Laboratory of Molecular Genetics, Frontier Research Unit, Institute of Medical Science, The University of Tokyo, Tokyo, Japan.
Division of Molecular Cell Signaling, Institute of Medical Science, The University of Tokyo, Tokyo, Japan.
EMBO J. 2020 Mar 2;39(5):e103444. doi: 10.15252/embj.2019103444. Epub 2020 Feb 3.
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.
丝裂原活化蛋白激酶（MAPK）Hog1 是酵母渗透适应的核心调节剂。当细胞暴露在高渗透压下时，功能冗余的 Sho1 和 Sln1 渗透压感受器分别激活 Ste11-Pbs2-Hog1 MAPK 级联和 Ssk2/Ssk22-Pbs2-Hog1 MAPK 级联。在典型的 MAPK 级联中，MAP3K 通过磷酸化激活环中的两个保守 Ser/Thr 残基来激活 MAP2K。在这里，我们报告 Ste11 仅磷酸化 Pbs2 中的一个激活磷酸化位点（Thr-518），而在最佳渗透压应激条件下，MAP3Ks Ssk2/Ssk22 可以磷酸化 Ser-514 和 Thr-518。除非 Pbs2 和 Hog1 之间的反应受到渗透压应激的增强，否则单磷酸化的 Pbs2 不能磷酸化 Hog1。Pbs2-Hog1 反应的渗透压增强缺乏会抑制基础 MAP3K 活性对 Hog1 的激活，并防止在没有渗透压应激的情况下，交配信息素到 Hog1 的串扰。我们还报告说，在轻度高渗透压下快速和瞬时的 Hog1 激活动力学以及在严重高渗透压下缓慢和延长的激活动力学都是由共同的反馈机制引起的。
