Abstract #22

AgSwe1p regulates mitosis in response to morphogenesis and nutrients in multinucleated A. gossyppii cells. Amy Gladfelter¹, Hanspeter Helfer². 1) Department of Biology, Dartmouth College, Hanover, NH; 2) Molecular Microbiology, Biozentrum-University of Basel, Basel, Switzerland.
   Nuclei in the filamentous, multinucleated fungus A. gossyppii divide asynchronously. We have investigated what internal and external signals spatially direct mitosis within these hyphal cells. Mitoses are most common near cortical septin rings found at growing tips and branch points. In septin mutants, mitoses are no longer concentrated at branch points suggesting that the septin rings function to locally promote mitosis near new branches. Similarly, cells lacking AgSwe1p kinase (a Wee1 homologue), AgHsl1p (a Nim1-related kinase) and AgMih1p (the phosphatase that likely counteracts AgSwe1p activity) also have mitoses distributed randomly in the hyphae as opposed to at branch points. Surprisingly, however, no phosphorylation of the CDK tyrosine 18 residue, the conserved substrate of Swe1p kinases, was detected in normally growing cells. In contrast, abundant CDK tyrosine phosphorylation was apparent in starving cells resulting in diminished nuclear density. This starvation-induced, CDK phosphorylation is AgSwe1p dependent and overexpressed AgSwe1p is sufficient to delay nuclei even in rich nutrient conditions. In starving cells lacking septins or AgSwe1p negative regulators, the nuclear density is further diminished compared to wild-type. We have generated a model in which AgSwe1p may regulate mitosis in response to cell intrinsic morphogenesis cues and external nutrient availability in multinucleated cells.

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