Abstract
The yeast high-osmolarity glycerol (HOG) stress-activated protein kinase Hog1 is activated in response to hyperosmotic stress, inducing the production and retention of glycerol to restore osmotic balance. Hog1 promotes retention of glycerol through closure of the plasma-membrane glycerol channel Fps1. Treatment of yeast with the toxic trivalent metalloid arsenite (As(III)) also activates Hog1 as part of a protective response in which Hog1 closes Fps1, the main entry port for As(III). In this study, we investigated how cells treated with As(III) avoid creating a new stress caused by the accumulation of glycerol in the absence of hyperosmotic stress conditions. We found that As(III) treatment did not induce glycerol accumulation and, in fact, blocked the accumulation of glycerol induced by constitutive Hog1 activity. We demonstrated that As(III) blocks glycerol production indirectly after its metabolic activation to methylarsenite (MAs(III)), which is a potent inhibitor of glycerol-3-phosphate dehydrogenase. Finally, we used a biotinylated arsenic probe to establish that Cys306 of yeast Gpd1, a highly conserved residue within the active site, is the key target of MAs(III). Conservative mutations at this residue greatly diminished Gpd1 activity. This study offers insight into mechanisms by which SAPK outputs are tailored to specific stressors.