A systems-biology analysis of feedback inhibition in the Sho1 osmotic-stress-response pathway

Background: A common property of signal transduction systems is that they rapidly lose their ability to respond to a given stimulus. For instance in yeast, the mitogen-activated protein (MAP) kinase Hog1 is activated and inactivated within minutes, even when the osmotic-stress stimulus is sustained. Results: Here, we used a combination of experimental and computational analyses to investigate the dynamic behavior of Hog1 activation in vivo. Computational modeling suggested that a negative-feed back loop operates early in the pathway and leads to rapid attenuation of Hog1 signaling. Experimental analysis revealed that the membrane-bound osmosensor Sho1 is phosphorylated by Hog1 and that phosphorylation occurs on Ser-166. Moreover, Sho1 exists in a homo-oligomeric complex, and phosphorylation by Hog1 promotes a transition from the oligomeric to monomeric state. A phosphorylation-site mutation (Shol S166E) diminishes the formation of Sho1-oligomers, dampens activation of the Hog1 kinase, and impairs growth in high-salt or sorbitol conditions. Conclusions: These findings reveal a novel phosphorylation-dependent feedback loop leading to diminished cellular responses to an osmotic-stress stimulus.