Researchers discover structure of Nature's 'circuit breaker'

st work has revealed many new details about the architecture and inner workings of potassium channels -- the voltage-sensing mechanism remained a frustrating mystery. "The general principle was understood that the electric field on the membrane somehow moved 'gating charges' on the channel that would somehow open it to allow potassium to flow," he said.

Figuring out the location and function of the charge-carrying structure remained a major problem, he said. "This has been a central project for me and members of my lab for a very long time," said MacKinnon. "We've been working on this for almost six years."

Indirect experiments in other laboratories suggested that the channel included some sort of voltage-sensing mechanism buried deep within the protein that slid back and forth with changing membrane charge to open or shut the channel.

However, the only way to reveal the mechanism definitively, said MacKinnon, was to obtain high-resolution x-ray crystallographic images of the structure. Such images are produced by beaming x-rays through purified protein crystals of the channels and then using a computer to analyze the patterns of diffracted x-rays to deduce the structure of the channel.

Producing the crystals necessary for such studies presented a major challenge, said MacKinnon. "Membrane proteins are notoriously hard to crystallize because they are embedded in this oily membrane, and to get them out of the membrane and crystallize them, you have to use detergents. And these detergents form a sort of 'life jacket' around the proteins, making them mushy and not amenable to crystallizing.

"What's more, we had another strike against us in crystallizing the potassium channel protein, because it had many moving parts, which really prevented crystallization," he said. After years of testing many different potassium channel proteins and crystallization techniques, the researchers finally found one protein -- from the thermophilic

Contact: Jim Keeley
Howard Hughes Medical Institute

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