Ras regulates assembly of mitogenic signalling complexes through the effector protein IMP

The signal transduction cascade comprising Raf, mitogen-activated protein (MAP) kinase kinase (MEK) and MAP kinase is a Ras effector pathway that mediates diverse cellular responses to environmental cues and contributes to Ras-dependent oncogenic transformation. Here we report that the Ras effector protein Impedes Mitogenic signal Propagation (IMP) modulates sensitivity of the MAP kinase cascade to stimulus-dependent activation by limiting functional assembly of the core enzymatic components through the inactivation of KSR, a scaffold/adaptor protein that couples activated Raf to its substrate MEK. IMP is a Ras-responsive E3 ubiquitin ligase that, on activation of Ras, is modified by auto-polyubiquitination, which releases the inhibition of Raf-MEK complex formation. Thus, Ras activates the MAP kinase cascade through simultaneous dual effector interactions: induction of Raf kinase activity and derepression of Raf-MEK complex formation. IMP depletion results in increased stimulus-dependent MEK activation without alterations in the timing or duration of the response. These observations suggest that IMP functions as a threshold modulator, controlling sensitivity of the cascade to stimulus and providing a mechanism to allow adaptive behaviour of the cascade in chronic or complex signalling environments.     

Synergistic response to oncogenic mutations defines gene class critical to cancer phenotype

Understanding the molecular underpinnings of cancer is of critical importance to the development of targeted intervention strategies. Identification of such targets, however, is notoriously difficult and unpredictable. Malignant cell transformation requires the cooperation of a few oncogenic mutations that cause substantial reorganization of many cell features and induce complex changes in gene expression patterns. Genes critical to this multifaceted cellular phenotype have therefore only been identified after signalling pathway analysis or on an ad hoc basis. Our observations that cell transformation by cooperating oncogenic lesions depends on synergistic modulation of downstream signalling circuitry suggest that malignant transformation is a highly cooperative process, involving synergy at multiple levels of regulation, including gene expression. Here we show that a large proportion of genes controlled synergistically by loss-of-function p53 and Ras activation are critical to the malignant state of murine and human colon cells. Notably, 14 out of 24 'cooperation response genes' were found to contribute to tumour formation in gene perturbation experiments. In contrast, only 1 in 14 perturbations of the genes responding in a non-synergistic manner had a similar effect. Synergistic control of gene expression by oncogenic mutations thus emerges as an underlying key to malignancy, and provides an attractive rationale for identifying intervention targets in gene networks downstream of oncogenic gain- and loss-of-function mutations.     

Ha-Ras augments c-Jun activity and stimulates phosphorylation of its activation domain

Ha-Ras augments c-Jun-mediated transactivation by potentiating the activity of the c-Jun activation domain. Ha-Ras also causes a corresponding increase in phosphorylation of specific sites in that part of the c-Jun protein. A Ha-Ras-induced protein kinase cascade resulting in hyperphosphorylation of the c-Jun activation domain could explain how these oncoproteins cooperate to transform rat embryo fibroblasts.     

A Raf-induced allosteric transition of KSR stimulates phosphorylation of MEK

In metazoans, the Ras-Raf-MEK (mitogen-activated protein-kinase kinase)-ERK (extracellular signal-regulated kinase) signalling pathway relays extracellular stimuli to elicit changes in cellular function and gene expression. Aberrant activation of this pathway through oncogenic mutations is responsible for a large proportion of human cancer. Kinase suppressor of Ras (KSR) functions as an essential scaffolding protein to coordinate the assembly of Raf-MEK-ERK complexes. Here we integrate structural and biochemical studies to understand how KSR promotes stimulatory Raf phosphorylation of MEK (refs 6, 7). We show, from the crystal structure of the kinase domain of human KSR2 (KSR2(KD)) in complex with rabbit MEK1, that interactions between KSR2(KD) and MEK1 are mediated by their respective activation segments and C-lobe αG helices. Analogous to BRAF (refs 8, 9), KSR2 self-associates through a side-to-side interface involving Arg?718, a residue identified in a genetic screen as a suppressor of Ras signalling. ATP is bound to the KSR2(KD) catalytic site, and we demonstrate KSR2 kinase activity towards MEK1 by in vitro assays and chemical genetics. In the KSR2(KD)-MEK1 complex, the activation segments of both kinases are mutually constrained, and KSR2 adopts an inactive conformation. BRAF allosterically stimulates the kinase activity of KSR2, which is dependent on formation of a side-to-side KSR2-BRAF heterodimer. Furthermore, KSR2-BRAF heterodimerization results in an increase of BRAF-induced MEK phosphorylation via the KSR2-mediated relay of a signal from BRAF to release the activation segment of MEK for phosphorylation. We propose that KSR interacts with a regulatory Raf molecule in cis to induce a conformational switch of MEK, facilitating MEK's phosphorylation by a separate catalytic Raf molecule in trans.     

