Abstract
Kinesin-5 family members including human Eg5/KSP contribute to the plus-end-directed force necessary for the assembly and maintenance of the bipolar mitotic spindle. We have used monomeric Eg5-367 in the nucleotide-free state to evaluate the role of microtubules at each step in the ATPase cycle. The pre-steady-state kinetic results show that the microtubule-Eg5 complex binds MgATP tightly, followed by rapid ATP hydrolysis with a subsequent slow step that limits steady-state turnover. We show that microtubules accelerate the kinetics of each step in the ATPase pathway, suggesting that microtubules amplify the nucleotide-dependent structural transitions required for force generation. The experimentally determined rate constants for phosphate product release and Eg5 detachment from the microtubule were similar, suggesting that these two steps are coupled with one occurring at the slow rate after ATP hydrolysis followed by the second step occurring more rapidly. The rate of this slow step correlates well with the steady-state k(cat), indicative that it is the rate-limiting step of the mechanism.