ROCKEFELLER UNIVERSITY PHOTO

Roderick MacKinnon, a biophysicist and self-taught X-ray crystallographer at Rockefeller University, was awarded the 2003 Nobel Prize in Chemistry for elucidating the chemical mechanisms of potassium ion channels. Potassium ion channel proteins span cell membranes, rapidly shepherding potassium ions in and out of cells during muscle contraction and nerve impulses. Remarkably, these channels don't allow sodium ions--which have an identical charge and are only a bit smaller than potassium ions--across the membrane.

MacKinnon and others spent more than a decade trying to determine the chemical basis of potassium channels' exquisite selectivity by carefully mutating individual amino acids in the protein. "Eventually, it became very clear to me that I would never understand it without seeing the channel. That's what drove me to structural biology," MacKinnon says. Here, he tells C&EN about his most memorable "Aha!" moment:

"The resolution of our first X-ray crystallographic structure of a potassium ion channel was only 3.2 . At that resolution, the protein skeleton was visible, but we couldn't discern individual potassium ions in the channel. The 'Aha!' experiment was the one in which we soaked the crystals in a solution containing rubidium ions, which are similar in size to potassium but much more electron dense. I knew from working with potassium channels for a long time that rubidium would go through the channel. We collected X-ray data from crystals in which potassium had been replaced by rubidium and used the phases from the protein structure to calculate a difference Fourier [electron density map].

"To me, that was really the most memorable moment of my career. In the difference map, you could see the rubidiums lined up along the ion pathway. I was very well aware of [Alan L.] Hodgkin and [Andrew F.] Huxley's 1955 paper, in which they studied radioisotopic fluxes across potassium ion channels in a cuttlefish axon. From these experiments, they came to the conclusion that whatever the basic mechanism of potassium transport was, it somehow involved two or three ions, lined up in a queue. It was a brilliant set of experiments that they did--a beautiful prediction. And to see it--it was fantastic, really a privilege, to be in a position to see that."

CRYSTAL CLEAR In MacKinnon's high-resolution structures, a queue of potassium ions (green; their electron density is shown as blue mesh) can be seen in ion pathway of the potassium channel protein KcsA.
COURTESY OF R. MACKINNON

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