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.