Single kinesin molecules stressed with optical tweezers.

Using the optical tweezers to pull on microtubules, we have stretched and twisted single kinesin molecules adsorbed to glass surfaces. Preliminary measurements suggest that the mechanical system is very compliant, with an apparent stretch of 120 nm with < 2 pN of force. Although measurements of the series compliance of the bead-microtubule structure are still in progress, the kinesin attachment site does not slip with stretch. However, under torsional stress, kinesin appears to slip. With torques < 2 pN-microns approximately 1 Hz in 2 mM AMP-PNP, there is no apparent limit to the number of revolutions that the microtubule can rotate around the kinesin attachment site (n = 44). Preliminary data from other nucleotide conditions are similar. Although there are rare instances of torsional elasticity where the attachment site unwinds, the restoring forces are not constant with angular position, also indicating slippage. Mechanisms of mechanochemical transduction must account for linear force generation in the presence of angular "slippage."