title: MEK1 mediates a positive feedback on Raf-1 activity independently of Ras and Src.
abstract: Growth factor stimulated receptor tyrosine kinases activate a protein kinase cascade via the serine/threonine protein kinase Raf-1. Direct upstream activators of Raf-1 are Ras and Src. This study shows that MEK1, the direct downstream effector of Raf-1, can also stimulate Raf-1 kinase activity by a positive feedback loop. Activated MEK1 mediates hyperphosphorylation of the amino terminal regulatory as well as of the carboxy terminal catalytic domain of Raf-1. The hyperphosphorylation of Raf-1 correlates with a change in the tryptic phosphopeptide pattern only at the carboxy terminus of Raf-1 and an increase in Raf-1 kinase activity. MEK1-mediated Raf-1 activation is inhibited by co-expression of the MAPK specific phosphatase MKP-1 indicating that the MEK1 effect is exerted through a MAPK dependent pathway. Stimulation of Raf-1 activity by MEK1 is independent of Ras, Src and tyrosine phosphorylation of Raf-1. MEK1 can however synergize with Ras and leads to further increase of the Raf-1 kinase activity. Thus, MEK1 can mediate activation of Raf-1 by a novel positive feedback mechanism which allows fast signal amplification and could prolong activation of Raf-1.
authors.affiliation: Institute of Medical Virology, University of Zurich, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Zimmermann
authors.forename: C
authors.initials: C
authors.lastname: Rommel
authors.forename: A
authors.initials: A
authors.lastname: Ziogas
authors.forename: J
authors.initials: J
authors.lastname: Lovric
authors.forename: K
authors.initials: K
authors.lastname: Moelling
authors.forename: G
authors.initials: G
authors.lastname: Radziwill
mesh_terms: D002461:Cell Line, Transformed; D004789:Enzyme Activation; D005246:Feedback; D006801:Humans; D048369:MAP Kinase Kinase 1; D020929:Mitogen-Activated Protein Kinase Kinases; D010449:Peptide Mapping; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf / Q000737:chemistry / Q000378:metabolism*; D014162:Transfection; D018631:ras Proteins / Q000378:metabolism; D019061:src-Family Kinases / Q000378:metabolism
chemical_list: D011505:Protein-Tyrosine Kinases; D019061:src-Family Kinases; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D048369:MAP Kinase Kinase 1; C482125:MAP2K1 protein, human; D020929:Mitogen-Activated Protein Kinase Kinases; D018631:ras Proteins