Contribution of phosphatidylserine to Rho1- and Pkc1-related repolarization of the actin cytoskeleton under stressed conditions in Saccharomyces cerevisiae

Actin polymerization and depolymerization in developing vertebrates

Development is a complex process that occurs throughout the life cycle. F-actin, a major component of the cytoskeleton, is essential for the morphogenesis of tissues and organs during development. F-actin is formed by the polymerization of G-actin, and the dynamic balance of polymerization and depolymerization ensures proper cellular function. Disruption of this balance results in various abnormalities and defects or even embryonic lethality. Here, we reviewed recent findings on the structure of G-actin and F-actin and the polymerization of G-actin to F-actin. We also focused on the functions of actin isoforms and the underlying mechanisms of actin polymerization/depolymerization in cellular and organic morphogenesis during development. This information will extend our understanding of the role of actin polymerization in the physiologic or pathologic processes during development and may open new avenues for developing therapeutics for embryonic developmental abnormalities or tissue regeneration.

Role of RhoGAP Rgd1 in Pkc1 signaling-related actin repolarization under heat shock stress in Saccharomyces cerevisiae

BACKGROUND

A serine/threonine kinase Pkc1 is the sole protein kinase C in the budding yeast Saccharomyces cerevisiae, and plays an important role in the regulation of polarized growth and stress responses such as those due to heat shock. Exposure of cells to high temperature transiently arrests polarized growth and leads to depolarization of the actin cytoskeleton, followed by actin repolarization during adaptation to heat shock stress. Actin repolarization is ensured by the activation of Pkc1 signaling; however, the molecular mechanisms underlying this phenomenon remain poorly understood.

METHODS

Using an overexpression construct of a constitutively active mutant of Pkc1 (Pkc1^R398P), we explored the Pkc1 target molecules involved in actin repolarization.

RESULTS

PKC1^R398P overexpression as well as heat shock stress increased the phosphorylation levels of Rho GTPase-activating protein (RhoGAP) Rgd1. Rgd1 was found to contribute to Pkc1-signaling-related actin repolarization during adaptation to heat shock stress in a GAP activity-independent manner, with Ser148 in Rgd1 playing a crucial role. Furthermore, Rgd1 was involved in the maintenance of phosphorylation status of the mitogen-activated protein (MAP) kinase Mpk1, a downstream effector of Pkc1, under heat shock stress.

CONCLUSIONS

Rgd1 is a target of Pkc1 signaling under conditions of heat shock stress, and required for the normal process of actin repolarization during adaptation to heat shock stress.

GENERAL SIGNIFICANCE

Our results provide insights into the molecular mechanism underlying Pkc1-mediated modulation of actin repolarization under heat shock stress.

Toxicity of dihydroxyacetone is exerted through the formation of methylglyoxal in Saccharomyces cerevisiae: effects on actin polarity and nuclear division

Dihydroxyacetone (DHA) is the smallest ketotriose, and it is utilized by many organisms as an energy source. However, at higher concentrations, DHA becomes toxic towards several organisms including the budding yeast Saccharomyces cerevisiae. In the present study, we show that DHA toxicity is due to its spontaneous conversion to methylglyoxal (MG) within yeast cells. A mutant defective in MG-metabolizing enzymes (glo1Δgre2Δgre3Δ) exhibited higher susceptibility to DHA. Intracellular MG levels increased following the treatment of glo1Δgre2Δgre3Δ cells with DHA. We previously reported that MG depolarized the actin cytoskeleton and changed vacuolar morphology. We herein demonstrated the depolarization of actin and morphological changes in vacuoles following a treatment with DHA. Furthermore, we found that both MG and DHA caused the morphological change in nucleus, and inhibited the nuclear division. Our results suggest that the conversion of DHA to MG is a dominant contributor to its cytotoxicity.

The TORC2-Dependent Signaling Network in the Yeast Saccharomyces cerevisiae

To grow, eukaryotic cells must expand by inserting glycerolipids, sphingolipids, sterols, and proteins into their plasma membrane, and maintain the proper levels and bilayer distribution. A fungal cell must coordinate growth with enlargement of its cell wall. In Saccharomyces cerevisiae, a plasma membrane-localized protein kinase complex, Target of Rapamicin (TOR) complex-2 (TORC2) (mammalian ortholog is mTORC2), serves as a sensor and master regulator of these plasma membrane- and cell wall-associated events by directly phosphorylating and thereby stimulating the activity of two types of effector protein kinases: Ypk1 (mammalian ortholog is SGK1), along with a paralog (Ypk2); and, Pkc1 (mammalian ortholog is PKN2/PRK2). Ypk1 is a central regulator of pathways and processes required for plasma membrane lipid and protein homeostasis, and requires phosphorylation on its T-loop by eisosome-associated protein kinase Pkh1 (mammalian ortholog is PDK1) and a paralog (Pkh2). For cell survival under various stresses, Ypk1 function requires TORC2-mediated phosphorylation at multiple sites near its C terminus. Pkc1 controls diverse processes, especially cell wall synthesis and integrity. Pkc1 is also regulated by Pkh1- and TORC2-dependent phosphorylation, but, in addition, by interaction with Rho1-GTP and lipids phosphatidylserine (PtdSer) and diacylglycerol (DAG). We also describe here what is currently known about the downstream substrates modulated by Ypk1-mediated and Pkc1-mediated phosphorylation.