G-alphat - activate cGMP phosphodiesterase
- includes:
- G-alphaq/11 - activate phospholipase C (PLC - cleaves PIP2 to IP3 and DAG)
- G-alpha12/13 - activate Ras (GTPase activating protein) and some tyrosine kinases
Mechanism of G-protein activation
- Rest
- G-protein closely associated with GPCR
- Heterotrimeric
- High affinity of alpha for GDP - Activation:
- G-protein binds to GPCR
- alpha subunit loses GDP
- alpha has high affinity for GTP
- G-protein dissociate to alpha + beta-gamma - Termination
- Free alpha is an intrinsic GTPase
- Hydrolyze bound-GTP to GDP
- Heterotrimer reassembles
Addition of ____ prolongs activation
Addition of Gpp(NH)p prolongs activation
- non-hydrolysable analog of GTP
- Irreversible binding to G-protein - stays dissociated/activated
How is G-protein interaction abolished?
- Pertussis toxin (G-alphai and G-alphao proteins)
- inactivates G-proteins
- Places an ADP-ribose onto the GTP-binding site
whooping cough inactivates Galpha i/Galpha o to prevent these signaling pathways
What is direct modulation of GPCR
- G-protein binds directly to ion channel
- activation of GIRK and KATP channel
- Inhibit N- and P/Q type VDCC
- Inhibit Na+ channels
- Activate Ca++ channels (heart and muscle)
- Galpha or Gbeta-gamma can both use this pathway
VDCC = voltage-dependent calcium channel
Effects of G-protein binding directly onto the ion channel?
- activation of GIRK and KATP channel
- Inhibit N- and P/Q type VDCC
- Inhibit Na+ channels
- Activate Ca++ channels (heart and muscle)
- Galpha or Gbeta-gamma can both use this pathway
VDCC = voltage-dependent calcium channel
What is indirect modulation of ion channels by G-proteins?
- G-protein regulate second messengers that alter ion channel activity
-beta-adrenergic activation of L-type VDCC
- via cAMP-dependent phosphorylation - Muscarinic receptor inhibition of M-current
-via decrease in PIP2 levels
2 examples of how G-protein regulates second messengers to alter ion channel activity (Indirect modulation)
- beta-adrenergic activation of L-type VDCC
- via cAMP-dependent phosphorylation - Muscarinic receptor inhibition of M-current
- via decrease in PIP2 levels
Provide evidence for direct modulation by G-protein
- Membrane-delimitation
- Beta-gamma complex remains very closely associated with the membrane - Cell-attached mode
- ACh mimics effect when pipette-applied but not when bath-applied
- response is restricted to small part of the cell
Beta-adrenergic receptors increase ____ current
Beta-adrenergic receptors increase L-type VDCC current
- activation of Gs pathway
- Channel phosphorylation
Evidence of indirect modulation?
- Cell-attached mode : bath-application mimics effect of agonist
- Second messengers can diffuse through cytoplasm
- Can signal to the entire cell from a local input
How is rhodopsin (in the retina) indirectly modulated?
- Photon hits rhodopsin and activates tranducin (Galphat)
- alpha-transducin activates cGMP phosphodiesterase
- Hydrolyses cGMP on non-inactivating Na+ channe; - Na+ channel closes
- Photoreceptor hyperpolarizes = stops releasing glutamate
- Loss of transmitter signal activates downstream neuron
What is the advantage of indirect modulation?
- Different GPCR’s can produce the same change in an ion channel
- K+ channels in hippocampal neuron can be modulated by GABAB and/or 5-HT1A receptor
- Diffusable messengers allow signaling to entire neuron from a local point
- Control
- Each GPCR can act at more than one effector to amplify the desired response
- 5-HT1A receptor in dorsal raphe neurons can activate K+ channels and inhibit VDCCs - Direct ion channel modulation localizes and restricts the response to small part of the neuron
Advantages of Indirect Modulation:
- Different GPCR’s can produce the same change in an ion channel
- K+ channels in hippocampal neuron can be modulated by ______ and/or______ receptor
- Diffusable messengers allow ________
- Control
- Each GPCR can act at more than one effector to _______
- 5-HT1A receptor in dorsal raphe neurons can activate **_ ** and inhibit **_ **
- Each GPCR can act at more than one effector to _______
- Direct ion channel modulation localizes and restricts the response to small part of the neuron
- Different GPCR’s can produce the same change in an ion channel
- K+ channels in hippocampal neuron can be modulated by GABAB and/or 5-HT1A receptor
- Diffusable messengers allow signaling to entire neuron from a local point
- Control
- Each GPCR can act at more than one effector to amplify the desired response
- 5-HT1A receptor in dorsal raphe neurons can activate K+ channels and inhibit VDCCs
- Each GPCR can act at more than one effector to amplify the desired response
- Direct ion channel modulation localizes and restricts the response to small part of the neuron
What is the significance of GPCR’s for neurons?
GPCR-mediated effects are slower than those from ionotropic receptors but are longer-lasting
What is the implication of having longer lasting effects on neurons (as from GPCR mediated effects)
- changes to neuron behaviour can be induced for an extended period
- 100s of msec to hours
- Ionotropic - signaling stops when NT is cleared
- Metabotropic - signal is prolonged due to delayed onset/offset of second messenger synthesis/catalysis
What class of receptors are important for neuromodulation?
NPY receptor
What are the 5 subtypes of NPY receptors?>
- Y1
- Y2
- Y4
- Y5
- Y6
