University College London Cancer Institute, University College London, London WC1E 6DD, United Kingdom.
Helen Diller Family Comprehensive Cancer Center, University of California, San Francisco, CA 94158
Proc Natl Acad Sci U S A. 2018 Nov 6;115(45):E10576-E10585. doi: 10.1073/pnas.1720352115. Epub 2018 Oct 22.
Dephosphorylation of the inhibitory "S259" site on RAF kinases (S259 on CRAF, S365 on BRAF) plays a key role in RAF activation. The MRAS GTPase, a close relative of RAS oncoproteins, interacts with SHOC2 and protein phosphatase 1 (PP1) to form a heterotrimeric holoenzyme that dephosphorylates this S259 RAF site. MRAS and SHOC2 function as PP1 regulatory subunits providing the complex with striking specificity against RAF. MRAS also functions as a targeting subunit as membrane localization is required for efficient RAF dephosphorylation and ERK pathway regulation in cells. SHOC2's predicted structure shows remarkable similarities to the A subunit of PP2A, suggesting a case of convergent structural evolution with the PP2A heterotrimer. We have identified multiple regions in SHOC2 involved in complex formation as well as residues in MRAS switch I and the interswitch region that help account for MRAS's unique effector specificity for SHOC2-PP1. MRAS, SHOC2, and PPP1CB are mutated in Noonan syndrome, and we show that syndromic mutations invariably promote complex formation with each other, but not necessarily with other interactors. Thus, Noonan syndrome in individuals with SHOC2, MRAS, or PPPC1B mutations is likely driven at the biochemical level by enhanced ternary complex formation and highlights the crucial role of this phosphatase holoenzyme in RAF S259 dephosphorylation, ERK pathway dynamics, and normal human development.
RAF 激酶上抑制性“S259”位点的去磷酸化（CRAF 上的 S259、BRAF 上的 S365）在 RAF 激活中起着关键作用。MRAS GTPase 是 RAS 癌蛋白的近亲，与 SHOC2 和蛋白磷酸酶 1（PP1）相互作用形成异三聚体全酶，使该 S259 RAF 位点去磷酸化。MRAS 和 SHOC2 作为 PP1 的调节亚基发挥作用，为该复合物提供了针对 RAF 的惊人特异性。MRAS 还作为靶向亚基发挥作用，因为膜定位对于细胞中 RAF 的有效去磷酸化和 ERK 途径调节是必需的。SHOC2 的预测结构与 PP2A 的 A 亚基显示出显著的相似性，表明与 PP2A 异三聚体的结构进化具有趋同特征。我们已经确定了 SHOC2 中参与复合物形成的多个区域，以及 MRAS 开关 I 和开关间区域中的残基，这些残基有助于解释 MRAS 对 SHOC2-PP1 的独特效应物特异性。Noonan 综合征中存在 MRAS、SHOC2 和 PPP1CB 的突变，我们表明综合征突变总是促进彼此之间的复合物形成，但不一定与其他相互作用者形成复合物。因此，SHOC2、MRAS 或 PPP1CB 突变个体中的 Noonan 综合征很可能在生化水平上是由增强的三元复合物形成驱动的，这突显了这种磷酸酶全酶在 RAF S259 去磷酸化、ERK 途径动力学和正常人类发育中的关键作用。
