RESEARCH ARTICLE◥

STRUCTURAL BIOLOGY

Activation mechanism of a humanSK-calmodulin channel complexelucidated by cryo-EM structuresChia-Hsueh Lee and Roderick MacKinnon*

Small-conductance Ca2+-activated K+ (SK) channels mediate neuron excitability andare associated with synaptic transmission and plasticity. They also regulate immuneresponses and the size of blood cells. Activation of SK channels requires calmodulin (CaM),but how CaM binds and opens SK channels has been unclear. Here we report cryo–electronmicroscopy (cryo-EM) structures of a human SK4-CaM channel complex in closed andactivated states at 3.4- and 3.5-angstrom resolution, respectively. Four CaM molecules bindto one channel tetramer. Each lobe of CaM serves a distinct function: The C-lobe binds tothe channel constitutively, whereas the N-lobe interacts with the S4-S5 linker in aCa2+-dependent manner. The S4-S5 linker, which contains two distinct helices, undergoesconformational changes upon CaM binding to open the channel pore. These structuresreveal the gating mechanism of SK channels and provide a basis for understandingSK channel pharmacology.

Ca2+ is arguably one of the most crucial cel-lular signals, and it affects virtually everyaspect of a cell. Gárdos first discovered alink between Ca2+ and K+ permeability in1958 when he recognized that Ca2+ can en-

hance the K+ permeability of human erythrocytes(1). Since then, several studies have reported asimilar “Gárdos effect” in various types of neu-rons (2). We now know that this effect is medi-ated by a family of Ca2+-activated K+ channels(3). These channels were historically cataloged asintermediate- or small-conductance Ca2+-activatedK+ (SK) channels to distinguish them from thewell-studied large-conductance Ca2+-activatedK+ (BK) channels (4–8). Widely expressed in neu-rons of the central nervous system, SK channelscontribute to the after-hyperpolarization follow-

ing an action potential and mediate the intrin-sic excitability of neurons (9). SK channels arealso implicated in synaptic transmission and plas-ticity. In T lymphocytes, SK channels modulatethe activation of immune responses (7, 10, 11).In erythrocytes, they regulate cell volume, andtheir dysfunction causes cell dehydration andhemolysis (8, 12–15).Given the physiological importance of SK

channels, it is important to understand how theywork at a molecular level. Although both SK andBK channels are activated by Ca2+, their aminoacid sequence identities are low, and their chan-nel gating mechanisms are completely different.Ca2+ ions open BK channels directly, whereasCa2+ ions open SK channels via calmodulin (CaM)(4, 16). CaM opens SK channels in a coopera-

tive manner with high Ca2+ sensitivity (medianeffective concentration, around 100 to 400 nM)(4–7, 17 ). The structures of BK channels havebeen studied in molecular detail (18, 19), but theCa2+-CaM gating mechanism of SK channels hasremained a mystery. To understand the structuralbasis of gating in SK channels, we determinedstructures of a full-length human SK channelin closed and activated states by using single-particle cryo–electron microscopy (cryo-EM).

Characterization of a human SK4-CaMchannel complex

After an initial screening of 25 SK proteins fromdifferent species by fluorescence-detection size-exclusion chromatography (20), we identifiedhuman SK4 (also known as IK, KCa3.1, KCNN4,or Gárdos channel) as a promising candidatefor structural studies. We then expressed andpurified human SK4 from mammalian cells. Pu-rified full-length SK4 channels, although havinga predicted molecular mass of 48 kDa, migrateat about 37 kDa by SDS–polyacrylamide gel elec-trophoresis (SDS-PAGE) (Fig. 1A). CaM was co-purified with the channel in both the presenceand absence of Ca2+. This observation suggeststhat Ca2+ is not necessary for the SK-CaM inter-action and that CaM constitutively binds to thechannel.To examine the function of the purified SK4-

