MUTANTS OF THE DROSOPHILA NCD MICROTUBULE MOTOR PROTEIN CAUSE CENTROSOMAL AND SPINDLE POLE DEFECTS IN MITOSIS

Nonclaret disjunctional (ncd) is a kinesin-related microtubule motor p rotein required for meiotic and early mitotic chromosome distribution in Drosophila. ncd translocates on microtubules with the opposite pola rity to kinesin, toward microtubule minus ends, and is associated with spindles in chromosome/spindle preparations. Here we report a new mut ant of ncd caused by partial deletion of the predicted coiled-coil cen tral stalk. The mutant protein exhibits a velocity of translocation an d ability to generate torque in motility assays comparable to near ful l-length ncd, but only partially rescues a null mutant for chromosome mis-segregation. Antibody staining experiments show that the partial l oss-of-function and null mutants cause centrosomal and spindle pole de fects, including centrosome splitting and loss of centrosomes from spi ndle poles, and localize ncd to centrosomes as well as spindles of wil d-type embryos. Association of ncd with spindles and centrosomes is mi crotubule- and cell cycle-dependent: inhibition of microtubule assembl y with colchicine abolishes ncd staining and centrosomal staining is o bserved in prometaphase, metaphase and anaphase, but diminishes in lat e anaphase/telophase. The cell cycle dependence of centrosomal stainin g and the defects of mutants provide clear evidence for activity of th e ncd motor protein near or at the spindle poles in mitosis. The ncd m otor may interact with centrosomal microtubules and spindle fibers to attach centrosomes to spindle poles, and mediate poleward translocatio n (flux) of kinetochore fibers, a process that may underlie poleward m ovement of chromosomes in mitosis. Together with previous work, our fi ndings indicate that ncd is important in maintaining spindle poles in mitosis as well as in meiosis.