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
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
when ligand binds to the receptor, pore opens
have large extracellular terminus
4 transmembrane domains
2nd transmembrane domain found inside pore determines size and what can pass through
2 intracellular loops
big loop important for regulation and trafficking
protein phosohorylation
3 amino acids that can be phosphorylated: ser, thr, tyr
protein kinase can
reversible hydrolysis reaction by protein phophatase
why is protein phosophorylation important?
phosphate group is large and negtaively charged which impacts environment of amino acids=different properties
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 prtein structure…
ion channels alter their conductance of ions
enzymes are switched on or off
change shape of AS
disrupting or enhancing a protein-protein interaction
receptors interacting with trafficking proteins
regulation of a multi-protein signalling complex
GABAaR subunits
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
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)= most are GluA1/2 or GluA2/3
kainate (GluK1-5)= KAR are more varied but
GluK4 and GluK5
must associate with GluK1,2 or 3
NMDA (GluN1, GluN2A-D) = NMDARs require
at least 1 GluN1
and 1 GluN2
orphan (GluD1-2)
gating of ligand-gated ion channel (e.g nAChR)
uses alpha, beta, gamma
when ACh binds, ion pore opens which permebale to sodium and potassium
sodium therefore flows into cell down electrochemical gardient= depolarisation
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 but remains shut even if ligand is still bound
GABAa and GlyR ligand
chloride-selective
channels, so Cl- flux, not Na+
LGIC may be excitatory or inhibitory depending on ion sensitivity
influx of -ve ions= membrane hyperpolarisation=VSSC inactivated=action pot’l less likely
influx of +ve ions= membrane depolarisation=VSSC activated= action pot’l more likely
(VSSC=voltage sensitive cation channels)
excitatation or inhibition in GABA and GlyR
leads to influx of calcium-less liklely to fire action potential
importance of calcium influx in membranes
important for downstream intracellular signalling events
calcium senstive proteins
calcineurin B = phophatase
PKC= protein kianse
synaptotagmin = exocytosis
PICK1 = receptor trafficking
maintenance of intracelluluar calcium
keeping the cytosolic [Ca2+] very low compared to extracellular space means cell noties change
storage organelles (e.g mitochondria and ER) means that Ca2+ can be used as an efficient signal
sodium-calcium antiporters= allow sodium in and calcium out
calcium pump= ATP to pump calcium out
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
large +ve charged arg (R) residue in pore prevents Ca2+ influx in AMPA receptors
RNA editing of inotropic 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