CaM complex, we reconstituted it into liposomesand monitored K+ flux by means of a fluorescence-based assay (21). Proteoliposomes were reconsti-tuted with a high concentration of K+ (150 mMKCl), then diluted into K+-free solution whileionic strength was maintained with 150 mMNaCl. In the presence of Ca2+, a fluorescencedecrease was observed owing to K+ efflux outof the liposomes (Fig. 1A), which could be in-hibited using two different SK4 channel blockers,NS6180 and senicapoc (22, 23). Thus, the purified

RESEARCH

Lee et al., Science 360, 508–513 (2018) 4 May 2018 1 of 6

Laboratory of Molecular Neurobiology and Biophysics, TheRockefeller University, Howard Hughes Medical Institute,1230 York Avenue, New York, NY 10065, USA.*Corresponding author. Email: mackinn@rockefeller.edu

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Fig. 1. Functional characterization and architecture of the Ca2+-freeSK-CaM channel complex. (A) Purified SK-CaM channel complexand fluorescence-based liposome flux assay. Left, SK-CaM complexin the presence or absence of Ca2+ (2 mM CaCl2 or 5 mM EGTA),analyzed by SDS-PAGE. Right, SK-CaM complex–mediated flux.Fluorescence changes due to K+ flux were monitored over time

(mean ± SEM; n = 4 to 8). Proteoliposomes in the presence of 2 mMCaCl2 showed robust flux, which could be blocked by 10 mM NS6180or senicapoc. CCCP, carbonyl cyanide m-chlorophenylhydrazone.(B to D) Cryo-EM structure of the Ca2+-free SK-CaM complex. Eachchannel subunit is shown in a different color. Purple, CaM C-lobein surface representation.

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SK4-CaM protein complex (hereafter referredto as SK-CaM) recapitulates the functional Ca2+-activated K+ efflux and pharmacological inhibi-tion characteristic of native channels.

Architecture of Ca2+-free SK-CaMchannel complex

We first determined the structure of SK-CaM inthe absence of Ca2+ to a resolution of 3.4 Å (figs. S1and S2). We found that four SK subunits forma fourfold-symmetric tetramer that is ~95 Å inlength and 120 Å in width when viewed fromwithin the plane of the membrane (Fig. 1B).Transmembrane helices S5 and S6 form the ionchannel pore, which is surrounded bymembrane-embedded helices S1 to S4. The domain com-prising these four helices interacts with the poredomain from the same subunit (Fig. 1C). Thisarrangement is similar to that of the BK channel(also known as Slo1) but different from that ofdomain-swapped Kv1 to Kv7 channels, wherehelices S1 to S4 interact with a neighboring poredomain (18, 24, 25).At the C-terminal end of S6, the helix unwinds

near the inner leaflet of the cell membrane. Thisallows the polypeptide to make a sharp turn be-fore two helices, HA and HB, that run almostparallel to the membrane plane. HB is followedby the HC helix (Fig. 1D). The HC helices fromfour SK subunits make up a coiled coil locatedat the center of the channel, which is importantfor channel assembly and trafficking (26, 27). Be-cause this region of the channel is flexible, thelocal structure is not as well resolved as otherregions, and the last 41 residues are invisible inthe structure. Consistent with previous studies(28, 29), the peripheral ends of HA and HB formthe binding site for the CaM C-lobes, which arevisible in the cryo-EM map (Fig. 1D). On thebasis of light scattering and analytical ultra-centrifugation experiments performed on CaMsand channel fragments, Halling et al. suggestedthat two to eight CaM molecules may bind toone channel (30). In the context of a full-lengthchannel, we observed that one CaM binds to oneSK subunit, resulting in four CaMs per channeltetramer.

