Isolation and Characterisation of Regulators of the Septation Initiation Network Regulators in Fission Yeast

The septation initiation network (SIN) in Schizosaccharomyces pombe regulates cytokinesis. Cdc7p encodes the first kinase in the core SIN pathway. In this study, I performed a genetic screen to identify novel regulators of the SIN pathway by isolating spontaneous revertants of cdc7-24 that displayed a cold-sensitive phenotype. The screen yielded a cold-sensitive allele of ppa2, the major PP2A catalytic subunit, and a mutant in ypa2, one of the two S. pombe PTPA orthologues. I characterised both the PTPA genes in the fission yeast, namely, ypa1 and ypa2. It was determined that both ypa1 and ypa2 are essential for survival at low temperatures. The double null mutant was found to be inviable at all temperatures, indicating that the two gene products have at least one essential overlapping function. It was also established that Ypa2p is the major form of PTPA in S. pombe. Characterisation of the phenotype of the ppa2 and ypa2 mutants suggest that the PP2A complex is involved in establishment of cell morphogenesis, determining site of division plane, the G2/M transition and separation of the divided cells. Genetic interactions of the ypa2 and ppa2 mutants with the SIN mutants suggest that reduction in PP2A activity rescues SIN mutants. It was also determined that ypa2 and ppa2 play a role in regulating asymmetric localisation of Cdc7p during mitosis and association of Sid2p with the contractile actomyosin ring.