von Kriegsheim Alex, Pitt Andrew, Grindlay G Joan, Kolch Walter, Dhillon Amardeep S
The Beatson Institute for Cancer Research, Garscube Estate, Switchback Road, Bearsden, Glasgow G61 1BD, UK.
Nat Cell Biol. 2006 Sep;8(9):1011-6. doi: 10.1038/ncb1465. Epub 2006 Aug 6.
The Raf-MEK-ERK pathway couples growth factor, mitogenic and extracellular matrix signals to cell fate decisions such as growth, proliferation, migration, differentiation and survival. Raf-1 is a direct effector of the Ras GTPase and is the initiating kinase in this signalling cascade. Although Raf-1 activation is well studied, little is known about how Raf-1 is inactivated. Here, we used a proteomic approach to identify molecules that may inactivate Raf-1 signalling. Protein phosphatase 5 (PP5) was identified as an inactivator that associates with Raf-1 on growth factor stimulation and selectively dephosphorylates an essential activating site, Ser 338. The PP5-mediated dephosphorylation of Ser 338 inhibited Raf-1 activity and downstream signalling to MEK, an effect that was prevented by phosphomimetic substitution of Ser 338, or by ablation of PP5 catalytic function. Furthermore, depletion of endogenous PP5 increased cellular phospho-Ser 338 levels. Our results suggest that PP5 is a physiological regulator of Raf-1 signalling pathways.
Raf-MEK-ERK信号通路将生长因子、促有丝分裂和细胞外基质信号与细胞命运决定（如生长、增殖、迁移、分化和存活）联系起来。Raf-1是Ras GTP酶的直接效应器，是该信号级联反应中的起始激酶。尽管对Raf-1的激活已进行了充分研究，但对于Raf-1如何失活却知之甚少。在此，我们采用蛋白质组学方法来鉴定可能使Raf-1信号失活的分子。蛋白磷酸酶5（PP5）被鉴定为一种失活剂，在生长因子刺激下与Raf-1结合，并选择性地使一个关键的激活位点Ser 338去磷酸化。PP5介导的Ser 338去磷酸化抑制了Raf-1活性以及向MEK的下游信号传导，这种效应可通过Ser 338的磷酸模拟取代或PP5催化功能的缺失来阻止。此外，内源性PP5的缺失会增加细胞中磷酸化Ser 338的水平。我们的结果表明，PP5是Raf-1信号通路的生理调节因子。