Dynamic CaM N-lobe as a Ca2+ sensor

When C4 symmetry was imposed during cryo-EM reconstruction, the CaM N-lobes of thechannel complex exhibited poor density, whichprecluded model building of this portion of CaM(Fig. 1, B to D, and fig. S1C). This suggests thatthe N-lobes exhibit static disorder, consistentwith high mobility in the absence of Ca2+. Whenthe structure was reconstructed without impos-ing symmetry, only one of the four CaM N-lobeswas visible (fig. S1C). To analyze the static dis-order of the CaM N-lobes further, we expandedthe data set and reoriented each subunit onto asingle position according to the C4 point group(31). We then performed focused classificationand subsequent refinement (fig. S3A). Throughthis strategy, three distinct conformations ofCaM with improved N-lobe density were iden-tified (Fig. 2).

In these reconstructions, the differences in theCaM C-lobes are subtle, resulting from a slightsliding along the HA and HB helices (fig. S3, Band C). In contrast, the CaM N-lobes exhibitlarge positional variations. It is evident that

the N-lobe can swing from the periphery of thechannel (Fig. 2, red) all the way to the center ofthe channel, close to the coiled coil (Fig. 2, blue).The N- and C-lobes of CaM are connected by acentral linker, which has the capacity tomaintain

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Fig. 3. Transmembrane domain and ion channel pore. (A) Comparison of single SK and BKsubunits. Gray bars represent approximate boundaries of the membrane bilayer. For clarity, only partof the BK intracellular domain is shown (Protein Data Bank ID, 5TJI). (B) Interface between theS45B and S6 helices, shown in surface representation. Gray, S6 helix from the same subunit as S45B.Blue, S6 and HA helices from an adjacent subunit. (C) Channel pore with only two subunitsshown for clarity. Pore radius: red, <1.15 Å; green, 1.15 to 2.30 Å; purple, >2.30 Å. Throughout thefigures, single-letter abbreviations for the amino acid residues are as follows: A, Ala; C, Cys;D, Asp; E, Glu; G, Gly; I, Ile; K, Lys; L, Leu; N, Asn; Q, Gln; R, Arg; S, Ser; T, Thr; V, Val; and Y, Tyr.

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Fig. 2. Conformational dynamics of CaM N-lobe. (A and B) Cryo-EM reconstruction of the Ca2+-freeSK-CaM complex. CaM N-lobe densities (blue, yellow, and red) from three classes after focusedclassification are superimposed on the consensus 3.4-Å-resolution map.

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an a-helical structure or unwind into a loop,allowing CaM to adopt multiple conformations(32). Under Ca2+-free conditions in SK-CaM, thisflexible linker permits the N-lobe to undergolong-range, rigid-body–like motions, travelingfrom the bottom of the S2 helix to the bottom ofthe S4-S5 linker (Figs. 2B and 1D), while theC-lobemaintains its interactionwith theHA andHB helices (fig. S3C). Previous mutagenesis ex-periments suggested that the two lobes of CaMserve distinct functions in the SK channel com-plex (33–35): The C-lobe interacts with SK chan-nels in a Ca2+-independentmanner, whereas theN-lobe senses Ca2+ and gates the channel. Ourstructure supports these findings by demonstrat-ing a permanent bound CaM C-lobe and a dy-namic N-lobe. This molecular plasticity of theN-lobe seems ideal for rapid detection and re-sponse to local Ca2+ signals.

Transmembrane domain and ionconduction pore

Although the SK channel has a similar topologyto the BK channel, we noticed two interestingdifferences. First, the S1 and S2 helices in SK aremuch longer than those in BK (18). Each about60 Å in length, S1 and S2 in SK extend beyondthe membrane boundary, into the cytoplasmicspace (Fig. 3A). The second difference involvesthe S4-S5 linker, which in SK consists of twoa-helices, S45A and S45B, rather than the short

turn observed in BK and other non–domain-swapped members of the six-transmembraneion channel superfamily, including the Kv10 toKv12, Slo2, and HCN channels (18, 36–39). Involtage-gated ion channels—whether domain-swapped or not—the S4-S5 linker plays a criticalrole in channel gating by couplingmovements ofthe voltage sensor to opening of the pore. ButSK is voltage-insensitive (5, 6, 17). Its particularS4-S5 linker structure is apparently suited toconfer CaM-mediated Ca2+ sensitivity to the SKchannel gate.S45B is wedged in between HA and S6, thus

