The first x-ray structures using the crystals revealed a startling surprise about the voltage-dependent potassium channel, said MacKinnon. Instead of being embedded deep within the protein, the voltage-sensing gating charges appeared to be incorporated in "paddles" on the outside of the protein.
Working from the x-ray structure, the researchers theorized that these positively charged paddles would flip-flop back and forth from inside to outside the membrane, according to the charge across the membrane. When the membrane became negatively charged on the outside, the paddles would be attracted and would flip toward the outside, opening the channel and allowing potassium to flow out, restoring the membrane charge to its resting state. And when the inside of the membrane became negatively charged, the paddles would flip back, snapping the channel shut.
Indeed, in the second Nature paper, MacKinnon and his colleagues proved that the paddles actually functioned in the way suggested by the structure. In those biochemical experiments, they used antibodies and other chemicals to grab the paddles on one or the other side of the membrane, proving that they flopped back and forth with changing membrane charge.
"So, we've shown that the membrane voltage decides whether the channel's open, because the paddle feels the voltage and goes one way or the other," said MacKinnon. "It's a feedback loop. The channel sets the membrane voltage, but the membrane voltage decides whether the channel's open. This is precisely the kind of feedback loop that you need in sodium and potassium channels to propagate a nerve impulse."