AbstractApplying a new in vitro motility assay system for microtubules and 22S dynein, we recently reported on an ATP‐induced extrusion of microtubules from microtubule‐dynein α‐ and β‐complexes [Mimori and Miki‐Noumura, 1994:Cell Motil. Cytoskeleton27:180–191]. In the present study, we prepared a γ‐complex by copolymerizing porcine brain tubulin andTetrahymenaciliary 22S dynein, and examined the ATP‐induced microtubule movement from the γ‐complex. The extrusion process appeared quite similar to that of the β‐complex. The sliding velocity was 18.39 ± 2.20 m̈m/sec, which was a value comparable to that of trypsin‐digested flagellar axonemes [Yano and Miki‐Noumura, 1980:J. Cell Sci.44:169–186]. Higher velocity may be due to a densely arranged dynein‐track with the same polarity, which was detached from the γ‐complex and absorbed in rows on a glass surface of the slide. Sometimes a free‐floating microtubule in the perfusion chamber was observed riding and sliding on the dynein‐track remaining on the slide after extrusion.Unexpectedly, we found that when the front part of the microtubule was fixed to a glass surface, a continuous sliding microtubule at the rear part on the dyneintrack often transformed into a left‐handed helix, and subsequently a twisted helix with several turns. The helix formation may be due to some rigidity in the microtubule and a right‐handed torque component in the sliding force of 22S dynein. The addition of ATP may release some distortion accumulated in the complex structure during copolymerization of tubulin and 22S dynein, inducing reverse rotati