providing lateral contacts between the pore andthe cytoplasmic structural elements that ulti-mately attach CaM to the channel (Fig. 3B). Hy-drogen bonds formed between Lys197 on S45B andGlu295 on HA from a neighboring subunit, andbetweenAsn201 on S45B andArg287 on S6 from thesame subunit, appear to “glue” these structuralelements together. Mutations of Arg287, possiblyby interfering with this interaction, change theintrinsic open probability of SK channels in theabsence of Ca2+ (40). The inter-subunit connec-tivity of this interface (each S4-S5 linker interactsstructurally with S6 from two subunits) could bethe structural underpinning of the high cooper-ativity of Ca2+ activation in SK (4–7, 17).In the absence of Ca2+, the SK channel is func-

tionally closed. The structure determined in theabsence of Ca2+ also appears closed. Residues

Val282 from each of the four S6 helices form aconstricted gate with a radius less than 1 Å (Fig.3C). This finding is in good agreement withstudies that have used thiol-reactive methane-thiosulfonate (MTS) reagents to assess the re-activity of site-directed cysteine residues placedalong the S6 helix (41–44). Furthermore, it hasbeen shown that replacement of Val282 by Glyproduces a “leaky” channel that conducts currentin the absence of Ca2+ (45). The structure alsoprovides information on the molecular basis ofthe channelopathy known as hereditary xero-cytosis, a type of hemolytic anemia associatedwith human SK4 mutations Val282→Glu andVal282→Met (14, 15, 46, 47). These two gain-of-functionmutations involve precisely that residuewhich forms the narrowest constriction withinthe pore of the closed SK channel.

Structures of Ca2+-bound SK-CaMchannel complex

To visualize an open conformation and under-stand howCa2+ activation occurs, we determinedthe structure of the SK-CaM complex in the pres-ence of Ca2+ to a resolution of 3.5 Å (figs. S4 andS5). We again observed that four CaMs bind to achannel tetramer (Fig. 4, A and B), just as in theCa2+-free state. However, the density for the CaMN-lobewas significantly improved relative to thatin the Ca2+-free structure, permitting the build-ing of an entire CaMmodel. Ca2+ binding altersthe conformation of the CaM N-lobe and causesit to attach firmly to the channel. The structurereveals several previously unidentified interac-tions between Ca2+-boundCaM and the channelsubunits. First, cytosolic portions of the S1 andS2 helices, which extend into the cytoplasmicspace, directly contact CaM (Fig. 4, C and D).Such interactions provide a plausible justifica-tion for the extraordinary length of S1 and S2.Second, the N-lobe interacts with HA and HCfrom an adjacent subunit (Fig. 4, C andD, yellowsubunit). Through these newly formed interac-tions, Ca2+-bound CaMand the SK channel forman extensive interaction network, with each CaMmolecule communicatingwith three channel sub-units (Fig. 4, C and D, green, blue, and yellowsubunits). This distributed interaction could po-tentially permit conformational changes broughtabout through the binding of one CaM N-lobeto influence the neighboring CaMmolecules. Itseems likely that this structural property couldgive rise, at least in part, to the high cooperativitythat is characteristic of SK channel activation.Twomajor differences between the full-length

cryo-EMstructure and the previously determinedcrystal structure of CaM in complexwith channelfragments (HA, HB, and part of HC) (29) deserveattention. First, in the cryo-EM structure, theCaMN-lobe binding pocket recognizes S45A, thefirst helix of the S4-S5 linker (Fig. 4, C and D),whereas in the crystal structure, theN-lobe bindsto HC rather than S45A (S45A was not includedin the crystallization construct). Many of the res-idues forming the HC binding site in the crystalstructure are buried at the center of the coiledcoil in the cryo-EM structure (fig. S6). The buried

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Fig. 4. Structure of the Ca2+-bound SK-CaM channel complex. (A and B) Structure of theCa2+-bound SK-CaM channel complex, colored as in Fig. 1 (purple, CaM). Ca2+ ions are shown aspink spheres. (C and D) Interactions between CaM (surface representation) and SK subunits.

