Cytoplasmic Dynein is not a Classical Duty Ratio Motor

Abstract

Cytoplasmic dynein is not a classical duty ratio motorThe mechanical cross bridge cycle of cytoplasmic dynein has often been compared with that of myosin. Cytoplasmic dynein and myosin 5 are both organelle bound motors responsible for transport of their cargo over a long distance. It has also been demonstrated that myosin 5 is a processive motor and that its processivity is regulated by the duty ratio; i.e., the ratio between bound and free state during the cross bridge cycle. As the binding of ATP leads to the dissociation of the motor filament complex a decrease of the ATP concentration results in an increased duty ratio.It was the aim of our study to investigate whether the processivity of cytoplasmic dynein is also governed by the duty ratio. With the optical trap single molecule measurements were carried out in a two bead dumbbell approach. At 100 μM ATP consecutive 8 nm steps up to stall force were observed often resulting in repeated 8nm forward and backward steps at stall force. To our surprise however, at low ATP concentrations dynein underwent only single binding events with an apparent working stroke of 8nm.These results can not be explained by a simple one site model for ATP binding where processivity is governed by the duty ratio. In contrast to myosin, dynein possesses two essential ATP binding sites. At low ATP concentrations we hypothesis that only one of the ATP binding sites is occupied, thereby resulting in a loss of processivity.