CONFORMATIONS OF KINESIN - SOLUTION VS. CRYSTAL-STRUCTURES AND INTERACTIONS WITH MICROTUBULES

Recently, the molecular structures of monomeric and dimeric kinesin co nstructs in complex with ADP have been determined by X-ray crystallogr aphy (Kull et al. 1996; Kozielski et al. 1997a; Sack et al. 1997). The ''motor'' or ''head'' domains have almost identical conformations in the known crystal structures, yet the kinesin dimer is asymmetric: the orientation of the two heads relative to the coiled-coil formed by th eir neck regions is different. We used small angle solution scattering of kinesin constructs and microtubules decorated with kinesin in orde r to find out whether these crystal structures are of relevance for ki nesin's structure under natural conditions and for its interaction wit h microtubules. Our preliminary results indicate that the crystal stru ctures of monomeric and dimeric kinesin are similar to their structure s in solution, though in solution the center-of-mass distance between the motor domains of the dimer could be slightly greater. The crystal structure of dimeric kinesin can be interpreted as representing two eq uivalent conformations. Transitions between these or very similar conf ormational states may occur in solution. Binding of kinesin to microtu bules has conformational effects on both, the kinesin and the microtub ule, Solution scattering of kinesin decorated microtubules reveals a p eak in intensity that is characteristic for the B-surface lattice and that can be used to monitor the axial repeat of the microtubules under various conditions. In decoration experiments, dimeric kinesin dissoc iates, at least partly, leading to a stoichiometry of 1:1 (one kinesin head per tubulin dimer; Thormahlen et al. 1998a) in contrast to the s toichiometry of 2:1 reported for dimeric ncd. This discrepancy is poss ibly due to the effect of steric hindrance between kinesin dimers on a djacent binding sites.