MCAK (Kinesin-13) has an Unconventional ATP Hydrolysis Cycle Adapted for Microtubule Depolymerization

Abstract

Unlike members of the kinesin-1 subfamily, the microtubule-depolymerising kinesin-13, MCAK, has no translocation activity. Rather it diffuses on the microtubule lattice to accelerate targeting to both ends, where it carries out ATP-dependent catalytic depolymerisation. The ATP hydrolysis cycle of MCAK has been largely overlooked. However, it may hold the key to the strikingly different behavior of kinesin-13 proteins compared to the conventional translocating kinesins that move directionally on the lattice. We have elucidated the ATP hydrolysis cycle of MCAK in solution and in the presence of both free tubulin dimers and microtubules. In contrast to most other kinesins and also myosins, for which product release is rate-limiting, ATP cleavage limits the hydrolysis cycle of MCAK in solution. Therefore MCAK meets the microtubule from solution in the ATP-containing state which binds tightly. Lattice-stimulated ATP cleavage drives MCAK into a weakly-bound nucleotide state, which diffuses on the lattice to target the microtubule end. An end-specific feature of the microtubule acts as a nucleotide exchange factor, promoting exchange of ADP for ATP by increasing the rate constant for ADP dissociation by more than 20-fold over the equivalent process in solution. Nucleotide exchange triggers tight binding of ATP-MCAK at the microtubule end, deforming the bound tubulin dimer causing lattice destabilization, leading to depolymerization. Tubulin-stimulated ATP hydrolysis is required to allow dissociation of tubulin-MCAK complexes released from the MT end, thereby allowing catalytic depolymerization. The altered ATP hydrolysis cycle of MCAK, relative to kinesin-1, tailors its affinity for tubulin to produce the characteristic weakly-bound diffusive interaction with the microtubule lattice and the strong microtubule end-dependent binding that promotes depolymerization.