Measurement of the Isometric Force Exerted by a Single Kinesin Molecule

The past year has brought remarkable advances in instru-mentation, making it possible to measure the movements of a single motor protein and the associated forces with nanom-eter and piconewton precision on a millisecond time scale (Svoboda et al., 1993; Finer et al., 1994; Simmons et al., 1994). Discrete displacement steps were discovered for both kinesin (Svoboda et al., 1993) and myosin (Finer et al., 1994; Ishijima et al., 1994), and the isometric force transients ex-erted by myosin were measured (Finer et al., 1994). The studies of Svoboda et al. and Finer et al. were made possible by the use of a laser trap that attenuated the Brownian move-ment of the microscopic position-marking bead tethered to the motor or to its associated filament (for review, see Svoboda and Block, 1994). Without a laser trap, the displace-ment steps would be buried in the Brownian noise (Malik et al.