The p150-glued Ssm4p regulates microtubular dynamics and nuclear movement in fission yeast

During vegetative growth of the fission yeast Schizosaccharomyces pombe, microtubules nucleate from multiple microtubule organising centres (MTOCs) close to the nucleus, polymerising until they reach the end of the cell and then shrinking back to the cell centre. In response to mating pheromone, S. pombe undergoes a morphological switch from a vegetative to a shmooing growth pattern. The switch in growth mode is paralleled by a switch in microtubular dynamics. Microtubules nucleate mostly from a single MTOC and pull on the ends of the cell to move the nucleus back and forth. This movement continues after cellular and nuclear fusion in the zygote and is important to ensure correct chromosome pairing, recombination and segregation during meiosis. Here we show that Ssm4p, a p150-Glued protein, is induced specifically in response to pheromone and is required for this nuclear movement. Ssm4p is associated with the cytoplasmic dynein complex and together with the CLIP-170 homologue Tip1p regulates dynein heavy chain localisation. We also show that Ssm4p collaborates with Tip1p in establishing the shmooing microtubular array.