title: Phosphorylation of Rap1 by cAMP-dependent Protein Kinase (PKA) Creates a Binding Site for KSR to Sustain ERK Activation by cAMP.
abstract: Cyclic adenosine monophosphate (cAMP) is an important mediator of hormonal stimulation of cell growth and differentiation through its activation of the extracellular signal-regulated kinase (ERK) cascade. Two small G proteins, Ras and Rap1 have been proposed to mediate this activation. Using HEK293 cells as a model system, we have recently shown that both Ras and Rap1 are required for cAMP signaling to ERKs. However, cAMP-dependent Ras signaling to ERKs is transient and rapidly terminated by PKA phosphorylation of the Raf isoforms C-Raf and B-Raf. In contrast, cAMP-dependent Rap1 signaling to ERKs and Rap1 is potentiated by PKA. We show that this is due to sustained binding of B-Raf to Rap1. One of the targets of PKA is Rap1 itself, directly phosphorylating Rap1a on serine 180 and Rap1b on serine 179. We show that these phosphorylations create potential binding sites for the adaptor protein 14-3-3 that links Rap1 to the scaffold protein KSR. These results suggest that Rap1 activation of ERKs requires PKA phosphorylation and KSR binding. Because KSR and B-Raf exist as heterodimers within the cell, this binding also brings B-Raf to Rap1, allowing Rap1 to couple to ERKs through B-Raf binding to Rap1 independently of its Ras-binding domain.
authors.affiliation: From the Vollum Institute, Oregon Health & Science University, Portland, Oregon 97239-3098.
authors.forename: Maho
authors.initials: M
authors.lastname: Takahashi
authors.affiliation: From the Vollum Institute, Oregon Health & Science University, Portland, Oregon 97239-3098.
authors.forename: Yanping
authors.initials: Y
authors.lastname: Li
authors.affiliation: From the Vollum Institute, Oregon Health & Science University, Portland, Oregon 97239-3098.
authors.forename: Tara J
authors.initials: TJ
authors.lastname: Dillon
authors.affiliation: From the Vollum Institute, Oregon Health & Science University, Portland, Oregon 97239-3098 stork@ohsu.edu.
authors.forename: Philip J S
authors.initials: PJ
authors.lastname: Stork
mesh_terms: D000818:Animals; D000242:Cyclic AMP / Q000235:genetics / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation / Q000235:genetics; D048049:Extracellular Signal-Regulated MAP Kinases / Q000235:genetics / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D008250:Lysine-tRNA Ligase / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D048493:Proto-Oncogene Proteins B-raf / Q000235:genetics / Q000378:metabolism; D020668:rap GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D000242:Cyclic AMP; C482119:BRAF protein, human; C482120:Braf protein, mouse; D048493:Proto-Oncogene Proteins B-raf; D017868:Cyclic AMP-Dependent Protein Kinases; D048049:Extracellular Signal-Regulated MAP Kinases; D020668:rap GTP-Binding Proteins; C000613080:KARS1 protein, human; D008250:Lysine-tRNA Ligase
keywords: 14-3-3 protein; Raf kinase; Ras-related protein 1 (Rap1); extracellular-signal-regulated kinase (ERK); protein kinase A (PKA)