ANRV288-CB22-02 ARI 30 August 2006 17:1 How Does Voltage Open an Ion Channel? Francesco Tombola,1,4 Medha M. Pathak,2 and Ehud Y. Isacoff1,2,3,# 1Department of Molecular and Cell Biology, University of California, Berkeley, California 94720; email: [email protected] 2Biophysics Graduate Group, University of California, Berkeley, California 94720; email: [email protected] 3Physical Bioscience Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720; email: [email protected] 4Department of Biomedical Sciences, University of Padova, Padova, Italy 35121 Annu. Rev. Cell Dev. Biol. 2006. 22:23–52 Key Words First published online as a Review in potassium channels, voltage gating, coupling, cooperativity, lipid Advance on May 5, 2006 by CAPES on 08/30/07. For personal use only. The Annual Review of Abstract Cell and Developmental Biology is online at http://cellbio.annualreviews.org Neurons transmit information through electrical signals generated by voltage-gated ion channels. These channels consist of a large This article’s doi: superfamily of proteins that form channels selective for potassium, 10.1146/annurev.cellbio.21.020404.145837 sodium, or calcium ions. In this review we focus on the molecular Copyright c 2006 by Annual Reviews. mechanisms by which these channels convert changes in membrane All rights reserved Annu. Rev. Cell Dev. Biol. 2006.22:23-52. Downloaded from arjournals.annualreviews.org voltage into the opening and closing of “gates” that turn ion con- 1081-0706/06/1110-0023$20.00 ductance on and off. An explosion of new studies in the last year, #To whom correspondence should be including the first X-ray crystal structure of a mammalian voltage- addressed. gated potassium channel, has led to radically different interpreta- tions of the structure and molecular motion of the voltage sensor. The interpretations are as distinct as the techniques employed for the studies: crystallography, fluorescence, accessibility analysis, and electrophysiology. We discuss the likely causes of the discrepant re- sults in an attempt to identify the missing information that will help resolve the controversy and reveal the mechanism by which a voltage sensor controls the channel’s gates. 23 ANRV288-CB22-02 ARI 30 August 2006 17:1 They “gate” rapidly, within one or a few mil- Contents liseconds, and when open they selectively con- duct specific ions. The coordinate function of INTRODUCTION................. 24 these channels produces signals (∼one-tenth THE ACTIVATION GATE......... 26 of a volt) in remarkably brief spurts (as short as Where Is the Gate Located? ...... 26 one millisecond) that can repeat at very high Which Parts of the Protein Form rates (up to 1000 per sec) and travel rapidly the Gate? ..................... 26 (∼100 meters per sec), even in extremely thin VSD ORGANIZATION AROUND (∼1-micron-diameter) processes, for long dis- THE PORE DOMAIN .......... 28 tances (meters) without decrement. Remark- GATING CHARGE able. How do they do it? We review here the MOVEMENT ................... 29 molecular properties of these channels, focus- VOLTAGE-SENSING ARGININES 30 ing mainly on voltage-gated potassium (Kv) MODELS OF VOLTAGE channels, which have been the objects of the SENSING ....................... 31 most extensive investigation in this area and Transporter Model ............... 33 have had a veritable explosion of progress in Helical Screw Model ............. 33 the past year. Paddle Model .................... 33 Kv channels are made of four subunits, Are We Ready for a Unified each containing six transmembrane segments Model? ....................... 34 that are named S1 through S6 (Figure 1a,b). VOLTAGE-GATED CHANNELS Helices S5 and S6 of the four subunits, as HAVE FOUR ADDITIONAL well as the P-helix and the loop that connects PORES .......................... 37 the helices, assemble together to form a cen- CREVICES WITHIN THE VSD tral pore domain, which contains the channel’s AND FOCUSING OF THE K+-selective pathway and gates. Four voltage- MEMBRANE ELECTRIC sensing domains (VSDs), each made of he- FIELD .......................... 39 lices S1–S4, surround the pore domain and COUPLING OF VOLTAGE control its gates. Subtype-selective assembly SENSING TO GATING ........ 40 of the channel-forming subunits is controlled WORKING TOGETHER TO by an intracellular N-terminal tetrameriza- OPEN THE GATE .............. 42 by CAPES on 08/30/07. For personal use only. tion domain (T1) (Figure 1a,b, green), which LIPID: THE MISSING PLAYER? . 