THREE-DIMENSIONAL STRUCTURE OF THE MOTOR DOMAIN OF NCD, A KINESIN-RELATED MOTOR WITH REVERSED POLARITY OF MOVEMENT. Elena P. Sablin^*, F. Jon Kull^*, Roger Cooke^*, Ronald D. Vale^*@#, and Robert J. Fletterick^*#. Departments of ^*Biochemistry/Biophysics and ^#Pharmacology and the Howard Hughes Medical Institute@ University of California, San Francisco, CA USA

The motor domain of the kinesin homolog NCD that is required formeiotic chromosome segregation in Drosophila has been crystallized in the presence of MgATP using polyethylene glycol as the precipitant. The crystals belong to the orthorhombic space group I222 with unit cell dimensions a=127.1 Å, b=122.26 Å and c=68.0 Å, and there is one NCD molecule per asymmetric unit. The structure of the NCD motor domain complexed with MgADP was solved using multiple isomorphous replacement method. X-ray diffraction data for the native NCD crystals and five isomorphous derivatives were measured at -170 C using R-AXIS IIC image plate detector. The current NCD model has been refined to 2.5 Å with an R value 22.4% and includes 321 amino acid residues along with MgADP and 66 water molecules. Structural comparison between the NCD and kinesin motor domains show that they are remarkably similar in structure and likely share a common microtubule binding site. Moreover, structural and functional comparisons of NCD, kinesin, myosin and G proteins reveal that these NTP-ases may utilize a similar strategy of changing conformation between NTP and NDP states. A general model for converting a common gamma-phosphate sensing mechanism into opposite polarities of movement for kinesin and NCD is proposed.