A structural pathway for activation of the kinesin motor ATPase

Molecular motors move along actin or microtubules by rapidly hydrolyzing AT P and undergoing changes in filament-binding affinity with steps of the nuc leotide hydrolysis cycle. It is generally accepted that motor binding to it s filament greatly increases the rate of ATP hydrolysis, but the structural changes in the motor associated with ATPase activation are not known. To i dentify the conformational changes underlying motor movement on its filamen t, we solved the crystal structures of three kinesin mutants that decouple nucleotide and microtubule binding by the motor, and block microtubule-acti vated, but not basal, ATPase activity. Conformational changes in the struct ures include a disordered loop and helices in the snitch I region and a vis ible switch II loop, which is disordered in wild-type structures. Switch I moved closer to the bound nucleotide in two mutant structures, perturbing w ater-mediated interactions with the Mg2+. This could weaken Mg2+ binding an d accelerate ADP release to activate the motor ATPase, The structural chang es we observe define a signaling pathway within the motor for ATPase activa tion that is likely to be essential for motor movement on microtubules.