A model of the microtubule-kinesin complex based on electron cryomicroscopy and X-ray crystallography

Motor proteins of the kinesin superfamily play an organising role in eukaryotic cells and participate in many crucial phases of the cell cycle by moving along microtubules and thereby changing the position of attached organelles. In their 'standard' form, kinesin motors are elongated heterotetrameric protein complexes composed of two identical heavy chains and two light chains; the central regions of the heavy chains intertwine, forming a coiled coil, with the globular 'heads' of the microtubule-interacting motor domains at one end. In order to understand how kinesin motors interact with and move along microtubules, we have combined electron cryomicroscopy and X-ray crystallographic data to build a model of the complex.

Using electron cryomicroscopy and image reconstruction, we have obtained three-dimensional maps of complexes of kinesin motor domain dimers and microtubules. Motor domain dimers interact one to one with tubulin dimers, with one head attached--lying along the microtubule protofilament--and the other unattached--pointing sideways and upwards towards the microtubule plus end. Using currently available crystallographic data, we have built an atomic resolution model of the motor domain dimer, which can be successfully 'docked' into the three-dimensional framework of the maps from electron cryomicroscopy.

Docking the atomic resolution model into the map of the microtubule-kinesin complex with the coiled coil of kinesin pointing away from the microtubule surface shows that the attached and unattached heads have similar relative positions on the microtubule and in the crystal. Three regions of the attached head appear likely to interact with the microtubule.