Post on 16-Jul-2020
IonChannelsareGatedPores
JimHuettner
LectureOverview
• Review of cellular electricity • Recording cellular electricity • Theories about channel selectivity • Channel structures and selectivity • Voltage-dependent gating • Channel-lipid interactions • Ligand-gated channels • Aquaporins and connexins
IonConcentrations
Na 117 K 3 Cl 120 Anions 0 Total 240
Na 30 K 90 Cl 4 Anions 116 Total 240
[+ charge] = [- charge] 0 mV
[+ charge] = [- charge] -89 mV
Channelsarecatalysts–Enzymesforthetranslocationofions
Channelshavetwofundamentalfeatures:
(1)Poreproperties(2)Gating
RecordingMembranePotential
howtomeasurecurrentA voltage clamp can control the voltage across the membrane and record the current that passes through the membrane. Ohm’s Law says: Voltage = Current * Resistance Circuitry in the voltage clamp controls Voltage
When channels open, Resistance will change and the
device will measure Current. Resistance = 1 / Conductance so another form of Ohm’s Law is: Current = Conductance * Voltage
patch-clampversus2-electrodeclamp
patchclamp twomicroelectrodevoltageclamp
Vm = Vc
Im
Vc
Vm
Vc Im
formingatightseal
Sakmann,B(1992)Neuron8:613-629
whole-cellandoutside-outpatch
10 µm Sakmann,B(1992)Neuron8:613-629
Howdoesachannelachieveselectivity?
Howdoesachannelachieveselectivity?
ChannelNomenclature
• Namedbyfunction:delayedrectifier,inwardrectifier,L-typecalciumchannel,NMDAreceptorchannel,etc.
• Namedbystructure:Kv1.1,Kir6.2,Cav1.1,NR1,etc.
ChannelSubunitTopology
KcsA, a bacterial K channel, the first channel crystalized
ChannelDendrogram
https://www.guidetopharmacology.org/GRAC/ReceptorFamiliesForward?type=IC
Rationalefordiversity?• Permeability• Kinetics• Modulation• TraffickingSignalIntegration
StructureofaPotassiumChannel
Doyleetal.,(1998)Science280:69-77
SelectivityFilterandSpace-FillingModel
Doyleetal.,(1998)Science280:69-77
ioncoordinationinthe“selectivityfilter”
GouauxandMacKinnon(2005)Science310:1461-1465
viewsofthechannelopenandshut
Jiang et al., (2002) Nature 417:515-522
voltage-gatedchannels
Long et al., (2005) Science 305:897-903
viewfromtheextracellularspace
Long et al., (2005) Science 305:897-903
voltage-dependentgating
Jiang et al., (2003) Nature 423:33-48
Longetal.,(2007)Nature450:376-382
lipidsco-crystallizedwiththechannel
Lipidco-crystallizedwithKcsA
Valiyaveetiletal.,(2002)Biochemistry41:10771
PIP2isrequiredforeukaryoticKchannelactivitybutinhibitsabacterialKchannel
D’Avanzoetal.,(2010)JBC285:37129
Suh & Hille (2008) Ann. Rev. Biophys. 37:175-195
Kir2.2withoutandwithPIP2
Hansenetal.,(2011)Nature477:495
familiesofligand-gatedchannels
AChRHydropathyPlot
AChRsubunittopology
M2HelixDeterminesConductance
Sakmann,B(1992)Neuron8:613-629
imagesofthenicotinicAChR
ToyoshimaandUnwin,(1988)Nature336:247-250
α4β2nicotinicreceptor
Morales-Perezetal.(2016)Nature538:411-415
Sobolevskyetal.,(2009)Nature462:745-756
GluA2glutamate(AMPA)receptor
P2X4purinergicreceptor
Kawateetal.,(2009)Nature460:592-598
Aquaporinwaterchannel
Luckey,2nded.Fig.12.6
AquaporinSelectivity
Albertsetal.,6thed.
IntercellularConnexinsChannels
Meyersetal.,(2018)Nature564:372-377
AdditionalReading• Sakmann,B.(1992)Elementarystepsinsynaptictransmission
revealedbycurrentsthroughsingleionchannels.Neuron8:613-629.
• DoyleDA,MoraisCabralJ,PfuetznerRA,KuoA,GulbisJM,CohenSL,ChaitBT,MacKinnonR.(1998)Thestructureofthepotassiumchannel:molecularbasisofK+conductionandselectivity.Science280:69-77.
• Gouaux,EandMacKinnonR.(2005)Principlesofselectiveiontransportinchannelsandpumps.Science310:1461-1465.