Courtesy of Steven Block

It had long been assumed that motor proteins would advance in discrete steps. However, the steps posed a challenge for measurement because they were small enough to be comparable to the background noise due to the Brownian (thermal) motion. During 1993, together with graduate student Karel Svoboda, postdoc Christoph Schmidt, and kinesin codiscoverer Bruce Schnapp, I set out to show that kinesin moved in a stepwise fashion along microtubules, and to measure the size of those steps. We built an optical trapping apparatus that could capture and manipulate a microscopic bead to which a single kinesin molecule had been bound. The added mechanical stiffness imposed by the optical trap suppressed some of the random noise. When we first saw the steps on the printout, it was a truly satisfying 'Aha!' moment.

- Steven Block, professor of biological sciences and applied physics, Stanford University

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