Structural comparison of dimeric Eg5, Neurospora kinesin (Nkin) and Ncd head-Nkin neck chimera with conventional kinesin

Cryo-electron microscopy and 3D image reconstruction of microtubules satura ted with kinesin dimers has shown one head bound to tubulin, the other free . The free head of rat kinesin sits on the top right of the bound head (wit h the microtubule oriented plus-end upwards) in the presence of 5'-adenylyl imido-diphosphate (AMPPNP) and on the top left in nucleotide-free solutions . To understand the relevance of this movement, we investigated other dimer ic plus-end-directed motors: Neurospora kinesin (Nkin); Eg5, a slow non-pro cessive kinesin; and a chimera of Ncd heads attached to Nkin necks. In the AMPPNP (ATP-like) state, all dimers have the free head to the top right, In the absence of nucleotide, the free head of an Nkin dimer appears to occup y alternative positions to either side of the bound head. Despite having th e Nkin neck, the free head of the chimera was only seen to the top right of the bound head. Eg5 also has the free head mostly to the top right. We sug gest that processive movement may require kinesins to move their heads in a lternative ways.