Catalysis of the microtubule on-rate is the major parameter regulating the depolymerase activity of MCAK

MCAK is a mitotic kinesin that binds to and slides along microtubules (MT) to reach their ends, where MCAK disassembles MTs into tubulin dimers. Now the kinetics of MCAK-MT interaction are directly observed in a single-molecule setup, dissecting the contributions of different MCAK regions to on-rates and tubulin disassembly activity. The kinesin-13, MCAK, is a critical regulator of microtubule dynamics in eukaryotic cells. We have functionally dissected the structural features responsible for MCAK's potent microtubule depolymerization activity. MCAK's positively charged neck enhances its delivery to microtubule ends not by tethering the molecule to microtubules during diffusion, as commonly thought, but by catalyzing the association of MCAK to microtubules. On the other hand, this same positively charged neck slightly diminishes MCAK's ability to remove tubulin subunits once at the microtubule end. Conversely, dimerization reduces MCAK delivery but improves MCAK's ability to remove tubulin subunits. The reported kinetics for these events predicts a nonspecific binding mechanism that may represent a paradigm for the diffusive interaction of many microtubule-binding proteins.