An Atomic-Level Engine that Accounts for Kinesin Motility and Catalysis

Abstract

Kinesin motor proteins convert the energy of ATP hydrolysis into stepping movement along microtubules. In this process, the microtubule can be considered as kinesin's regulatory partner, responsible for activating the enzyme's functional behavior. In the absence of atomic resolution structures describing the kinesin-microtubule complex, the mechanism of this activation has remained unknown. We use cryo-electron microscopy to derive atomic models describing the complete, microtubule-attached, kinetic cycle of a kinesin motor. The resolution of our reconstructions (∼8A) enabled us to unambiguously build crystallographically-determined conformations of kinesin's key subcomponents into the density maps. The resulting models reveal novel arrangements of kinesin's nucleotide-sensing switch loops and of its microtubule binding element known as the switch II helix. Based on these models, we present a detailed molecular mechanism accounting for kinesin's force generation cycle. In this mechanism, the switch loops control a seesaw-like movement of the catalytic domain relative to the switch II helix, which remains fixed on the microtubule surface. Microtubules couple the seesaw movement to ATP binding by stabilizing the formation of extra coils at the N terminus of the switch II helix, which interact directly with the switch loops. Tilting of the seesaw to assume the ATP-bound orientation in turn elicits a power stroke by the motor domain's force-delivering element known as the neck linker. This sequence of events accounts for the essential mechanics of kinesin's force-delivery cycle, and also yields a new model for the catalytically active conformation of kinesin's ancestral relative, myosin.