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residues are therefore likely not accessible tothe CaM N-lobe in the full-length channel. Weconclude that S45A is the functional CaMN-lobebinding site.Second, in the crystal structure, the channel

fragments and CaMs form a dimeric, twofold-symmetric structure. On the basis of that ob-servation, a mechanism was proposed in whichchannel activation breaks down the fourfold sym-metry, causing the channel to become a twofold-symmetric dimer-of-dimers. This transitionwasproposed to generate a rotary force to open thechannel (29). Although this was a reasonableidea given the structures of channel fragments,the full-length structure shows that the activatedSK-CaM complex remains fourfold-symmetricthroughout (Fig. 4, A and B, and fig. S4C).

Structural basis for channel activation

Although the amino acid sequence of S45A doesnot correspond to aknowncanonical CaM-bindingmotif (48), S45A is highly conserved, and residuesdirectly facing the CaMN-lobe pocket (Ala, Ser,and Leu) are identical across the SK channel fam-ily (Fig. 5A), suggesting that this region is crucialto channel function.When theCaMN-lobe binds,it pulls the S45A helix downward and displacesit by 4 Å (as measured at the C-terminal end ofS45A) (Fig. 5B, activated state I). This in turncauses S45B tomove outward, away from the poreaxis. Because S45B is tightly coupled to the pore-lining S6 helix (Fig. 3B), this displacement of S45Bexpands the S6 helical bundle and enlarges theradius of the cytoplasmic pore entrance to 5 Å(Fig. 5, B and D). In addition, the channel gateformed by Val282 expands. Even after this expan-sion, the Val282 side chain still constricts the poreto a radius of∼1.6 Å (Fig. 5, C andD), which is toonarrow to permit the flow of hydratedK+ ions. Inthis activated state I, the channel is undergoingmovements toward opening, although the poreappears to remain nonconductive. The existenceof such a channel conformation is supported byfunctional studies, which indicate that the max-imum channel-open probability of SK4 remainslow (49, 50), usually 0.1 to 0.3, at saturating con-centrations of Ca2+. Thus, even under maximalstimulation, the channel is still more often in anonconductive conformation.Through three-dimensional classification, we

identified another activated state (Fig. 5, B to D,activated state II) anddetermined the correspond-ing structure atmoderate resolution (4.7 Å) (figs.S4C and S7). Compared with activated state I,the CaM N-lobe binding more dramatically re-arranges S45A, S45B, and S6 in state II. S45A andS45B are displaced by an additional 2 Å (Fig. 5Band fig. S7D), which causes S6 to move furtheraway from the pore axis. This expands the chan-nel gate at the level of Val282 to a radius of ∼3.5 Å(Fig. 5, C and D), which would allow permeationof partially hydrated K+ ions. State II is less pop-ulated than state I (fig. S4C), which is consistentwith open probability measurements showingthat activated channels occupy a conductive statewith lower probability than they occupy the non-conductive states. These analyses reinforce the

notion that activated state II likely represents anopen, conductive state of SK channels, althoughfuture higher-resolution structures will be re-quired to reach an unambiguous conclusion.Our structures lead us to propose that the

binding of the CaMN-lobe to S45A initiates chan-nel activation. This proposal predicts that pre-

venting the interaction between CaM and S45Awould preclude channel opening. To test thisprediction, we introduced a cysteine at position181 on the S45A helix. Ser181 is located in themiddle of S45A and points directly into the CaMN-lobe pocket. We reasoned that labeling thisposition with a negative-charged MTS reagent

