Preparation of a flexible, porous polyacrylamide substrate for mechanical studies of cultured cells.

velocity drops to ~86 nm/sec and individual steps become apparent. The mean size of steps may be determined by taking the power spectrum of the autocorrelation function of the record.18,31,36This spectrum (Fig. 7B, inset) displays a prominent peak at a spatial frequency of 0.121 nm -1, corresponding to a step size of 8.26 nm for this particular record. This value-which involves no elastic corrections-is identical, within experimental error, to the kinesin step size of 8.3 :::t::0.2 nm estimated in earlier work using a fixed trap, to which a 19% adjustment for series compliance had been applied.18 The force clamp enables us, for the first time, to acquire stepping data for kinesin molecules subjected to high loads over distances as great as 200-300 nm, a feat previously impossible with a stationary trap.