Jcb_201409087 1..12

Controlling nuclear positioning within cells is crucial for many cellular processes including cell division, polarity, and motility in unicellular and multicellular organisms. Pronuclear migration, during which male and female pronuclei migrate toward the center of the zygote, is a key step for sexual reproduction and embryogenesis (Reinsch and Gönczy, 1998). In zygotes of most species, including human, a microtubule (MT) aster is nucleated from the centrosome associated with the male pronucleus, and migration is dependent on the minus end– directed motor dynein (Clift and Schuh, 2013). Dynein accumulates on the female pronuclear envelope mediating translocation toward the aster center. Simultaneously, the male pronucleus is pulled to the cell center in a MT length– and dynein-dependent manner (Kimura and Onami, 2005; Wühr et al., 2010). In yeast, nuclear congression is observed after fusion of two haploid cells and requires another minus end–directed motor, kinesin-14 Kar3 (Meluh and Rose, 1990). Its localization along MTs proposes that Kar3 pulls nuclei together either by sliding overlapping antiparallel MTs nucleated from spindle pole bodies (SPBs; Meluh and Rose, 1990) or by cross-linking depolymerizing MT plus ends (Molk et al., 2006). A recent study supports a model whereby pulling forces are generated by SPBanchored Kar3 (Gibeaux et al., 2013). Distinct MT-dependent mechanisms underlying nuclear movements have been characterized in the fission yeast Schizosaccharomyces pombe. In interphase, first, the nucleus is tightly associated with MT minus ends on multiple sites along the nuclear envelope including the SPB, whereas dynamic MT plus ends grow toward the cell tips, where they exert, upon growth against the cortex, repulsive pushing forces centering the nucleus (Drummond and Cross, 2000; Tran et al., 2001). Organization of interphase MT bundles partly depends on kinesin-14 Klp2, which mediates sliding of MTs of opposite polarity relative to each other and groups MT minus ends at the cell center (Carazo-Salas et al., 2005; Janson et al., 2007). Moreover, Klp2 dissociates from MTs at the end of mitosis to prevent Klp2dependent clustering of daughter cell nuclei in the cell middle that occurs in telophase, when Klp2 dissociation from MTs is impaired (Mana-Capelli et al., 2012). This demonstrates that Klp2 can directly generate forces to translocate nuclei. Microtubules (MTs) and associated motors play a central role in nuclear migration, which is crucial for diverse biological functions including cell division, polarity, and sexual reproduction. In this paper, we report a dual mechanism underlying nuclear congression during fission yeast karyogamy upon mating of haploid cells. Using microfluidic chambers for long-term imaging, we captured the precise timing of nuclear congression and identified two minus end–directed motors operating in parallel in this process. Kinesin-14 Klp2 associated with MTs may cross-link and slide antiparallel MTs emanating from the two nuclei, whereas dynein accumulating at spindle pole bodies (SPBs) may pull MTs nucleated from the opposite SPB. Klp2-dependent nuclear congression proceeds at constant speed, whereas dynein accumulation results in an increase of nuclear velocity over time. Surprisingly, the light intermediate chain Dli1, but not dynactin, is required for this previously unknown function of dynein. We conclude that efficient nuclear congression depends on the cooperation of two minus end–directed motors. Microtubule minus end motors kinesin-14 and dynein drive nuclear congression in parallel pathways