Posas F, Saito H
Dana-Farber Cancer Institute and Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115, USA.
EMBO J. 1998 Mar 2;17(5):1385-94. doi: 10.1093/emboj/17.5.1385.
Exposure of yeast cells to increased extracellular osmolarity induces the HOG1 mitogen-activated protein kinase (MAPK) cascade, which is composed of SSK2, SSK22 and STE11 MAPKKKs, PBS2 MAPKK and HOG1 MAPK. The SSK2/SSK22 MAPKKKs are activated by a 'two-component' osmosensor composed of SLN1, YPD1 and SSK1. The SSK1 C-terminal receiver domain interacts with an N-terminal segment of SSK2. Upon hyperosmotic treatment, SSK2 is autophosphorylated rapidly, and this reaction requires the interaction of SSK1 with SSK2. Autophosphorylation of SSK2 is an intramolecular reaction, suggesting similarity to the mammalian MEKK1 kinase. Dephosphorylation of SSK2 renders the kinase inactive, but it can be re-activated by addition of SSK1 in vitro. A conserved threonine residue (Thr1460) in the activation loop of SSK2 is important for kinase activity. Based on these observations, we propose the following two-step activation mechanism of SSK2 MAPKKK. In the first step, the binding of SSK1 to the SSK1-binding site in the N-terminal domain of SSK2 causes a conformational change in SSK2 and induces its latent kinase activity. In the second step, autophosphorylation of SSK2 renders its activity independent of the presence of SSK1. A similar mechanism might be applicable to other MAPKKKs from both yeast and higher eukaryotes.
将酵母细胞暴露于细胞外渗透压升高的环境中会诱导HOG1丝裂原活化蛋白激酶（MAPK）级联反应，该级联反应由SSK2、SSK22和STE11 MAPKKK、PBS2 MAPKK以及HOG1 MAPK组成。SSK2 / SSK22 MAPKKK由由SLN1、YPD1和SSK1组成的“双组分”渗透压感受器激活。SSK1的C末端接收结构域与SSK2的N末端片段相互作用。在高渗处理后，SSK2迅速发生自磷酸化，并且该反应需要SSK1与SSK2相互作用。SSK2的自磷酸化是一种分子内反应，这表明它与哺乳动物的MEKK1激酶相似。SSK2的去磷酸化会使激酶失活，但在体外添加SSK1可使其重新激活。SSK2激活环中一个保守的苏氨酸残基（Thr1460）对激酶活性很重要。基于这些观察结果，我们提出了以下SSK2 MAPKKK的两步激活机制。第一步，SSK1与SSK2 N末端结构域中的SSK1结合位点结合会导致SSK2发生构象变化并诱导其潜在的激酶活性。第二步，SSK2的自磷酸化使其活性独立于SSK1的存在。类似的机制可能适用于酵母和高等真核生物的其他MAPKKK。
