The Mechanism of Processivity and Directionality of Cytoplasmic Dynein
- Author(s): Cleary, Frank Banta
- Advisor(s): Yildiz, Ahmet
- et al.
Cytoplasmic dynein is a dimeric motor that transports intracellular cargoes towards the minus-end of microtubules (MTs). In contrast to other processive motors, microtubule release of the dynein heads is not precisely coordinated and the mechanism of dynein processivity remains unclear. By engineering the mechanical and catalytic properties of the motor we show that dynein processivity minimally requires a single active motor domain (head) and a second inert MT binding domain. The AAA+ ring and the linker of the other head are dispensable, suggesting that processivity arises from a high ratio of MT bound to unbound time, and not from interhead communication. Additionally, nucleotide-dependent microtubule release is gated by tension on the linker domain.
We find that dynein releases rapidly from the MT when force is applied in the forward direction (towards the minus-end), but remains bound under backward forces. This finding suggests that the asymmetric release properties of the microtubule binding domain (MTBD) control dynein directionality. Consistent with this hypothesis, replacing the MTBD with a catalytically inactive kinesin motor domain, which favors release towards the plus end, results in reversal of motor directionality. Furthermore, a dynein dimer maintains its minus-end directed motility when the released monomer is allowed to orient freely during the search for a new binding site. The results rule out directionality models based on a swinging mechanism of the linker domain. We propose that the mechanism of dynein directionality is fundamentally distinct from kinesin and myosin motors and determined by asymmetric release and binding properties of its MT binding interface. We develop a quantitative model for dynein stepping that reproduces the velocity and stepping characteristics of dynein motors and their response to chemical and mechanical perturbation.