Osmostress enhances activating phosphorylation of Hog1 MAP kinase by mono-phosphorylated Pbs2 MAP2K.

Kazuo Tatebayashi,Haruo Saito,Yuji Tokunaga,Akiko Nishimura,Satomi Adachi‐Akahane,Taichiro Tomida,Hiroko Kozuka‐Hata,Katsuyoshi Yamamoto,Masaaki Oyama,Tomomi Takayama

doi:10.15252/embj.2019103444

Abstract

The MAP kinase (MAPK) Hog1 is the central regulator of osmoadaptation in yeast. When cells are exposed to high osmolarity, the functionally redundant Sho1 and Sln1 osmosensors, respectively, activate the Ste11‐Pbs2‐Hog1 MAPK cascade and the Ssk2/Ssk22‐Pbs2‐Hog1 MAPK cascade. In a canonical MAPK cascade, a MAPK kinase kinase (MAP3K) activates a MAPK kinase (MAP2K) by phosphorylating two conserved Ser/Thr residues in the activation loop. Here, we report that the MAP3K Ste11 phosphorylates only one activating phosphorylation site (Thr‐518) in Pbs2, whereas the MAP3Ks Ssk2/Ssk22 can phosphorylate both Ser‐514 and Thr‐518 under optimal osmostress conditions. Mono‐phosphorylated Pbs2 cannot phosphorylate Hog1 unless the reaction between Pbs2 and Hog1 is enhanced by osmostress. The lack of the osmotic enhancement of the Pbs2‐Hog1 reaction suppresses Hog1 activation by basal MAP3K activities and prevents pheromone‐to‐Hog1 crosstalk in the absence of osmostress. We also report that the rapid‐and‐transient Hog1 activation kinetics at mildly high osmolarities and the slow and prolonged activation kinetics at severely high osmolarities are both caused by a common feedback mechanism.

Highlights

The MAP kinase (MAPK) Hog1 is the central regulator of osmoadaptation in yeast
Since each of the strains used in those studies expressed at least one MAP3K in the highosmolarity glycerol (HOG) pathway (Ssk2, Ssk22, or Ste11), the results were interpreted as evidence for an alternative mechanism for MAP3K activation following osmostress
To examine if Hog1 could be activated by osmostress in the absence of known upstream osmosensor signaling, we constructed a mutant yeast strain that lacked all four transmembrane proteins involved in the SHO1 branch (Sho1, Opy2, Hkr1, and Msb2) as well as the two MAP3Ks essential for the SLN1 branch (Ssk2 and Ssk22)

Summary

Introduction

When cells are exposed to high osmolarity, the functionally redundant Sho and Sln osmosensors, respectively, activate the Ste11-Pbs2-Hog MAPK cascade and the Ssk2/Ssk22Pbs2-Hog MAPK cascade. We report that the MAP3K Ste phosphorylates only one activating phosphorylation site (Thr-518) in Pbs, whereas the MAP3Ks Ssk2/Ssk can phosphorylate both Ser-514 and Thr-518 under optimal osmostress conditions. Mono-phosphorylated Pbs cannot phosphorylate Hog unless the reaction between Pbs and Hog is enhanced by osmostress. The lack of the osmotic enhancement of the Pbs2-Hog reaction suppresses Hog activation by basal MAP3K activities and prevents pheromone-to-Hog crosstalk in the absence of osmostress. We report that the rapid-andtransient Hog activation kinetics at mildly high osmolarities and the slow and prolonged activation kinetics at severely high osmolarities are both caused by a common feedback mechanism

Methods

Results

Conclusion