How Ion Channels Sense Membrane Potential

COMMENTARY How ion channels sense membrane potential Richard Horn* Department of Physiology, Institute of Hyperexcitability, Jefferson Medical College, Philadelphia, PA 19107 nderstanding the inner work- ings of an ion channel has, in the last several years, boiled down to a hunt for dynamic Ustructural cartoons that capture the es- sence of the channel’s biophysical behav- ior. The main approach to this end has been the ‘‘structure–function’’ study in which selected residues are mutated and the functional consequences are explored electrophysiologically. Functionally impor- tant regions of the ion channel protein are often revealed from this type of experi- ment. Nevertheless, these studies are typi- cally devoid of anything approaching real structural data. Therefore, the cartoon models depicting, for example, the way ion channels open and close are largely fantasies, however much insight they pro- vide. This deficiency changed dramatically with the first crystal structure of a bacte- rial potassium channel (1). The signifi- cance of this scientific breakthrough included the sobering realization that standard structure–function studies can result in absurd structural predictions, Fig. 1. Models of topology and voltage sensor movement. The membrane is indicated by horizontal highlighting the importance of atomic lines. (A) Topology of a potassium channel subunit with six transmembrane segments (S1–S6). The S3 level structures to accompany functional segment is divided into two helices, S3a and S3b. The S4 segment is in red. (B) Conventional model of studies. voltage sensor movement. Two of the four subunits are shown (sliced open to expose the ‘‘gating pore’’) The most captivating property of the with the ion permeation path between them. The foreground and background subunits are removed for ion channels responsible for action poten- clarity. Gray represents protein. Depolarization (Right) moves the extracellular portion of the S4 segment tials in excitable cells (nerve and muscle (red) outward through a short hydrophobic gating pore, opening the permeation pathway. Most of the cells) is their exquisite sensitivity to small S4 segment is surrounded by hydrophilic vestibules. The transmembrane electric ﬁeld falls mainly across changes of membrane potential. This is the gating pore. (C) Paddle model, which is adapted from ref. 4. Two opposing subunits are shown. Depolarization moves the paddle outward through lipid, pulling the cytoplasmic activation gate open. arguably the most highly scrutinized fea- The transmembrane electric ﬁeld felt by S4 arginines falls mainly across the lipid. B and C are adapted from ture of these proteins since Hodgkin and ref. 11. Huxley (2) first described the sodium and potassium currents of the squid giant axon in 1952. However, despite a wealth of principal voltage sensor of the channel aqueous vestibules surrounding the S4 electrophysiological studies and a surfeit (9). Every third residue of S4 is basic, usu- segment (9). In all of these models, the of discarded cartoons, the atomic struc- ally an arginine, and neutralizing any of side chains of the S4 arginines transfer ture of a voltage-gated ion channel re- the first four of these arginines reduces their charges through proteinaceous ter- mained elusive until 2003, when the the channel’s sensitivity to changes of rain. In the paddle model, the S4 segment MacKinnon laboratory obtained a crystal membrane potential. The consensus opin- and the C-terminal helix of the S3 seg- structure of the potassium channel KvAP ion is that the positively charged S4 segment (S3b) form a compact structure (the (3). A functional study of KvAP by the ment can be electrophoresed across the voltage sensor paddle) that extends into same group led to a proposal, known as membrane electric field in response to lipid at the periphery of the channel pro- the ‘‘voltage sensor paddle model’’ (4), changes of membrane potential; this tein (Fig. 1C). In response to a change of that was so surprising and apparently con- membrane potential, the paddle moves a movement controls whether the perme- tradictory with a wide variety of previous large distance (15–28 Å; refs. 4 and 10) ation pathway is available for the flux of experimental data that it generated an through the lipid, schlepping its cargo of extensive controversy (e.g., see refs. 5–7) ions. Exactly how the S4 segment moves positive charges along with S3b. and a flurry of new experimental studies. distinguishes the paddle model from all What’s so contentious about the paddle The results of most of these studies are at others. In many BP-era models, the S4 model? That depends on how it is de- variance with the paddle model, including segment is largely surrounded by aqueous fined. In the originally published version those in the article by Gonzalez et al. (8) solutions and moves its charge through a (figure 5 in ref. 4), the S2 and S3a seg- in this issue of PNAS. hydrophobic gasket made primarily of ments