Abstract
Kinesin and cytoplasmic dynein are mechanochemical ATPases thought to transport vesicular organelles within cells by moving them along microtubules. Molecules from purified enzyme preparations will bind to plastic microspheres (beads) and move them along microtubules in vitro at rates of ∽0.5 μm s-1. Bead movement can be directly observed by light microscopy in an assay that provides an opportunity to study motions driven by small numbers of enzyme molecules. Measurement of such motions with nanometer-scale precision yields information about the molecular events which drive bead movement.We devised a method to measure the relative position o£ an enzyme-driven 150nm bead with a precision better than 1 nm at 30 Hz2. This method is based on digital processing of bead images in differential interference contrast microscopy using a cross-correlation/centroid algorithm.