title: Ras regulates assembly of mitogenic signalling complexes through the effector protein IMP.
abstract: 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.
authors.affiliation: Department of Cell Biology, UT Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75390-9039, USA.
authors.forename: Sharon A
authors.initials: SA
authors.lastname: Matheny
authors.forename: Chiyuan
authors.initials: C
authors.lastname: Chen
authors.forename: Robert L
authors.initials: RL
authors.lastname: Kortum
authors.forename: Gina L
authors.initials: GL
authors.lastname: Razidlo
authors.forename: Robert E
authors.initials: RE
authors.lastname: Lewis
authors.forename: Michael A
authors.initials: MA
authors.lastname: White
mesh_terms: D000818:Animals; D002460:Cell Line; D004789:Enzyme Activation; D006801:Humans; D020935:MAP Kinase Signaling System; D051379:Mice; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism; D008969:Molecular Sequence Data; D011485:Protein Binding; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism; D051381:Rats; D025801:Ubiquitin / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases / Q000737:chemistry / Q000378:metabolism*; D029867:Xenopus Proteins / Q000737:chemistry / Q000378:metabolism*; D014982:Xenopus laevis; D018631:ras Proteins / Q000378:metabolism*
chemical_list: D025801:Ubiquitin; D029867:Xenopus Proteins; C480973:Brap protein, rat; C480974:IMP protein, Xenopus; D044767:Ubiquitin-Protein Ligases; D011494:Protein Kinases; C097199:KSR-1 protein kinase; D019908:Proto-Oncogene Proteins c-raf; D020929:Mitogen-Activated Protein Kinase Kinases; D018631:ras Proteins