STRUCTURAL AND FUNCTIONAL DOMAINS OF THE DROSOPHILA NCD MICROTUBULE MOTOR PROTEIN

Nonclaret disjunctional (ncd) is a kinesin-related microtubule motor p rotein that is required for proper chromosome distribution in Drosophi la. Despite its sequence similarity to kinesin heavy chain, ncd transl ocates with the opposite polarity as kinesin, toward microtubule minus ends. We have expressed different regions of the protein in bacteria and analyzed the proteins for function. Results indicate that ncd cons ists of three domains: a basic, proline-rich N-terminal ''tail,'' a ce ntral alpha-helical coiled-coil stalk, and a C-terminal motor domain. The ncd N terminus proteins bundle microtubules in motility assays and show ATP-independent binding to microtubules in solution. Truncated p roteins, lacking the tail but containing the predicted motor domain an d differing lengths of the stalk, did not support microtubule gliding in in vitro assays but showed microtubule-stimulated MgATPase activity in solution. Addition of a nonspecific N terminus to two of the trunc ated proteins restored directional gliding and rotation of microtubule s in motility assays, demonstrating that these properties map to the p redicted mechanochemical domain of ncd. Physical properties of the C t erminus proteins indicate that the stalk region is important for dimer ization and that the ncd protein probably exists and functions as a di mer.