Torque Generation by Axonemal Outer-Arm Dynein

Outer-arm dynein is the main engine providing the motive force in cilia. Using three-dimensional tracking microscopy, we found that contrary to previous reports Tetrahymena ciliary three-headed outer-arm dynein (alpha beta gamma) as well as proteolytically generated two-headed (beta gamma) and one-headed (alpha) subparticles showed clockwise rotation of each sliding microtubule around its longitudinal axis in nnicrotubule corkscrewing assays. By measuring the rotational pitch as a function of ATP concentration, we also found that the microtubule corkscrewing pitch is independent of ATP concentration, except at low ATP concentrations where the pitch generated by both three-headed alpha beta gamma and one-headed alpha exhibited significantly longer pitch. In contrast, the pitch driven by two-headed beta gamma did not display this sensitivity. In the assays on lawns containing mixtures of alpha and beta gamma at various ratios, the corkscrewing pitch increased dramatically in a nonlinear fashion as the ratio of a in the mixture increased. Even small proportions of alpha-subparticle could significantly increase the corkscrewing pitch of the mixture. Our data show that torque generation does not require the three-headed outer-arm dynein (alpha beta gamma) but is an intrinsic property of the subparticles of axonennal dyneins and also suggest that each subparticle may have distinct mechanical properties.