Opening Potassium Channels to Scrutiny

Opening Up Kv Structure

Kv channels have three essential parts: the pore, the gate, and the voltage sensor. The voltage sensor detects membrane voltage, and with a change in conformation - involving a key helix called S4 - it opens the gate. The loose protein structure of Kv channels makes getting an atomic-level picture of its structure difficult, since many channels will not form usable crystals. After five years of work, MacKinnon's group coaxed KvAP to form crystals by binding it to an antibody.1 But to many "this structure was distorted," says Richard Horn (who was not involved in the study) from Thomas Jefferson University in Philadelphia, leading to debate and a flurry of research on Kv channels.

Stephen Long, first author on the Kv1.2 paper and now at Memorial Sloan-Kettering Cancer Center, says he and his colleagues did away with the antibody for Kv1.2. Instead, they "used a combination of lipids and detergents," says Long, a new technique that gave them crystals reminiscent of a lipid-bilayer, as detailed in the first Hot Paper. In the second Hot Paper, the resulting structure shows the voltage sensors as upright, independent domains. This wasn't predicted before hand, but it is consistent with previous results and therefore suggests that the protein is in a native state. Horn says the structure made a lot more sense.

The idea of voltage sensors as independent domains was recently supported by David Clapham from Harvard Medical School and Yasushi Okamura from Okazaki Institute for Integrative Bioscience, who independently discovered voltage-sensor proteins without pores.4,5 These proteins conduct H+ ions and might be the long sought after voltage-gated H+ channels involved in processes such as oxidative bursts in leukocytes. Most people thought of the Kv channel structure as requiring a pore to hold the voltage sensors together, since they are highly charged yet span the membrane, says Michael Grabe from the University of Pittsburgh, who was not involved in the study. "That you can chop these proteins up, and they work, no one would have believed five years ago," says Grabe.

Closing in on the Details

Despite the insight gleaned from the Kv1.2 structure, it shows only the channel in an open state. "To understand how the voltage sensor moves from one state to another, we need to see it in the down [closed] state," says Grabe. Seeing the closed state would help solve another controversy: How far does the S4 segment move between states?

Recently, Grabe and colleagues took a closer look at the S4 segment in the closed state by using conditional lethal/second-site suppressor yeast screens.6 Their works suggests that the movement of S4 is quite large, on the order of 12 Ångstroms. Other work by Francisco Bezanilla at the University of Chicago and colleagues, using histidine scanning mutagenesis to determine the constraints of the S4 segments, suggests its movement is smaller (6.5 Å).7 "There is still some discrepancy," says Horn, "but each side is kind of morphing towards the other." Horn and others agree that a closed crystalline structure will likely appear soon.

The Kv1.2 structure also left other things unanswered, since some of the structure (such as the voltage sensors) is blurred and at low resolution, says Roux. "So there is a bit of debate about details." Roux and his colleagues recently used the Kv1.2 structure in computer simulations to model these low resolutions areas, surprisingly predicting that the membrane is thinner around the ion channel.8 This could make the electric field stronger around the channel, and voltage sensing easier.

In terms of working out the details, "it's a lot of work to basically climb this one Kv1.2 mountain," says Roux, but it's leading the way to other Kv structure determinations and aiding scientists in understanding pathologies associated with ion channels.

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