F-ACTIN DISTRIBUTION AND FUNCTION DURING SEXUAL-DIFFERENTIATION IN SCHIZOSACCHAROMYCES-POMBE

Sexual differentiation in Schizosaccharomyces pombe is induced from th e G(1) phase of the cell cycle by nitrogen starvation and the presence of mating pheromone, We describe the distribution of F-actin during s exual differentiation. Cortical F-actin dots have previously been show n to be restricted to one end of the rod shaped cell during the Gr pha se of the cell cycle. Within half an hour of nitrogen starvation the d istribution of cortical F-actin dots switched from being monopolar to bipolar, This was then reversed as the F-actin cytoskeleton repolarize d so that cortical F-actin dots accumulated towards the projection tip at one end of the cell. Following cell fusion, F-actin dots were rand omly scattered during the horsetail movement that precedes meiosis I a nd remained scattered until prometaphase or metaphase of meiosis II, w hen they concentrated around the nucleus. F-actin was seen on the lagg ing face of the nuclei which faced the partner nucleus during anaphase B of meiosis II. Early on in this anaphase F-actin was also seen on t he opposite side of the nucleus, near the spindle pole body. F-actin a ccumulated within the spores in the mature ascus, Treatment with the a ctin depolymerising drug Latrunculin A showed that F-actin is required for cell fusion and spore formation. Latrunculin A treatment extended all stages from karyogamy to meiosis I. The S. pombe homologue of the actin binding protein profilin, Cdc3, was shown to be required for co njugation, Cdc3 co-localized with the formin related molecule Fus1 at the projection tip. The polarization of F-actin cortical dots to the p rojection tip was unaffected in the cdc3.124 mutant, but cdc3.124 muta nt cells were unable to break down the cell walls between the two cell s following agglutination.