Slt2 Is Required to Activate ER-Stress-Protective Mechanisms through TORC1 Inhibition and Hexosamine Pathway Activation.

Isabel E Sánchez-Adriá,Gemma Sanmartín,Francisca Randez-Gil,Francisco Estruch,Jose A Prieto

doi:10.3390/jof8020092

Abstract

Slt2, the MAPK of the cell wall integrity (CWI) pathway, connects different signaling pathways and performs different functions in the protective response of S. cerevisiae to stress. Previous work has evidenced the relation of the CWI pathway and the unfolded protein response (UPR), a transcriptional program activated upon endoplasmic reticulum (ER) stress. However, the mechanisms of crosstalk between these pathways and the targets regulated by Slt2 under ER stress remain unclear. Here, we demonstrated that ectopic expression of GFA1, the gene encoding the first enzyme in the synthesis of UDP-GlcNAc by the hexosamine biosynthetic pathway (HBP) or supplementation of the growth medium with glucosamine (GlcN), increases the tolerance of slt2 mutant cells to different ER-stress inducers. Remarkably, GlcN also alleviates the sensitivity phenotype of cells lacking IRE1 or HAC1, the main actors in controlling the UPR. The exogenous addition of GlcN reduced the abundance of glycosylated proteins and triggered autophagy. We also found that TORC1, the central stress and growth controller, is inhibited by tunicamycin exposure in cells of the wild-type strain but not in those lacking Slt2. Consistent with this, the tunicamycin-induced activation of autophagy and the increased synthesis of ATP in response to ER stress were absent by knock-out of SLT2. Altogether, our data placed Slt2 as an essential actor of the ER stress response by regulating the HBP activity and the TORC1-dependent signaling.

Highlights

The fungal cell wall is an external rigid structure that gives shape and integrity to the cell [1]
Tunicamycin induced the activation of a UPRE::lacZ reporter [43] both in wild-type and slt2 mutant cells, the activation levels were lower in the latter (Figure 1B)
Similar behavior was observed when analyzing the induction of Hac1 and Kar2 by tunicamycin in wild-type and slt2 mutant cells (Figure S1A)

Summary

Introduction

The fungal cell wall is an external rigid structure that gives shape and integrity to the cell [1]. Made mainly of polysaccharides and glycoproteins, is continuously remodeled to allow the growth and the morphological changes required during the cell cycle [2,3,4]. Substantial changes in composition and thickness occur in response to environmental physical stress, such as osmotic and heat stresses, in order to avoid cell membrane rupture and lysis [5]. The biosynthetic pathways involved in its formation are strictly regulated in response to different signaling pathways [3,6]. In Saccharomyces cerevisiae, the cell wall integrity (CWI) pathway is the key pathway in controlling cell wall dynamics [7].

Methods

Results

Conclusion