UNC-83 coordinates kinesin-1 and dynein to move the nucleus along dynamic, polarized microtubules

Nuclear positioning is essential for cell division, cell migration, formation of polarized cells, and organization of syncytial cells. Failure in nuclear positioning leads to a variety of developmental defects and human diseases. Nuclear migration requires attachment of the cytoskeleton to the nuclear envelope. A nuclear-envelope bridge created by inner nuclear membrane SUN proteins and outer nuclear membrane KASH proteins functions to transfer forces generated in the cytoplasm to the nuclear lamina. Nuclear migration events in the Caenorhabditis elegans embryonic hypodermal precursor cells provide an excellent system for determining the molecular mechanisms of nuclear migration. The KASH protein UNC-83 and the SUN protein UNC-84 function together during this nuclear migration. Here I describe the mechanism of force generation and the dynamics of bidirectional nuclear migration.;The kinesin light chain KLC-2 and three dynein regulators were identified in a yeast two-hybrid screen for cytoplasmic binding partners of UNC-83. Two complexes of dynein regulators are recruited to the nuclear envelope. One consists of the NudE homolog NUD-2 and the NudF/Lis1 homolog LIS-1, and the other includes dynein light chain DLC-1, the BicaudalD homolog BICD-1, and the Egalitarian homolog EGAL-1. Kinesin-1 mutants had severe nuclear migration defects, whereas disruption of dynein regulatory complexes or dynein heavy chain resulted in weaker defects. This data predicted a model in which UNC-83 acts as a cargo adaptor between the nuclear envelope and microtubule motors. In this model kinesin-1 provides the major forces to move the nucleus, while dynein is involved in regulation of bidirectional transport.;Time-lapse imaging of nuclear migration events was used to characterize the kinetics of nuclear migration and determine the mechanism of microtubule motor coordination. Dynein regulatory mutants lacked normal bidirectional movements and had defects in nuclear rolling, which was established as a mechanism to bypass accumulations of cytoplasmic granules. Therefore, the role of dynein in nuclear migration is to resolve cellular roadblocks that kinesin-1 cannot. Microtubules in the hypodermal cells are dynamic and polarize in the direction of migration. These data further support a model where kinesin-1 moves nuclei forward and dynein functions to move nuclei backwards for short stretches to resolve roadblocks.