The cell-end factor pom1p inhibits mid1p in specification of the cell division plane in fission yeast

Intrinsic spatial cues ensure the proper placement of the cell division plane. In the fission yeast Schizosaccharomyces pombe, the position of the nucleus helps to direct the medial positioning of contractilering assembly and subsequent cell division [1-3]. An important factor in this process is mid1p (anillin-like protein), which is a peripheral-membrane protein that forms a broad cortical band of dots overlying the nucleus in interphase and recruits myosin in early mitosis [4-8]. How mid1p localizes to this cortical band and tracks the nucleus is not clear, especially because its localization is independent of the cytoskeleton [2, 8]. Here, we used a combination of experimental and computational approaches to test mid1p localization mechanisms. We provide evidence that pom1p, a DYRK-family protein kinase [9] that forms a concentration gradient emanating from the nongrowing cell end, inhibits mid1p. In pom1 mutants, mid1p is distributed over half of the cell, covering the nongrowing cell end. This abnormal distribution is established in a dynamic manner in interphase and leads to the formation of misplaced or multiple contractile rings. Our computational and experimental results support a model in which both positive cues from the medial nucleus and negative cues from the cell tips specify the position of the division plane.