REEXAMINATION OF THE POLARITY OF MICROTUBULES AND SHEETS DECORATED WITH KINESIN MOTOR DOMAIN

Electron microscope images of microtubules and tubulin sheets decorate d with kinesin head domains have shown the main mass of the kinesin he ad domain to be superimposed on one subunit of each tubulin dimer. We have polymerized brain tubulin extensions on to the ends of flagellar axonemes under varied conditions, in order to check the polarity of th e tubulin-kinesin head complex. Since the polarity of axonemes incubat ed with normal brain tubulin may be ambiguous, we also tried 50% N-eth ylmaleimide-treated tubulin which specifically blocks minus ends. Our conclusion, which conflicts with recently published results, is that t he main mass of the kinesin head is associated with the tubulin subuni t closer to the plus end of a microtubule.