Solution structures of dimeric kinesin and ncd motors

The dimeric structure of the members of the kinesin family of motor protein s determines the individual characteristics of their microtubule-based moti lity. Crystal structures for ncd and kinesin dimers, which move in opposite directions on microtubules, show possible states of these dimers with ADP bound but give no information about these dimers in solution. Here, low-ang le X-ray and neutron scattering were used to investigate their solution str uctures. Scattering profiles of Drosophila ncd 281-700 (NCD281) and human k inesin 1-420 (hKIN420) were compared with models made from the crystallogra phically determined structures of NCD281 and rat kinesin 1-379 (rKIN379). F rom the low-angle region it was found that the radius of gyration (R-g) Of NCD281 is 3.60 +/- 0.075 nm, which is in agreement with the crystallography -based model. Scattering by longer ncd constructs (NCD250 and NCD224)is als o well fit by the appropriate crystallography-based models. However, the me asured R-g of hKIN420, 4.05 +/- 0.075 nm, is significantly smaller than tha t of the crystallography-based model. In addition, the overall scattering p attern of NCD281 is well fit by the model, but that of hKIN420 is poorly fi t. Model calculations indicate that the orientation of the catalytic cores is different from that observed in the rKIN379 crystal structure. Like the crystal structure, the best-fitting models do not show 2-fold symmetry abou t the neck axis; however, their overall shape more resembles a mushroom tha n the 'T"-like orientation of the catalytic cores found in the crystal stru cture. The center of mass separations of the catalytic cores in the best-fi tting models are 0.7-1 nm smaller than in the crystal structure.