LigandGated Receptors PCTH400

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Ligand-Gated ReceptorsPCTH400

• David A. Mathers, Ph.D.
• Cellular and Physiological Sciences, UBC
• Room 421, Pharmacology Building, UBC
• Tel [phone]
• E mail mathers_at_interchange.ubc.ca

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Ionotropic Receptor
Metabotropic Receptor
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P2X P2Y
P2X
P2Y
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Receptor subunit number varies across the
families of ionotropic receptors
Nicotinic AChR GABAAR GlyR
5-HT3R n 5
Ionotropic GluR n 4
P2X Purinergic n 3
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Nicotinic AChR may have evolved from bacterial
LGCs which have 4 trans-membrane domains
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M2 domains line pore of nAChR
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nACh receptors are non-selective cation channels

• Positively charged amino acids form 3 rings which
  are the ion selectivity filter exclude negative
  charged ions make the pore cation-selective
  (Na, Ca2 in, K out)

Levels 1 3 prefilters Level 2 main filter
Out
e
-
-
-
1
a
a
Ions pass through in single file binding to
charged wall sites
-
-
-
2
-
-
-
3
In
2.5 nm
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Neuromuscular nAChRs are a2ß?d in fetus
(long-open time), a2ßed in adult (short open
time) Neuronal nAChRs have a and ß subunits
only but retain pentameric symmetry
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Nicotinic AChR of Torpedo ray
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Structural insights on nAChR from cryo-electron
microscopy and X-ray crystallography

• Resolution at 4.6 Å reveals key structural
  features

ACh binding pocket (a subunits only) the two
are slightly different in shape, in accord with
their different ACh affinities. Tunnel probably
selects ACh over other cations. The dimeric
nature of rapsyn allows linkage of adjacent nAChRs
Outer vestibule 20 Angstroms wide
cations
b
a
Gate (M2)
Inner vestibule is linked to cytoplasm by tunnels
which are 10 Å wide
Rapsyn dimer
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Axial rotation of M2 domains opens nAChR channel
Closed channel
Open channel
All 5 M2 segments are kinked towards central
pore, closing it off
All 5 M2 segments rotated around pore axis so
that kink is pointed away from pore
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Glycine receptors are the major mediators of
postsynaptic inhibition in the spinal cord
Pentameric assemblies of a1-4 and ß subunits
forming Cl- selective channel
Gephyrin anchors receptor at synaptic sites
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GABAA receptor target for many drugs
Benzodiazepines
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GABAA and glycine receptor channels display
multiple (up to 8) conductance states with a
preferred main (m) state
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Ionotropic GluRs bind agonist between S1 and S2
domains and resemble GluR0 in containing an
inverted K channel
L-Glutamate
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AMPA Receptors for glutamate are tetrameric
assemblies of GluR1-4 subunits forming a
non-selective cation channel
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P2X1-7 receptors are cation-permeable ion
channels composed of three subunits, each with
two membrane spanning domains, M1 and M2
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G-protein coupled receptors share a 7
transmembrane a helical domain likely evolved
from bacterial rhodopsin
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G-protein coupled receptors bind agonists in
bi-lobular structures (Venus flytrap modules,
VFTMs) evolved from bacterial periplasmic binding
proteins
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The GABAB receptor B1 subunit binds GABA in the
leucine-isoleucine-valine binding protein
(LIV-BP) like domain of its VFTM
VFTM
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GABA binds to residues in both lobes of the VFTM
of GABAB1 subunit, closing two lobes around
itself
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GABA binding to the VFTM of GABAB1 subunit causes
rotation of VFTM in GABAB2 subunit, leading to
G-protein activation, reversible by
agonist-unbinding and regulators of G-protein
signalling (RGS)
GABA
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Metabotropic receptors influence multiple
intracellular signalling pathways via Gß? dimers
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Speculative Channel Evolution
Voltage-dependent channels
Na channels
Ionotropic ligand-gated channels
K channels
Ca2 channels
Gly
nAChR
GABAA
Glu
5-HT3
Metazoa
800 mya
Protists
2400 mya
Prokaryotes
Early K channels, glutamate receptors,
periplasmic binding proteins