Protein interaction switches coordinate Raf-1 and MST2/Hippo signalling

Abstract

Signal transduction requires the coordination of activities between different pathways. In mammalian cells, Raf-1 regulates the MST-LATS and MEK-ERK pathways. We found that a complex circuitry of competing protein interactions coordinates the crosstalk between the ERK and MST pathways. Combining mathematical modelling and experimental validation we show that competing protein interactions can cause steep signalling switches through phosphorylation-induced changes in binding affinities. These include Akt phosphorylation of MST2 and a feedback phosphorylation of Raf-1 Ser 259 by LATS1, which enables Raf-1 to suppress both MST2 and MEK signalling. Mutation of Raf-1 Ser 259 stimulates both pathways, simultaneously driving apoptosis and proliferation, whereas concomitant MST2 downregulation switches signalling to cell proliferation, transformation and survival. Thus, competing protein interactions provide a versatile regulatory mechanism for signal distribution through the dynamic integration of graded signals into switch-like responses.

miR-155-dependent regulation of mammalian sterile 20-like kinase 2 (MST2) coordinates inflammation, oxidative stress and proliferation in vascular smooth muscle cells

“…MST2 binds Raf-1 at two distinct sites that overlap with the MEK-binding domains [16], suggesting that MST2 competes with MEK to bind Raf-1 under certain conditions, thus affecting MAPK signaling. Consistent with this observation, when a disruptor peptide, i.e., stearoylated-MST2 was used to dissociate MST2 from Raf-1, binding of MEK to Raf-1 as well as activation of MEK and ERK1/2 was significantly increased [17]. We observed that miR-155 activates the ERK1/2 pathway by directly repressing the expression of MST2, consequently enhancing the interaction of between Raf-1 and MEK and promoting VSMC proliferation as well as injury-induced neointimal hyperplasia.…”

“…Alteration in the MST2-Raf-1 interaction affects the ERK1/2 pathway [17]. Therefore, we examined how miR-155 impinges on MST2-mediated regulation of the ERK1/2 pathway.…”

“…MST2 binds to Raf-1 at two distinct sites that partially overlap with the mitogen-activated protein kinase kinase (MEK)-binding domain on Raf-1 [16], suggesting that MST2 competes with MEK to bind to Raf-1 and affects the MAPK signaling cascade. Furthermore, the interaction between MST2 and Raf-1 regulates the ERK1/ 2 pathway and inhibits the pro-apoptotic activation of MST2 [15,17]. Although MST1/2 activates Ser/Thr protein kinases and regulates the Raf-1/ ERK pathway activity, the role of MST2 in regulation of inflammation and oxidative stress in VSMCs has not been studied.…”

The complexities and versatility of the RAS-to-ERK signalling system in normal and cancer cells

“…Besides their enzymatic role as kinases, RAF proteins were also shown to have MEK-independent functions including control of apoptotic pathways by crosstalk with the pro-apoptotic kinase ASK1 [48], and the Rho effector kinase ROKalpha [49]. In addition, RAF1 and ARAF were shown to efficiently control the pro-apoptotic MST2/Hippo pathway by binding and sequestration of MST2 [20,26,[50][51][52][53].…”

“…This is related to important gaps in our knowledge of the RAS activity regulation, the molecular mechanisms governing RAS signalling networks and their biological functions. Systems biology models can help us understand network functions, and several models focussing on different parts of the RAS network have been developed [20]. Yet, most of these models have limited coverage of the RAS signalling network, and future models will have to include critical missing RAS effectors and mechanisms.…”