ligand-gated ion channels Flashcards
what are ligand gated/channel-linked receptors?
an agonist binding site and associated ion channel incorporated into the same macromolecular complex
type of ion channel that open in response to the binding of a specific molecule (called a ligand) to the receptor
are ligand gated ion channel ionotropic or metabotropic?
ionotropic
what time scale do LGIC work in?
milliseconds
name 6 examples of LGICs
nictonic ACh receptor (nAChR)
ionotropic glutamate receptors (iGluR)
gabba-aminobutyric acid type A (GABAA) receptors
inhibitory glycine receptors (GlyR)
5-hydroxytryptamine type 3 (5-HT3; serotonin) receptors
P2X subtype of purinergic receptors
membrane topology of nAChR, GABAaR, GlyR and 5HT3R
pentameric structure as they are made from 5 subunits arranged around central pore
when ligand binds to the receptor, pore opens
4 transmembrane domains, second transmembrane domain (M2) found inside pore determines size of ion pore and what ions can pass through
extracellular domain contains ligand-binding sites
2 intracellular loops
big loop important for regulation and trafficking of receptors
usually contain 2 alpha subunits, a beta, gamma/epsilon and delta
protein phosohorylation
3 amino acids that can be phosphorylated: serine, threonine, tyrosine
protein kinase can catalyse phosophrylation- sticks to hydroxl of AA
reversible hydrolysis reaction by protein phophatase to return protein back to unphosphorlyated state
why is protein phosophorylation important?
phosphate group is large and negtaively charged which impacts environment of amino acids=different properties
this changes activity of protein and then can return
2 main categories of phosphorylation effects
- inducing a conformational change in the 3D protein structure
- disrupting or enhancing a protein-protein interaction
inducing a conformation change in the 3D protein structure…
ion channels alter their conductance of ions (e,g phosphate groups on receptors)
enzymes are switched on or off= change shape of AS by phosphate group changing structure
disrupting or enhancing a protein-protein interaction
receptors interacting with trafficking proteins (send to right synapse at right time)
regulation of a multi-protein signalling complex
GABAaR subunits (type of ionotropic receptor)
many subunits (6 alpha, 3 beta, 3 gamma etc)
most common composition is 2 alpha, 2 beta and 1 gamma
- need at least one alpha and one beta for a functional GABAa
- only beta binds GABA
different subunits convey specific pharmacology, channel
properties, modes of regulation, trafficking, etc.
how do GABAaR subunits associate?
non-covalent interactions
specific nature of GABAaR subunits and intracellular loops
specific interacting proteins bind the subunit intracellular loops
phosphorylation differentially affects specific subunit intracellular loops
helps control properties of GABAaR subunits
membrane topology of ionotropic glutamate receptors
tetrameric structure
4 transmembrane domains
loop is outside cell so no phosophorylation
means they have intracellular c-terminus
this determines trafficking and is regulated by phosphorylation
channel pore is lined by 2nd transmembrane domain
subunits of the ionotropic glutamate receptor family (iGluRs)
AMPA (GluA1-4)
kainate (GluK1-5)
NMDA (GluN1, GluN2A-D)
orphan (GluD1-2)
most AMPA receptors are…
GluA1/2 or GluA2/3
kainate receptors
KAR are more varied but GluK4 and GluK5
must associate with GluK1,2 or 3 to give glutamate receptor
NMDA receptors
NMDARs require
at least 1 GluN1
and 1 GluN2 to make glutamate receptor
gating of ligand-gated ion channels (e.g nAChR)
use of alpha, beta, gamma and delta subunits
when ligand binds=conformational change which opens ion channel
when ACh binds, ion pore opens which is permebale to sodium and potassium
sodium therefore flows into cell down electrochemical gardient= depolarisation of postsynaptic cell
important facts about ligand-gated ion channels
- channel gating is NOT voltage dependent
- channel desensitises while ligand still bound, i.e. channel opening is transient
because ligand binding and unbinding is slow- signals last miliseconds so receptor briefly opens when ligand binds but remains shut even if ligand is still bound
GABAa and GlyR ligand receptors
chloride-selective
channels, so Cl- flux, not Na+ (inhibitory)
LGIC may be excitatory or inhibitory depending on ion sensitivity
influx of -ve ions= membrane hyperpolarisation=VSSC inactivated=AP less likely
influx of +ve ions= membrane depolarisation=VSSC activated= AP more likely
(VSSC=voltage sensitive cation channels)
influx in GABA and GlyR
leads to influx of calcium-less liklely to fire action potential
inhibitory ligand-gated ion channels
GABAaR
GABAcR
GlyR
excitatory ligand-gated ion channels
5-HT3R
nAChR
AMPAR
KAR
NMDAR
importance of calcium influx in membranes
important for downstream intracellular signalling events
calcium senstive proteins
calcineurin B = phophatase
PKC= protein kinase
synaptotagmin = exocytosis
PICK1 = receptor trafficking
maintenance of intracelluluar calcium
Na-Ca antiporters let Na into cell and uses energy to push out an ion of calcium
Ca2+ pumps use ATP to pump calcium out
keeping the cytosolic (cellular cytoplasm) [Ca2+] very low compared to extracellular space means cell notices change
storage organelles (e.g mitochondria and ER) means that Ca2+ can be used as an efficient signal
calcium permability of LGIC: nAChr and 5-HT3R
Ca2+-permeability
varies according to
subunit composition
modulate neurotransmitter
release
calcium permability of LGIC: NMDAR
always calcium permeable
regulate AMPAR trafficking
during synaptic plasticity
calcium permability of LGIC: KAR
Ca2+-permeability
varies according to
subunit composition
modulate neurotransmitter release
calcium permability of LGIC: AMPAR
only GluA2-lacking
receptors are Ca2+-
permeable (or those
with unedited GluA2)
synaptic plasticity and cell death
GluA2 subunit determines Ca2+ permeability of AMPARs
2 transmembrane domain determines what goes through channel
in channel pore lining large +ve charged arginine (R) residue in pore prevents Ca2+ influx in AMPA receptors (repels calcium)
RNA editing of ionotropic glutamate receptors
critical channel positions contain amino acids not predicted from the genomic DNA sequence
eg. GluA2 DNA actually codes for a glutamine (Gln, Q) NOT an arginine (Arg, R)
the codon for Arg is created in the mRNA by a reaction catalysed by the enzyme ADAR, which changes
one of the bases in the mRNA (= RNA editing)
when brain RNA of adult rats is analysed by RT-PCR, the Q/R site of GluR2 is edited almost completely
(>99%) whereas GluR5 and GluR6 are edited to a lower extent (40% and 80%, respectively)
this helps to regulate calcium permeability
