Delayed Turnover of Unphosphorylated Ssk1 during Carbon Stress Activates the Yeast Hog1 Map Kinase Pathway.
In Saccharomyces cerevisiae, the Hog1 mitogen-activated protein kinase (MAPK) pathway coordinates the adaptation to osmotic stress and was recently reported to respond to acute changes in glucose levels. Similarly as in osmotic stress, glucose starvation leads to a transient accumulation of Hog1 in the nucleus. However, the kinetics and the mechanism of Hog1 activation are different for these stress conditions. During osmotic shock the activation of Hog1 can be transduced by either the Sho1 or the Sln1/Ypd1/Ssk1 branch. During glucose starvation the phosphorylation of Hog1 is slower and is completely dependent on Ssk1, but independent of Sho1. To characterize the mechanism of activation of Hog1 during carbon stress, we examined the turnover of Ssk1 protein levels upon glucose starvation in the presence of cycloheximide and monitored protein levels by western blotting. Our data demonstrate that unphosphorylated Ssk1 was quickly degraded during exponential growth and after osmotic stress but remained remarkably stable during glucose limitation. We conclude that glucose starvation induces a delay in the turnover of unphosphorylated Ssk1, which is sufficient to activate the Hog1 MAPK pathway. Although unphosphorylated Ssk1 is known to be degraded by the proteasome, its stabilization is apparently not due to changes in cellular localization or decrease in ubiquitination levels during glucose limitation.
Division of Nephrology & Hypertension, Oregon Health & Science University, Portland, Oregon, 97239, United States of America.
Milene Carmes
MC
Vallejo
Division of Nephrology & Hypertension, Oregon Health & Science University, Portland, Oregon, 97239, United States of America.
Peter
P
Mayinger
D003513:Cycloheximide / Q000494:pharmacology; D015966:Gene Expression Regulation, Fungal; D005947:Glucose / Q000172:deficiency / Q000494:pharmacology; D047908:Intracellular Signaling Peptides and Proteins; D007976:Leupeptins / Q000494:pharmacology; D008565:Membrane Proteins; D020928:Mitogen-Activated Protein Kinases / Q000235:genetics* / Q000378:metabolism; D009997:Osmotic Pressure; D010766:Phosphorylation / Q000187:drug effects; D046988:Proteasome Endopeptidase Complex / Q000187:drug effects / Q000378:metabolism; D011494:Protein Kinases; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D059748:Proteolysis / Q000187:drug effects; D012441:Saccharomyces cerevisiae / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics* / Q000378:metabolism; D015398:Signal Transduction; D012965:Sodium Chloride / Q000494:pharmacology; D013312:Stress, Physiological; D054875:Ubiquitination / Q000187:drug effects
D047908:Intracellular Signaling Peptides and Proteins; D007976:Leupeptins; D008565:Membrane Proteins; D011500:Protein Synthesis Inhibitors; C459787:SHO1 protein, S cerevisiae; C087199:SSK1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D012965:Sodium Chloride; D003513:Cycloheximide; D011494:Protein Kinases; C102035:YPD1 protein, S cerevisiae; C088589:HOG1 protein, S cerevisiae; D020928:Mitogen-Activated Protein Kinases; C083749:SLN1 protein, S cerevisiae; D046988:Proteasome Endopeptidase Complex; D005947:Glucose; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde