The Highly Conserved Protein Methyltransferase, Skb1, Is a Mediator of Hyperosmotic Stress Response in the Fission YeastSchizosaccharomyces pombe

Abstract

The p21-activated kinase, Shk1, is required for cell viability, establishment and maintenance of cell polarity, and proper mating response in the fission yeast, Schizosaccharomyces pombe. Previous genetic studies suggested that a presumptive protein methyltransferase, Skb1, functions as a positive modulator of Shk1. However, unlike Shk1, Skb1 is not required for viability or mating of S. pombe cells and contributes only modestly to the regulation of cell morphology under normal growth conditions. Here we demonstrate that Skb1 plays a more significant role in regulating cell growth and polarity under conditions of hyperosmotic stress. We provide evidence that the inability of skb1⌬ cells to properly maintain cell polarity in hyperosmotic conditions results from inefficient subcellular targeting of F-actin. We show that Skb1 localizes to cell ends, sites of septation, and nuclei of S. pombe cells. Hyperosmotic shock results in substantial delocalization of Skb1 from cell ends and nuclei, as well as stimulation of Skb1 protein methyltransferase activity. Taken together, our results demonstrate a new role for Skb1 as a mediator of hyperosmotic stress response in fission yeast. We show that the protein methyltransferase activity of the human Skb1 homolog, Skb1Hs, is also stimulated by hyperosmotic stress in fission yeast, providing evidence for evolutionary conservation of a role for Skb1-related proteins as mediators of hyperosmotic stress response, as well as mechanisms involved in regulating this novel class of protein methyltransferases.

Stress-regulated kinase pathways in the recovery of tip growth and microtubule dynamics following osmotic stress inS. pombe

“…5E,H). Increasing the levels and contrast of the fluorescence signal uniformly in each image to approach that of unstressed cells revealed the displacement of F-actin patches from cell tips that was reported by Rupes et al (Rupes et al, 1999;Bao et al, 2001) (Fig. 5F,I).…”

“…We have extended previous assessments of the impact of osmotic stress upon the cytoskeleton (Bao et al, 2001;Rupes et al, 1999;Tatebe et al, 2005) with the characterization of the kinetics with which microtubule dynamics and cell-tip growth resume following arrest in response to osmotic stress.…”

“…Exposures to osmotic, heat or centrifugation stress have all led to a transient dispersal of F-actin patches from cell tips (Bao et al, 2001;Petersen and Hagan, 2005;Rupes et al, 1999;Soto et al, 2007). Following heat and centrifugation stress, phosphorylation of serine 402 of the fission yeast polo kinase, Plo1, is required for the recruitment of F-actin patches back to cell tips to promote a return to tip growth (Petersen and Hagan, 2005).…”

Interplay between chromatin remodelers and protein arginine methyltransferases

“…Skb1 has also been shown to inhibit mitosis through cdc2, and is involved in maintaining cell polarity as well as mediating hyperosmotic stress response (Gilbreth et al, 1998;Bao et al, 2001;Wiley et al, 2003). As work on yeast Skb1 progressed, others were able to identify PRMT5 in a yeast two-hybrid experiment as Jak2 interacting protein highlighting the importance of PRMT5 in signaling (Pollack et al, 1999).…”

The Novel Rho GTPase-activating Protein Family Protein, Rga8, Provides a Potential Link between Cdc42/p21-activated Kinase and Rho Signaling Pathways in the Fission Yeast, Schizosaccharomyces pombe

“…scd2⌬ cells, like ras1⌬ and scd1⌬ mutants, are viable but ovoid in shape and mating-defective. Skb1 and Skb5 are apparently specialized regulators of Shk1, as they are required for proper control of cell polarity under conditions of high osmolarity but not under normal growth conditions (29,30). Shk1 is negatively regulated by an essential and highly conserved WD repeat protein, Skb15 (20).…”