Mtl1 is required to activate general stress response through Tor1 and Ras2 inhibition under conditions of glucose starvation and oxidative stress.

Mtl1 is a member of the cell wall integrity (CWI) pathway of Saccharomyces cerevisiae, which functions as a cell wall sensor for oxidative stress. Genome-wide transcriptional analysis revealed a cluster of genes that were down-regulated in the absence of Mtl1. Many of these genes were potentially regulated by the general stress response factor Msn2/Msn4. In response to rapamycin, caffeine, glucose starvation and oxidative stress provoked by H(2)O(2), mtl1 presents a significant loss of viability as well as a deficiency in the transcriptional response mediated by Msn2/Msn4. The Mtl1 function was required (i) to induce ribosomal gene repression, (ii) to induce the general stress response driven by the transcription factor Msn2/Msn4, and (iii) to activate the CWI pathway in response to both glucose starvation and oxidative stress. We also detected higher cAMP levels in the mtl1 mutant than in wild type cells indicative of up-regulated RAS2-PKA activity. Disruption of TOR1, disruption of RAS2, or hyperactivation of Rho1 restored both the viability and the transcriptional function (both ribosomal and Msn2/Msn4-dependent gene expression) in the mtl1 mutant to almost wild type levels when cells were starved of glucose or stressed with H(2)O(2). Taking our results together, we propose an essential role for Mtl1 in signaling oxidative stress and quiescence to the CWI pathway and to the general stress response through Rho1 and the inhibition of either the TOR1 or RAS2 functions. These mechanisms would be required to allow cells to adapt to both oxidative and nutritional stresses.

Sit4 is required for proper modulation of the biological functions mediated by Pkc1 and the cell integrity pathway in Saccharomyces cerevisiae.

Maintenance of cellular integrity in Saccharomyces cerevisiae is carried out by the activation of the protein kinase C-mediated mitogen-activated protein kinase (PKC1-MAPK) pathway. Here we report that correct down-regulation of both basal and induced activity of the PKC1-MAPK pathway requires the SIT4 function. Sit4 is a protein phosphatase also required for a proper cell cycle progression. We present evidence demonstrating that the G(1) to S delay in the cell cycle, which occurs as a consequence of the absence of Sit4, is mediated by up-regulation of Pkc1 activity. Sit4 operates downstream of the plasma membrane sensors Mid2, Wsc1, and Wsc2 and upstream of Pkc1. Sit4 affects all known biological functions involving Pkc1, namely Mpk1 activity and cell wall integrity, actin cytoskeleton organization, and ribosomal gene transcription.