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A more recent version of this article appeared on October 1, 2006
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Submitted on March 21, 2006
Revised on July 12, 2006
Accepted on July 31, 2006
*Molecular Microbiology, University of Basel Biozentrum, 4056 Basel, Switzerland; Department of Biology, Dartmouth College, Hanover, NH 03755
Monitoring Editor: Kerry Bloom
Nuclei in the filamentous, multinucleated fungus A. gossypii 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 phosphatase (the Cdc25 homologue 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 with 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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