Lee et al., Science 360, 508–513 (2018) 4 May 2018 4 of 6

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Fig. 5. Channel activation by Ca2+-bound CaM. (A) Binding of CaM N-lobe to SK S45A. Thecryo-EM density of S45A is shown (from the 3.5-Å-resolution map of the Ca2+-bound SK-CaMcomplex). Ca2+ ions are shown as green spheres. Inset, the amino acid sequence alignmentof S45A from four human SK channel subtypes. The region highlighted in cyan is absolutelyconserved. (B) Conformational changes in the S4-S5 linker and pore upon CaM N-lobe binding.Structures of different states are aligned at the selectivity filter. (C) Top-down view of conformationalchanges in the pore, shown from the extracellular side. (D) Radius of the pore in the activatedstates versus the closed state. The radius is plotted as a function of the distance along the poreaxis. V282 defines the narrowest constriction site of the cytoplasmic gate. (E) Preventing channelactivation by compromising the CaM N-lobe–S45A interaction. 10 mM Ca2+ or 2 mM MTSES wasapplied as indicated. Quantification of remaining currents after MTSES labeling is also shown(mean ± SD; n = 4 to 5; Student’s t test). WT, wild type.

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(MTSES) would reduce the affinity of the CaMN-lobe and thereby impede N-lobe binding (Fig.5E). After exposure to MTSES, the current fromthe Ser181→Cys mutant channels diminished sig-nificantly (7 ± 7% remaining current), whereasthe current fromwild-type channels showedonlya small reduction (81 ± 9% remaining current).These results support the proposed role of theCaMN-lobe–S45A interaction in channel gating.

Discussion

On the basis of the structural and functional analy-ses presentedhere,wepropose anewmodel for SKchannel activation (Fig. 6). In the absence of Ca2+,CaM preassociates with the channel through itsC-lobe. Meanwhile, the CaM N-lobe maintainsonly weak interactions with the channel and isconformationally flexible. In this resting state,the channel pore is closed. In the presence ofincreasing Ca2+ concentrations, the CaM N-lobebinds to Ca2+. This triggers a conformationalchange, which increases the affinity of theN-lobefor the S45A helix within the S4-S5 linker. UponCaMbinding, S45A is displaceddownward (towardthe cytoplasm), which causes S45B tomove awayfrom the pore axis. Such a movement rearrangesthe S6 helices, permitting the pore to open. As anaid to visualizing the structural transition of SKchannel activation, we animated the CaMmove-ment and the corresponding conformationalchanges in the SK channel upon Ca2+ binding.This animation illustrates howCaMopens the SKchannel and encapsulates the activation model(movie S1).Several studies have shown that the phospho-

rylation state of CaM regulates SK channel ac-tivity (51–53). When CaM is phosphorylated atposition Thr79, the apparent Ca2+ sensitivity ofthe SK channel is reduced, causing the channelto close more rapidly (52). One hypothesis to ex-plain this is that Thr79 phosphorylation disruptsinteractions between lipids and the SK-CaMcom-plex (53). Our study provides a basis for inter-preting these functional studies and perhaps forbuilding on the hypothesis. Thr79 is positionedwithin the hinge that connects the CaMN- andC-lobes, which is wedged between S2 and HB

and faces toward the lipid membrane (Fig. 4, Cand D). The addition of a phosphate group toThr79 would seem likely to influence the con-formation of the surrounding channel subunitsand/or influence lipid-channel interactions.Riluzole, the first U.S. Food and Drug

Administration–approved medication for amy-otrophic lateral sclerosis, has been suggested toact through SK channels (54, 55). Riluzole andrelated compounds (e.g., 1-EBIO) potentiate SKchannel activity and are proposed, on the basisof crystal structures, to bind to the interface be-tween the CaMN-lobe andHC (56–59). Becauseour results redefine the native CaMN-lobe bind-ing interface, we suggest that SK channel poten-tiators may instead bind in between the CaMN-lobe and S45A (fig. S8). Such a possibility needsfurther exploration.This study provides a plausible mechanism of

SK channel activation and highlights the role ofthe S4-S5 linker in coupling Ca2+-induced CaMbinding to channel opening. Our structures alsoprovide a foundation for the development oftherapeutic agents targeting SK channels.

