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. Preli minary measurements suggest that the mechanical system is very complia nt, with an apparent stretch of 120 nm with <2 pN of force. Although m easurements 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. Wi th torques <2 pN-mu m similar to 1 Hz in 2 mM AMP-PNP, there is no app arent limit to the number of revolutions that the microtubule can rota te around the kinesin attachment site (n = 44). Preliminary data from other nucleotide conditions are similar. Although there are rare insta nces of torsional elasticity where the attachment site unwinds, the re storing 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.''