title: Constitutive activation of the fission yeast pheromone-responsive pathway induces ectopic meiosis and reveals ste11 as a mitogen-activated protein kinase target.
abstract: In the fission yeast Schizosaccharomyces pombe, meiosis normally takes place in diploid zygotes resulting from conjugation of haploid cells. In the present study, we report that the expression of a constitutively activated version of the pheromone-responsive mitogen-activated protein kinase kinase kinase (MAP3K) Byr2 can induce ectopic meiosis directly in haploid cells. We find that the Ste11 transcription factor becomes constitutively expressed in these cells and that the expression of pheromone-responsive genes no longer depends on nitrogen starvation. Epistasis analysis revealed that these conditions bypassed the requirement for the meiotic activator Mei3. Since Mei3 is normally needed for inactivation of the meiosis-repressing protein kinase Pat1, this finding suggests that the strong Byr2 signal causes inactivation of Pat1 by an alternative mechanism. Consistent with this possibility, we found that haploid meiosis was dramatically reduced when Ste11 was mutated to mimic phosphorylation by Pat1. The mutation of two putative MAPK sites in Ste11 also dramatically reduced the level of haploid meiosis, suggesting that Ste11 is a direct target of Spk1. Supporting this, we show that Spk1 can interact physically with Ste11 and also phosphorylate the transcription factor in vitro. Finally, we demonstrate that ste11 is required for pheromone-induced G1 arrest. Interestingly, when we mutated Ste11 in the sites for Pat1 and Spk1 phosphorylation simultaneously, the cells could still arrest in G1 in response to pheromone, suggesting the existence of yet a third bifurcation of the signaling pathway.
authors.affiliation: Institute of Molecular Biology, University of Copenhagen, Øster Farimagsgade 2A, DK-1353 Copenhagen K, Denmark.
authors.forename: Søren
authors.initials: S
authors.lastname: Kjaerulff
authors.forename: Inger
authors.initials: I
authors.lastname: Lautrup-Larsen
authors.forename: Søren
authors.initials: S
authors.lastname: Truelsen
authors.forename: Morten
authors.initials: M
authors.lastname: Pedersen
authors.forename: Olaf
authors.initials: O
authors.lastname: Nielsen
mesh_terms: D020134:Catalytic Domain / Q000235:genetics / Q000502:physiology; D015870:Gene Expression; D006238:Haploidy; D020930:MAP Kinase Kinase Kinases / Q000235:genetics / Q000378:metabolism*; D008540:Meiosis / Q000235:genetics / Q000502:physiology*; D020928:Mitogen-Activated Protein Kinases / Q000235:genetics / Q000502:physiology*; D009154:Mutation / Q000235:genetics; D009584:Nitrogen / Q000378:metabolism; D010675:Pheromones / Q000502:physiology*; D010766:Phosphorylation; D012568:Schizosaccharomyces / Q000201:enzymology* / Q000235:genetics; D029702:Schizosaccharomyces pombe Proteins / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*
chemical_list: D010675:Pheromones; D029702:Schizosaccharomyces pombe Proteins; D014157:Transcription Factors; C071596:ste11 protein, S pombe; D020928:Mitogen-Activated Protein Kinases; C498189:Spk1 protein, S pombe; C077892:BYR2 protein, S pombe; D020930:MAP Kinase Kinase Kinases; D009584:Nitrogen