KINESIN FOLLOWS THE MICROTUBULES PROTOFILAMENT AXIS

We tested the hypothesis that kinesin moves parallel to the microtubul e's protofilament axis. We polymerized microtubules with protofilament s that ran either parallel to the microtubule's long axis or that ran along shallow helical paths around the cylindrical surface of the micr otubule. When gliding across a kinesin-coated surface, the former micr otubules did not rotate. The latter microtubules, those with supertwis ted protofilaments, did rotate; the pitch and handedness of the rotati on accorded with the supertwist measured by electron cryo-microscopy. The results show that kinesin follows a path parallel to the protofila ments with high fidelity. This implies that the distance between conse cutive kinesin-binding sites along the microtubule must be an integral multiple of 4.1 nm, the tubulin monomer spacing along the protofilame nt, or a multiple of 8.2 nm, the dimer spacing.