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KINESIN, a two-headed motor enzyme molecule, hydrolyses ATP to direct organelle transport along microtubules. As it moves along a microtubule, kinesin remains associated with, or 'tracks', micro-tubule protofilaments^1,2. We have prepared truncated kinesin derivatives that contain either two mechanochemical head domains³ or only a single head. Unlike intact kinesin and the two-headed derivatives, the one-headed enzyme frequently fails to track protofilaments, suggesting that it detaches from microtubules during movement. In this way, the one-headed kinesin derivative is similar to the motor enzyme myosin, which frequently detaches from the actin filament during movement⁴. For myosin (which has two heads), the consequence of this detachment is that single molecules do not appear to drive continuous movement along the filament⁵. Our observations suggest that the ability of single two-headed kinesin …