Conjugation in S. pombe: identification of a microtubule-organising centre, a requirement for microtubules and a role for Mad2

During the G1 phase of the cell cycle, cells of the fission yeast Schizosaccharomyces pombe can be induced to mate by nitrogen starvation and the presence of mating pheromones. Polarised growth towards cells of the opposite mating type (P or M) leads to the formation of a projection tip and, upon contact, localised cell wall degradation results in conjugation and cell fusion [1]. Here, we have investigated the role of microtubules in this process. We describe a previously unidentified microtubule-organising centre (MTOC) that forms at projection tips upon cell-to-cell contact, before cells fuse. Treatment of mating cells with the microtubule-destabilising drug thiabendazole (TBZ) showed that microtubule integrity was required for mating at two distinct stages: during projection tip formation and cell fusion. Projection tip formation requires filamentous (F) actin function [2] and microtubules are required for the localisation of F actin to the projection tip. We also identify a role during mating for Mad2--a mitotic checkpoint protein that is required in all eukaryotes to maintain the mitotic state in response to microtubule depolymerisation [3]. S. pombe mad2 mutant cells were compromised in their ability to mate upon removal of TBZ, indicating that in fission yeast, in the absence of microtubules, Mad2 is also required to maintain mating competence.