1996;Merdes et al. mediated by overlapping mechanisms involving both NuMA and HSET is essential for chromosome movement during mitosis. Keywords:chromosome, kinetochore, spindle pole, NuMA, HSET == Introduction == The spindle is a complex microtubule-based superstructure responsible for chromosome movement and segregation during mitosis and meiosis (McIntosh and Koonce 1989;Mitchison 1989a;Rieder 1991;Hyman and Karsenti 1996;Compton 2000). Chromosome movement on spindles NS-018 maleate during mitosis in cultured cells has been well documented (Gorbsky 1992;Rieder and Salmon 1994; Inou and Salmon 1995; Rieder and Salmon 1998;Khodjakov et al. 1999) and is driven by three different force-generating mechanisms (Mitchison 1989b;Gorbsky 1992;Rieder and Salmon 1994;Khodjakov and Rieder 1996;Khodjakov et al. 1999). Poleward chromosome movement is usually driven by forces derived from kinetochore-associated microtubule motor proteins and the continuous poleward flux of tubulin subunits within the spindle lattice. Kinetic analyses indicate that a majority (6070%) of poleward chromosome movement in cultured somatic cells is usually driven by forces generated by kinetochore-associated motors (Mitchison and Salmon 1992), and candidates for these motors are the kinesin-related proteins CENP-E and MCAK/XKCM1 as well as cytoplasmic dynein (Rieder and Alexander 1990;Pfarr et al. 1991;Steuer et al. 1991,Yen et al. 1991;Walczak et al. 1996;Schaar et al. 1997;Solid wood et al. 1997;Maney et al. 1998). Chromosome movement away from spindle poles during congression is usually driven by polar ejection forces (Rieder et al. 1986) that may be generated by chromosome-associated kinesin-related proteins (Antonio et al. 2000;Funabiki and Murray 2000). An implicit assumption in how these force-generating mechanisms cause chromosome movement is Rabbit polyclonal to APE1 that microtubule minus ends are strongly anchored at spindle poles. It has been suggested that this anchorage is necessary in anaphase to bear the load when chromosomes move, so that the chromosomes move toward the poles rather than the poles toward the chromosome (Nicklas 1989). In its extreme form, this idea posits that if the microtubule minus ends were not appropriately anchored at spindle poles, then the poleward forces generated by kinetochore-associated motors (coupled to microtubule depolymerization) would pull the microtubules in toward the chromosome rather than move the chromosome toward the pole. Indeed, direct observation of microtubule-chromosome interactions under defined in vitro NS-018 maleate conditions has exhibited that microtubules are reeled in toward the chromosomes in the absence of anchorage at minus ends (Koshland et al. 1988). Likewise, the polar ejection forces would extrude the microtubules past the region of the spindle pole toward the cell cortex instead of pushing the chromosome toward the spindle equator. Our understanding of the mechanisms for microtubule minus end NS-018 maleate anchorage at spindle poles has grown substantially in the past few years (Merdes and Cleveland 1997;Compton 1998). Centrosomes are the dominant site for microtubule nucleation and, when present, are located at spindle poles as a consequence of their function in microtubule nucleation. However, a variety of evidence demonstrates that centrosomes are neither necessary nor sufficient to act as functional spindle poles, and chromosome movement occurs normally in cells lacking centrosomes and in cells where centrosomes have been experimentally removed (Szollosi et al. 1972;Nicklas 1989;Heald et al. 1996;Gaglio et al. 1997;Khodjakov et al. 2000). These observations demonstrate that centrosomes cannot be the primary anchorage sites for microtubule minus ends at spindle poles in vertebrate cells. Thus, we hypothesize that noncentrosomal NS-018 maleate proteins provide the primary anchorage site for microtubule minus ends at poles to counterbalance the forces involved in chromosome movement. To test this hypothesis, we monitored chromosome movement in cultured cells after.