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A more recent version of this article appeared on August 1, 2002
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Submitted on January 18, 2002
Revised on March 22, 2002
Accepted on May 1, 2002
1 Department of Biochemistry and Molecular Biology, SUNY Upstate Medical University, 750 East Adams Street, Syracuse, New York 13210
2 Wadsworth Center, New York State Department and Department of Biological Sciences, School of Public Health, SUNY, Albany, New York 12208
* Corresponding author. E-mail address: ambergd{at}mail.upstate.edu.
S. cerevisiae adapts to osmotic stress through the activation of a conserved HOG (High Osmolarity Growth) MAP kinase pathway. Transmission through the HOG pathway is very well understood and yet other aspects of the cellular response to osmotic stress remain poorly understood, most notably regulation of actin organization. The actin cytoskeleton rapidly disassembles in response to osmotic insult and is only induced to reassemble after osmotic balance with the environment is re-established. Here we show that one of three MEK kinases of the HOG pathway, Ssk2p, is specialized to facilitate actin cytoskeleton re-assembly following osmotic stress. Within minutes of cells experiencing osmotic stress or catastrophic disassembly of the actin cytoskeleton through Latrunculin A treatment, Ssk2p concentrates in the neck of budding yeast cells and concurrently forms a 1:1 complex with actin. These observations suggest that Ssk2p has a novel, previously un-described function in sensing damage to the actin cytoskeleton. We further describe a second function for Ssk2p in facilitating reassembly of a polarized actin cytoskeleton at the end of the cell cycle, a prerequisite for efficient cell cycle completion. Loss of Ssk2p, its kinase activity, or ability to localize and interact with actin led to delays in actin recovery and a resulting delay in cell cycle completion. These unique capabilities of Ssk2p are activated by a novel mechanism that does not involve known components of the HOG pathway.
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