Activation of JNK by Epac is independent of its activity as a Rap guanine nucleotide exchanger.
Guanine nucleotide exchange factors (GEFs) and their associated GTP-binding proteins (G-proteins) are key regulatory elements in the signal transduction machinery that relays information from the extracellular environment into specific intracellular responses. Among them, the MAPK cascades represent ubiquitous downstream effector pathways. We have previously described that, analogous to the Ras-dependent activation of the Erk-1/2 pathway, members of the Rho family of small G-proteins activate the JNK cascade when GTP is loaded by their corresponding GEFs. Searching for novel regulators of JNK activity we have identified Epac (exchange protein activated by cAMP) as a strong activator of JNK-1. Epac is a member of a growing family of GEFs that specifically display exchange activity on the Rap subfamily of Ras small G-proteins. We report here that while Epac activates the JNK severalfold, a constitutively active (G12V) mutant of Rap1b does not, suggesting that Rap-GTP is not sufficient to transduce Epac-dependent JNK activation. Moreover, Epac signaling to the JNKs was not blocked by inactivation of endogenous Rap, suggesting that Rap activation is not necessary for this response. Consistent with these observations, domain deletion mutant analysis shows that the catalytic GEF domain is dispensable for Epac-mediated activation of JNK. These studies identified a region overlapping the Ras exchange motif domain as critical for JNK activation. Consistent with this, an isolated Ras exchange motif domain from Epac is sufficient to activate JNK. We conclude that Epac signals to the JNK cascade through a new mechanism that does not involve its canonical catalytic action, i.e. Rap-specific GDP/GTP exchange. This represents not only a novel way to activate the JNKs but also a yet undescribed mechanism of downstream signaling by Epac.
Hochbaum
Daniel
D
Laboratorio de Fisiología y Biología Molecular, Departamento de Fisiología y Biología Molecular y Celular, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, and IFYBINE-CONICET, Buenos Aires 1428, Argentina.
Tanos
Tamara
T
Ribeiro-Neto
Fernando
F
Altschuler
Daniel
D
Coso
Omar A
OA
D020816:Amino Acid Motifs; D015153:Blotting, Western; D002460:Cell Line; D004247:DNA / Q000378:metabolism; D018076:DNA, Complementary / Q000378:metabolism; D004789:Enzyme Activation; D017353:Gene Deletion; D016336:Gene Products, vpr / Q000378:metabolism*; D005799:Genes, Dominant; D005982:Glutathione Transferase / Q000378:metabolism; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases; D048670:MAP Kinase Kinase 4; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D009154:Mutation; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D015398:Signal Transduction; D013997:Time Factors; D014162:Transfection; D048051:p38 Mitogen-Activated Protein Kinases
D018076:DNA, Complementary; D016336:Gene Products, vpr; D020662:Guanine Nucleotide Exchange Factors; C116122:RAPGEF3 protein, human; D004247:DNA; D005982:Glutathione Transferase; D048031:JNK Mitogen-Activated Protein Kinases; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D048051:p38 Mitogen-Activated Protein Kinases; D048670:MAP Kinase Kinase 4; D020929:Mitogen-Activated Protein Kinase Kinases