Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, United States.
Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, United States.
Elife. 2017 Oct 24;6:e30278. doi: 10.7554/eLife.30278.
The Hippo pathway controls tissue growth and homeostasis through a central MST-LATS kinase cascade. The scaffold protein SAV1 promotes the activation of this kinase cascade, but the molecular mechanisms remain unknown. Here, we discover SAV1-mediated inhibition of the PP2A complex STRIPAK as a key mechanism of MST1/2 activation. SLMAP binding to autophosphorylated MST2 linker recruits STRIPAK and promotes PP2A-mediated dephosphorylation of MST2 at the activation loop. Our structural and biochemical studies reveal that SAV1 and MST2 heterodimerize through their SARAH domains. Two SAV1-MST2 heterodimers further dimerize through SAV1 WW domains to form a heterotetramer, in which MST2 undergoes trans-autophosphorylation. SAV1 directly binds to STRIPAK and inhibits its phosphatase activity, protecting MST2 activation-loop phosphorylation. Genetic ablation of SLMAP in human cells leads to spontaneous activation of the Hippo pathway and alleviates the need for SAV1 in Hippo signaling. Thus, SAV1 promotes Hippo activation through counteracting the STRIPAK PP2A phosphatase complex.
Hippo 通路通过中央 MST-LATS 激酶级联控制组织生长和动态平衡。支架蛋白 SAV1 促进了这个激酶级联的激活，但分子机制尚不清楚。在这里，我们发现 SAV1 介导的 PP2A 复合物 STRIPAK 的抑制作用是 MST1/2 激活的关键机制。SLMAP 与自磷酸化的 MST2 接头结合募集 STRIPAK，并促进 MST2 在激活环上的 PP2A 介导的去磷酸化。我们的结构和生化研究表明，SAV1 和 MST2 通过其 SARAH 结构域异二聚化。两个 SAV1-MST2 异二聚体进一步通过 SAV1 WW 结构域二聚化形成异四聚体，其中 MST2 发生转位自磷酸化。SAV1 直接结合 STRIPAK 并抑制其磷酸酶活性，保护 MST2 激活环磷酸化。在人类细胞中敲除 SLMAP 会导致 Hippo 通路的自发激活，并减轻 Hippo 信号传导中对 SAV1 的需求。因此，SAV1 通过拮抗 STRIPAK PP2A 磷酸酶复合物促进 Hippo 激活。