Elevated levels of diacylglycerol and decreased phorbol ester sensitivity in ras-transformed fibroblasts

Diacylglycerol (DG) plays a central role in phospholipid metabolism and is an endogenous activator of protein kinase C. We have suggested that constitutive activation of this kinase is one mechanism by which oncogenes transform cells. The ras-encoded proteins are similar to regulatory G-proteins and are candidates for the unknown G-protein that modulates phosphatidylinositol (PI) turnover. Differences in polyphosphoinositide metabolism have been reported for ras-transformed cells. But because these experiments were performed on confluent cultures of established cell lines, the differences are difficult to attribute to ras transformation. Here we show that exponentially growing NIH 3T3 fibroblasts recently transformed by Ha-ras or Ki-ras possess elevated DG concentrations without significant alterations in the levels of other polyphosphoinositide metabolites. The basal phosphorylation of protein kinase C substrate of relative molecular mass (Mr) 80,000 (80K) is significantly increased in all the ras-transformed cell lines. Surprisingly, however, further phosphorylation of this protein on addition of phorbol ester was greatly reduced. Ha-ras cells also show less binding of phorbol ester than control cells, suggesting that elevation of DG causes partial down-regulation in addition to activation of protein kinase C.     

Draper-dependent glial phagocytic activity is mediated by Src and Syk family kinase signalling

The cellular machinery promoting phagocytosis of corpses of apoptotic cells is well conserved from worms to mammals. An important component is the Caenorhabditis elegans engulfment receptor CED-1 (ref. 1) and its Drosophila orthologue, Draper. The CED-1/Draper signalling pathway is also essential for the phagocytosis of other types of 'modified self' including necrotic cells, developmentally pruned axons and dendrites, and axons undergoing Wallerian degeneration. Here we show that Drosophila Shark, a non-receptor tyrosine kinase similar to mammalian Syk and Zap-70, binds Draper through an immunoreceptor tyrosine-based activation motif (ITAM) in the Draper intracellular domain. We show that Shark activity is essential for Draper-mediated signalling events in vivo, including the recruitment of glial membranes to severed axons and the phagocytosis of axonal debris and neuronal cell corpses by glia. We also show that the Src family kinase (SFK) Src42A can markedly increase Draper phosphorylation and is essential for glial phagocytic activity. We propose that ligand-dependent Draper receptor activation initiates the Src42A-dependent tyrosine phosphorylation of Draper, the association of Shark and the activation of the Draper pathway. These Draper-Src42A-Shark interactions are strikingly similar to mammalian immunoreceptor-SFK-Syk signalling events in mammalian myeloid and lymphoid cells. Thus, Draper seems to be an ancient immunoreceptor with an extracellular domain tuned to modified self, and an intracellular domain promoting phagocytosis through an ITAM-domain-SFK-Syk-mediated signalling cascade.     

TNF-RII and c-IAP1 mediate ubiquitination and degradation of TRAF2

Tumour necrosis factor-alpha (TNF-alpha) is a proinflammatory mediator that exerts its biological functions by binding two TNF receptors (TNF-RI and TNF-RII), which initiate biological responses by interacting with adaptor and signalling proteins. Among the signalling components that associate with TNF receptors are members of the TNF-R-associated factor (TRAF) family. TRAF2 is required for TNF-alpha-mediated activation of c-Jun N-terminal kinase (JNK), contributes to activation of NF-kappaB, and mediates anti-apoptotic signals,. TNF-RI and TNF-RII signalling complexes also contain the anti-apoptotic ('inhibitor of apoptosis') molecules c-IAP1 and c-IAP2 (refs 5, 6), which also have RING domain-dependent ubiquitin protein ligase (E3) activity. The function of IAPs in TNF-R signalling is unknown. Here we show that binding of TNF-alpha to TNF-RII induces ubiquitination and proteasomal degradation of TRAF2. Although c-IAP1 bound TRAF2 and TRAF1 in vitro, it ubiquitinated only TRAF2. Expression of wild-type c-IAP1, but not an E3-defective mutant, resulted in TRAF2 ubiquitination and degradation. Moreover, E3-defective c-IAP1 prevented TNF-alpha-induced TRAF2 degradation and inhibited apoptosis. These findings identify a physiologic role for c-IAP1 and define a mechanism by which TNF-RII-regulated ubiquitin protein ligase activity can potentiate TNF-induced apoptosis.     

EPS8 and E3B1 transduce signals from Ras to Rac.

The small guanine nucleotide (GTP)-binding protein Rac regulates mitogen-induced cytoskeletal changes and c-Jun amino-terminal kinase (JNK), and its activity is required for Ras-mediated cell transformation. Epistatic analysis placed Rac as a key downstream target in Ras signalling; however, the biochemical mechanism regulating the cross-talk among these small GTP-binding proteins remains to be elucidated. Eps8 (relative molecular mass 97,000) is a substrate of receptors with tyrosine kinase activity which binds, through its SH3 domain, to a protein designated E3b1/Abi-1. Here we show that Eps8 and E3b1/Abi-1 participate in the transduction of signals from Ras to Rac, by regulating Rac-specific guanine nucleotide exchange factor (GEF) activities. We also show that Eps8, E3b1 and Sos-1 form a tri-complex in vivo that exhibits Rac-specific GEF activity in vitro. We propose a model in which Eps8 mediates the transfer of signals between Ras and Rac, by forming a complex with E3b1 and Sos-1.