43 also serves as a scaffold to bind accessory β subunits (Kvβs). Instead of four separate sub- units, voltage-gated sodium (Nav) and cal- cium (Cav) channels have four covalently con- INTRODUCTION nected domains; each domain has a secondary Annu. Rev. Cell Dev. Biol. 2006.22:23-52. Downloaded from arjournals.annualreviews.org The action potential, the secretion of hor- structure similar to that of a single subunit of mones and neurotransmitters, the heart beat, a potassium channel. Nav and Cav channels the reaction of an egg that prevents fertil- are evolutionarily related to each other and ization by multiple sperm, the contraction of to Kv channels (Yu & Catterall 2004). Bacte- skeletal muscle, and the control of transpira- rial voltage-gated sodium channels are closely tion from the leaves of a plant are diverse bio- related to both eukaryotic Cav and Nav chan- logical phenomena that have at least one thing nels, but they are made of four independent in common: They are all mediated by voltage- subunits, like potassium channels (Koishi et al. gated ion channels. These channels respond 2004, Ren et al. 2001). to changes in the gradient of voltage across The ion-conducting pathway of voltage- the membrane by opening and closing an ion gated channels allows permeation at a high conductance pathway across the membrane. rate, on the order of 106–108 ions per second, 24 Tombola · Pathak · Isacoff ANRV288-CB22-02 ARI 30 August 2006 17:1 Figure 1 (a) The architecture of a Kv channel subunit. Cylinders are helical segments. The pore domain is shown in blue, the voltage-sensing domain (VSD) in red, the S4-S5 linker in purple, and the tetramerization domain in green. (b) A single Kv1.2 subunit color coded as in a. Potassium ions are colored yellow. The Kv1.2 tetramer (c) top view (extracellular side) and (d ) side view. Each subunit is shown in a different color. Potassium ions are colored purple. Coordinates from Long et al. (2005a), PDB ID 2A79. All the molecular drawings have been created using by CAPES on 08/30/07. For personal use only. Swiss-Pdb viewer (http://www.expasy. org/spdbv/). and can discriminate between ions with re- cium and sodium channels, and Gouaux & markable efficiency. Potassium channels, for MacKinnon (2005) reviewed general princi- + Annu. Rev. Cell Dev. Biol. 2006.22:23-52. Downloaded from arjournals.annualreviews.org example, have a permeability ratio for K ples of ion transport by channels and pumps. over Na+ of >100:1, and calcium channels The process by which the ion-conducting select for Ca2+ over Na+ with a ratio of pathway of voltage-gated channels opens in >1000:1. Much progress has been recently response to changes in membrane poten- made in understanding the mechanism un- tial is called activation; it is the subject derlying ion permeation in potassium chan- of the present review. The activation gate, i.e., nels. MacKinnon (2003), Armstrong (2003), the element that physically opens and closes and Roux (2005) provide recent reviews on the transmembrane ion conduction pathway, permeation and selectivity of potassium chan- is discussed first. Then, the mechanism by nels. French & Zamponi (2005), Sather & which voltage-gated channels detect changes McCleskey (2003), and Yu & Catterall (2003) in membrane potential and control their acti- recently reviewed ion conduction in cal- vation gate is discussed in the context of the www.annualreviews.org • Voltage-Induced Channel Activation 25 ANRV288-CB22-02 ARI 30 August 2006 17:1 recent crystal structure of the Kv1.2 chan- ies on N-type inactivation in A-type (fast in- nel. The last section of this review deals with activating) potassium channels. Fast inactiva- the interaction between Kv channels and the tion involves the binding of the N-terminal Shaker: fast-inactivating membrane lipid. domain, also referred to as the ball, to its voltage-gated receptor on the intracellular side of the chan- potassium channel nel (Hoshi et al. 1990, Iverson & Rudy 1990, from Drosophila THE ACTIVATION GATE Zagotta et al. 1990). As with internal QA ions, melanogaster that is The pore domain of a voltage-gated ion chan- the N-terminal ball binds to the pore only the best known member of the nel contains the permeation pathway, which when the activation gate is open and acts via Shaker/Kv1 family is opened and closed by two distinct molec- a “foot-in-the-door” mechanism, making it Quaternary ular gates: the activation and slow inactiva- harder, or impossible, for the gate to close ammonium (QA): tion gates. We focus here on the activation while it is bound (Demo & Yellen1991). Thus, class of potassium gate. In most voltage-gated channels at rest- the activation gate appears to be located on channel blockers ing potential (∼−70 mV in neurons), the ac- the intracellular mouth of the ion-conducting NGK2: tivation gate is closed, and membrane depo- pore. voltage-gated larization causes a conformational change in potassium channel the VSDs that is transmitted to the pore do- from a main and results in opening of the gate. It has Which Parts of the Protein Form the neuroblastoma- Gate? glioma hybrid cell been estimated that the conductance of a sin- line. It belongs to the gle Shaker channel drops at least 105 times, Choi et al. (1993) reported that mutations in Shaw/Kv3 subfamily going from the open to the closed state (Soler- the S6 helix alter the internal binding of QA Tetraethyl Llavina et al.