were oriented in the same plane as In the BP (before paddle) era, voltage- protein (Fig. 1B). Although several varia- gated potassium channels were shown to tions on this theme have been proposed, be tetramers in which each subunit had six all of them involve a small physical move- See companion article on page 5020. transmembrane segments (Fig. 1A). The ment of the outer four arginines through *E-mail: [email protected]. fourth transmembrane segment, S4, is the an electric field highly focused by the © 2005 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0501640102 PNAS ͉ April 5, 2005 ͉ vol. 102 ͉ no. 14 ͉ 4929–4930 Downloaded by guest on September 28, 2021 the bilayer, with the S1–S2 and S2–S3 the cytoplasmic side. Moreover, be- movement of the S4 segment during linkers buried within lipid. In a modified cause S4 is moving its outermost charges charge movement. version (10), the S1–S4 segments were all through the electric field, a physically Gonzalez et al. (8) applied a second test rotated into transmembrane orientations, large S4 movement should carry any of the paddle model. If S4 shuttles its consistent with experimental data on charged residues on S3b at least partway charges a long distance across the electric potassium channel topology (5–7). The through the electric field. Small-movement field, then it should also move other pad- original paddle model also advanced the conventional models, by contrast, predict dle residues, whether charged or not, energetically unpalatable idea that the that S3b will remain near the extracellular through the electric field. Therefore, any side chains of charge-carrying arginines surface at all voltages and that charged changes in the total charge of paddle resi- cross the membrane in intimate associa- residues on S3b will not contribute to the dues should influence the voltage depen- tion with the hydrocarbon core of the bi- load carried by the S4 arginines. Gonzalez dence of gating. The number of charges layer rather than with protein. Subsequent et al. (8) tested the voltage-dependent moved completely through the electric experimental data suggest otherwise (11– accessibility of S3b residues by cysteine field during gating was estimated by the 13), and a newer variant of the model, scanning with extracellular methanethio- steepness of the voltage dependence of although only partially fleshed out, ac- sulfonate-ethyltrimethylammonium channel opening at very negative voltages, knowledges this accommodation (10). (MTSET), a permanently charged cys- the ‘‘limiting-slope’’ method (19, 20). Other controversial aspects of the paddle teine reagent that is excluded from the Gonzalez et al. (8) found that even drastic model are discussed elsewhere (5–7). hydrophobic lipid. Also, they tested changes in total charge, e.g., neutralization In the space of 2 years, the paddle whether the side chains of S3b residues of four consecutive acidic residues at the model has morphed into something more move through the electric field by altering extracellular end of S3b (a total of 16 ele- akin to the previous models. To be fair, the charge of these residues and determin- mentary charges for the four subunits), these older models have also taken on ing whether this affects the total number had no effect on the number of charges one feature of the paddle model. In many of charges coupled to channel opening. coupled to gating. Moreover, charge alter- of the previous proposals, including sev- The voltage-dependent accessibility of ations in the middle of S3b had no effect eral from my laboratory, the S4 segment cysteines introduced into S3b was exam- on limiting slope in the short-linker con- was completely insulated from lipid. We ined previously (16, 17), and, in both of struct. These results are consistent with an and others now acknowledge that, al- these studies, the cysteines were accessible earlier study (11) in which the charge of though the business district through which to extracellular MTSET at hyperpolarized paddle residues was altered not by muta- S4 transports its charges is proteinaceous, voltages. Gonzalez et al. (8) extended the tion but by attaching charged adducts to at least some of S4’s backside is exposed scan all of the way down to the C termi- introduced cysteines. All of these results to lipid. Nevertheless, the two classes of nus of S3a (residue I315) in Shaker potas- are inconsistent with a large movement of models continue to differ in one key sium channels. The mutant I315C was the S4 segment during gating, a hallmark aspect, the physical magnitude of S4 equally reactive to extracellular MTSET at feature of the paddle model. movement. The paddle model proposes a Ϫ110 mV as at ϩ100 mV. It could be Despite extensive efforts to understand substantially larger transmembrane dis- argued, however, that these results are not voltage sensor movement, there is no con- placement of the S4 segment, because the in direct contradiction with the paddle sensus model.

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