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Seeing little things by attaching bigger things to them

by Andre on 27 February 2006

The National Lecturer at this year’s Biophysical Society meeting was Kazuhiko Kinosita from Waseda University in Japan. I didn’t know much about his work before seeing the talk and that made it all the more enjoyable. The picture that the society used to promote the talk on all of the meeting’s printed materials shows an actin filament rotating due to the action of F1-ATPase, a rotary motor made out of protein. ATPase spins actin This experiment was the first to show conclusively that F1-ATPase was a rotary motor and Steve Block suggested in his introduction that it was a critical experiment that triggered the 1997 Nobel prize decision in favour of Boyer and Walker.

But he’s also done some interesting things since then using a similar approach. He presented some still unpublished work on myosin in which his students attached an actin filament to one of the legs of myosin-V to directly visualize its stepping. The actin is fluorescently labelled so that it’s visible under a light microscope. The resulting video was incredible. It was just a white bar moving around on a black background (just like his previous work on f1-ATPase was just a lumpy spinning rod) but what it demonstrated was great. There was a clear power stroke followed by a Brownian search as the unbound head of myosin tried to find a binding site on actin to complete its step. Unfortunately, I don’t have a copy of the video to share with you but I’ll be sure to pass on the details of the paper when it’s published.

Going back to F1-ATPase, Kinosita discussed some recent work, and in keeping with the theme of the talk, it involved attaching a microscale reporter (in this case magnetic beads) to amplify the motor’s nanoscale behaviour. Using magnetic tweezers, they were able not only to observe the rotary motion but also to apply a retarding torque to the beads. Since they were applying a known torque to the beads and could observe their rotation using an optical microscope, they could calculate the energy generated by the motor during a complete turn. Then, by comparing this calculated value with the energy released during the hydrolysis of three ATP molecules (that’s how many the motor uses to make a complete turn) they found that the motor’s efficiency was near one! But that’s not all, they were also able to apply torques great enough to run the motor in reverse and could actually show that ATP was created in the process (around two per rotation). That’s like rolling your car backwards down a hill to fill its gas tank. This is another example of our human scale intuition breaking down hard at the nanoscale. It’s also a great demonstration of mechanical biochemistry.

In addition to the results, I also appreciated Kinosita’s attitude towards his research: he wants to see some motion directly and his naive-seeming approach of attaching big sticks to things he wants to visualize is satisfyingly effective. I look forward to hearing more about his efforts in the future. If you can’t wait for more, check out his entertaining essay My Biophysics.

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