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Vol. 16, Issue 1, 358-371, January 2005

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The Nuclear Kinase Lsk1p Positively Regulates the Septation Initiation Network and Promotes the Successful Completion of Cytokinesis in Response to Perturbation of the Actomyosin Ring in Schizosaccharomyces pombe

Jim Karagiannis ^* , Andrea Bimbó ^*, Srividya Rajagopalan ^*, Jianhua Liu , and Mohan K. Balasubramanian ^*

^* Laboratory of Cell Division, Temasek Life Sciences Laboratory, Singapore 117604, Singapore; Genome Institute of Singapore, Singapore 138672, Singapore

Submitted June 21, 2004; Accepted November 1, 2004
Monitoring Editor: Randy Schekman

Cytokinesis in fission yeast requires the function of an actomyosin-based contractile ring whose constriction is dependent on a signaling module termed the septation initiation network (SIN). In response to minor perturbation of the ring, the duration of SIN signaling is extended concurrently with a delay in nuclear cycle progression. These mechanisms require the conserved phosphatase Clp1p/Flp1p and facilitate the successful completion of cytokinesis, thereby increasing cellular viability. To isolate novel components of this cytokinesis monitoring system, we screened a genome-wide bank of protein kinase deletion mutants and identified Lsk1p, a nuclear-localized protein kinase. Similar to clp1 mutants, and in contrast to wild type, lsk1 cells are unable to maintain the integrity of the actomyosin ring upon treatment with low doses (0.3 µM) of latrunculin A. However, unlike clp1 mutants, lsk1 cells are competent to delay nuclear cycle progression after cytokinetic failure. In addition, lsk1 mutants suppress the lethal, multiseptate phenotype conferred by hyperactivation of the SIN, demonstrating that Lsk1p is a positive regulator of this module. In this report, we demonstrate that Lsk1p acts in parallel to Clp1p to promote actomyosin ring stability upon checkpoint activation. Our studies also establish that actomyosin ring maintenance and nuclear cycle delay in response to cytokinetic perturbation can be genetically resolved into independent pathways.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

Corresponding author. E-mail address: jim{at}tll.org.sg.

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