Remodeling of the Fission Yeast Cdc42 Cell-Polarity Module via the Sty1 p38 Stress-Activated Protein Kinase Pathway

Abstract

SummaryThe Rho family GTPase Cdc42 is a key regulator of eukaryotic cellular organization and cell polarity [1]. In the fission yeast Schizosaccharomyces pombe, active Cdc42 and associated effectors and regulators (the “Cdc42 polarity module”) coordinate polarized growth at cell tips by controlling the actin cytoskeleton and exocytosis [2, 3, 4]. Localization of the Cdc42 polarity module to cell tips is thus critical for its function. Here we show that the fission yeast stress-activated protein kinase Sty1, a homolog of mammalian p38 MAP kinase, regulates localization of the Cdc42 polarity module. In wild-type cells, treatment with latrunculin A, a drug that leads to actin depolymerization, induces dispersal of the Cdc42 module from cell tips and cessation of polarized growth [5, 6]. We show that latrunculin A treatment also activates the Sty1 MAP kinase pathway and, strikingly, we find that loss of Sty1 MAP kinase signaling prevents latrunculin A-induced dispersal of the Cdc42 module, allowing polarized growth even in complete absence of the actin cytoskeleton. Regulation of the Cdc42 module by Sty1 is independent of Sty1’s role in stress-induced gene expression. We also describe a system for activation of Sty1 kinase “on demand” in the absence of any external stress, and use this to show that Sty1 activation alone is sufficient to disperse the Cdc42 module from cell tips in otherwise unperturbed cells. During nitrogen-starvation-induced quiescence, inhibition of Sty1 converts non-growing, depolarized cells into growing, polarized cells. Our results place MAP kinase Sty1 as an important physiological regulator of the Cdc42 polarity module.

A novel interplay between GEFs orchestrates Cdc42 activity during cell polarity and cytokinesis

“…To determine if ectopic Gef1 at the cortex in scd1Δ mutants or LatA-treatment of cells results in ectopic Cdc42 activation, we analyzed CRIB-3xGFP localization in these cells. We find that in cells treated with LatA, CRIB-3xGFP localizes randomly to the cortex, signifying ectopic Cdc42 activation, similar to previous reports (Mutavchiev et al, 2016). In mock DMSO-treated control cells, CRIB-3xGFP forms caps at the growing ends (Fig.…”

“…Since scd1 mutants display defects in actin organization, we posit that Scd1 promotes polarized Gef1 localization via the actin cytoskeleton. A recent report shows that ectopic Cdc42 activation in LatA-treated cells depends on the stress-activated MAP kinase Sty1 (Mutavchiev et al, 2016). We found that fission yeast cells treated with LatA did not display ectopic Cdc42 activation in the absence of sty1.…”

Cell Polarity in Yeast

“…According to this hypothesis, cell curvature affects the catchment area from which actin cables can recruit secretory vesicles: Flatter regions enable more vesicle recruitment, diluting and hence broadening the polarity site. This idea may be worth revisiting in light of new findings showing that the effects of perturbing actin on the distribution of Cdc42 result primarily from induction of a stress-activated pathway involving the MAPK Sty1 (Mutavchiev et al 2016). In sty1 mutants lacking this pathway, actin depolymerization had little or no effect on Cdc42 (or even, surprisingly, on polar growth) (Mutavchiev et al 2016).…”

“…This idea may be worth revisiting in light of new findings showing that the effects of perturbing actin on the distribution of Cdc42 result primarily from induction of a stress-activated pathway involving the MAPK Sty1 (Mutavchiev et al 2016). In sty1 mutants lacking this pathway, actin depolymerization had little or no effect on Cdc42 (or even, surprisingly, on polar growth) (Mutavchiev et al 2016). It would be interesting to know whether curvature scaling of Cdc42 clusters in germinating spores is still observed in sty1 mutants and, if so, whether that scaling is still actin dependent.…”

Optogenetics reveals Cdc42 local activation by scaffold-mediated positive feedback and Ras GTPase

“…However, a previously assumed major role of the actin cytoskeleton, which was revealed by the displacement of the Cdc42 feedback module from cell poles upon depolymerization of F-actin (Bendezu and Martin, 2011;Bendezu et al, 2015), is in fact a consequence of MAPK stress signaling rather than a direct effect on protein recruitment by the actin cytoskeleton (Mutavchiev et al, 2016). The finding that a For3-3GFP Scd1-GBP bridge was unable to restore viability to scd2∆ ras1∆ gef1∆ triple mutants suggests For3 is not directly implicated in Pak1 localization.…”