Ace2p contributes to fission yeast septin ring assembly by regulating mid2⁺ expression

The fission yeast Schizosaccharomyces pombe divides through constriction of an actomyosin-based contractile ring followed by formation and degradation of a medial septum. Formation of an organized septin ring is also important for the completion of S. pombe cell division and this event relies on the production of Mid2p. mid2⁺ mRNA and protein accumulate in mitosis. Recent microarray analyses identified mid2⁺ as a target of the Ace2p transcription factor, and ace2⁺ as a target of the Sep1p transcription factor. In this study, we find that Mid2p production is controlled by Ace2p functioning downstream of Sep1p. Consequently, both Sep1p and Ace2p are required for septin ring assembly and genetic analyses indicate that septin rings function in parallel with other Ace2p targets to achieve efficient cell division. Conversely, forced overproduction of Sep1p or Ace2p prevents septin ring disassembly. We find that Ace2p levels peak during anaphase and Ace2p is post-translationally modified by phosphorylation and ubiquitylation. Ace2p localizes symmetrically to dividing nuclei and functions independently of the septation initiation network.