title: Regulation of Raf-1 by direct feedback phosphorylation.
abstract: The Raf-1 kinase is an important signaling molecule, functioning in the Ras pathway to transmit mitogenic, differentiative, and oncogenic signals to the downstream kinases MEK and ERK. Because of its integral role in cell signaling, Raf-1 activity must be precisely controlled. Previous studies have shown that phosphorylation is required for Raf-1 activation, and here, we identify six phosphorylation sites that contribute to the downregulation of Raf-1 after mitogen stimulation. Five of the identified sites are proline-directed targets of activated ERK, and phosphorylation of all six sites requires MEK signaling, indicating a negative feedback mechanism. Hyperphosphorylation of these six sites inhibits the Ras/Raf-1 interaction and desensitizes Raf-1 to additional stimuli. The hyperphosphorylated/desensitized Raf-1 is subsequently dephosphorylated and returned to a signaling-competent state through interactions with the protein phosphatase PP2A and the prolyl isomerase Pin1. These findings elucidate a critical Raf-1 regulatory mechanism that contributes to the sensitive, temporal modulation of Ras signaling.
authors.affiliation: Laboratory of Protein Dynamics and Signaling, National Cancer Institute-Frederick, Frederick, MD 21702, USA.
authors.forename: Michele K
authors.initials: MK
authors.lastname: Dougherty
authors.forename: Jürgen
authors.initials: J
authors.lastname: Müller
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Ritt
authors.forename: Ming
authors.initials: M
authors.lastname: Zhou
authors.forename: Xiao Zhen
authors.initials: XZ
authors.lastname: Zhou
authors.forename: Terry D
authors.initials: TD
authors.lastname: Copeland
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Conrads
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Veenstra
authors.forename: Kun Ping
authors.initials: KP
authors.lastname: Lu
authors.forename: Deborah K
authors.initials: DK
authors.lastname: Morrison
mesh_terms: D000818:Animals; D002460:Cell Line; D015536:Down-Regulation; D004789:Enzyme Activation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D025461:Feedback, Physiological; D007527:Isoenzymes / Q000378:metabolism; D051379:Mice; D016297:Mutagenesis, Site-Directed; D000072340:NIMA-Interacting Peptidylprolyl Isomerase; D010455:Peptides / Q000235:genetics / Q000378:metabolism; D019696:Peptidylprolyl Isomerase / Q000378:metabolism; D010749:Phosphoprotein Phosphatases / Q000378:metabolism; D010766:Phosphorylation; D010982:Platelet-Derived Growth Factor / Q000378:metabolism; D019908:Proto-Oncogene Proteins c-raf / Q000235:genetics / Q000378:metabolism*; D017479:Receptors, Platelet-Derived Growth Factor / Q000378:metabolism; D015398:Signal Transduction; D014982:Xenopus laevis; D018631:ras Proteins / Q000378:metabolism
chemical_list: D007527:Isoenzymes; D000072340:NIMA-Interacting Peptidylprolyl Isomerase; D010455:Peptides; D010982:Platelet-Derived Growth Factor; D017479:Receptors, Platelet-Derived Growth Factor; D019908:Proto-Oncogene Proteins c-raf; D048049:Extracellular Signal-Regulated MAP Kinases; D010749:Phosphoprotein Phosphatases; D018631:ras Proteins; D019696:Peptidylprolyl Isomerase; C000606850:Pin1 protein, mouse