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ClosedClosed OpenOpenCa2+-boundCa2+-bound

S6 S4

HAHA

HBHB S45AS45A

S45BS45B

N-lobeN-lobe

CaMCaM

C-lobeC-lobe

Ca2+Ca2+

Fig. 6. Gating mechanism of the SK channel. In the absence ofCa2+ (left panel), the SK channel is closed. The CaM C-lobe staysassociated with the channel, whereas the CaM N-lobe barelybinds to the channel. The very flexible N-lobe can adopt multipleconformations, but its binding pocket remains closed (60, 61).

When Ca2+ binds to the CaM N-lobe (middle panel), the loberearranges into a more open conformation, allowing it to interactwith S45A. The N-lobe pulls the S45A helix downward, which displacesS45B away from the pore axis (right panel). This expands the S6 helicalbundle and eventually opens the pore.

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ACKNOWLEDGMENTSWe thank M. Ebrahim and J. Sotiris at the Evelyn Gruss LipperCryo-EM Resource Center of Rockefeller University for assistancewith data collection and members of the MacKinnon and Jue Chenlaboratories for helpful discussions. We gratefully acknowledgethe support of NVIDIA Corporation with the donation of the Titan XPascal GPU used for this research. Funding: This work wassupported in part by National Institutes of Health grant GM43949.C.-H.L. is supported by the Jane Coffin Childs Memorial Fundfellowship (#61-1632). R.M. is an investigator of the Howard

Hughes Medical Institute. Author contributions: C.-H.L. performedall experiments. C.-H.L. and R.M. analyzed the structures andwrote the manuscript. Competing interests: The authors declare nocompeting financial interests. Data and materials availability:Cryo-EM density maps of the SK-CaM complex have been depositedin the Electron Microscopy Data Bank under accession codesEMD-7537 (Ca2+-free state), -7538 (Ca2+-bound state I), and -7539(Ca2+-bound state II). Atomic coordinates have been deposited inthe Protein Data Bank under accession codes 6CNM (Ca2+-freestate), 6CNN (Ca2+-bound state I), and 6CNO (Ca2+-bound state II).

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/360/6388/508/suppl/DC1Materials and MethodsFigs. S1 to S8Table S1References (62–80)Movie S1

8 January 2018; accepted 16 March 201810.1126/science.aas9466

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structuresActivation mechanism of a human SK-calmodulin channel complex elucidated by cryo-EM

Chia-Hsueh Lee and Roderick MacKinnon

DOI: 10.1126/science.aas9466 (6388), 508-513.360Science 

, this issue p. 508Sciencebinds calcium. With calcium bound, it then binds to the channel and induces conformational changes that open the pore.tetramer binds constitutively to the C-lobe of calmodulin. The N-lobe of calmodulin is reasonably unconstrained until it calmodulin, a protein with two lobes, known as C and N, separated by a flexible region. Each monomer in the channelmicroscopy structures of human SK4-calmodulin channel complexes. Activation occurs when calcium binds to

electron−calcium opens the channels to conduct potassium across the cell membrane. Lee and MacKinnon report cryo both the intrinsic excitability of neurons and synaptic transmission. An increase in the concentration of intracellular

(SK) channels are expressed throughout the nervous system and affect +-activated K2+Small-conductance CaHow calcium gates a potassium channel

ARTICLE TOOLS http://science.sciencemag.org/content/360/6388/508

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CONTENTRELATED http://stke.sciencemag.org/content/sigtrans/11/558/eaat9093.full

REFERENCES

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