Structure of a fast kinesin: implications for ATPase mechanism and interactions with microtubules

We determined the crystal structure of the motor domain of the fast fungal kinesin from Neurospora crassa (NcKin). The structure has several unique fe atures. (i) Loop 11 in the switch 2 region is ordered and enables one to de scribe the complete nucleotide-binding pocket, including three inter-switch salt bridges between switch 1 and 2. (ii) Loop 9 in the switch 1 region be nds outwards, making the nucleotide-binding pocket very wide. The displacem ent in switch 1 resembles that of the G-protein ras complexed with its guan osine nucleotide exchange factor. (ii!) Loop 5 in the entrance to the nucle otide-binding pocket is remarkably long and interacts with the ribose of AT P. (iv) The linker and neck region is not well defined, indicating that it is mobile. (v) Image reconstructions of ice-embedded microtubules decorated with NcKin show that it interacts with several tubulin subunits, including a central beta -tubulin monomer and the two flanking alpha -tubulin monome rs within the microtubule protofilament. Comparison of NcKin with other kin esins, myosin and G-proteins suggests that the rate-limiting step of ADP re lease is accelerated in the fungal kinesin and accounts for the unusually h igh velocity and ATPase activity.