Inner-arm dynein c of Chlamydomonas flagella is a single-headed processive motor

Axonemal dyneins are force-generating ATPases that produce movement of eukaryotic cilia and flagella(1,2). Several studies indicate that inner-arm dyneins mainly produce bending moments in flagella(3,4) and that these motors have inherent oscillations in force and motility(5-8). Processive motors such as kinesins have high duty ratios of attached to total ATPase cycle (attached plus detached) times(9) compared to sliding motors such as myosin(10). Here we provide evidence that subspecies-c, a single-headed axonemal inner-arm dynein, is processive but has a low duty ratio. Ultrastructurally it is similar to other dyneins(11-14), with a single globular head, long stem and a slender stalk that attaches to microtubules. In vitro studies of microtubules sliding over surfaces coated with subspecies-c at low densities (measured by single-molecule fluorescence) show that a single molecule is sufficient to move a microtubule more than 1 mu m at 0.7 mu m s(-1). When many motors interact the velocity is 5.1 mu m s(-1), fitting a duty ratio of 0.14. Using optical trap nanometry, we show that beads carrying a single subspecies-c motor move processively along the microtubules in 8-nm steps but slip backwards under high loads. These results indicate that dynein subspecies-c functions in a very different way from conventional motor proteins, and has properties that could produce self-oscillation in vivo.