Activation of JNK by Epac is independent of its activity as a rap guanine nucleotide exchanger
Activation of JNK by Epac is independent of its activity as a rap guanine nucleotide exchanger
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DOI：
10.1074/jbc.m305208200
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发表时间：
2003-09-05
影响因子：
4.8
通讯作者：
Coso, OA
中科院分区：
文献类型：
--
作者：
Hochbaum, D;Tanos, T;Coso, OA
鸟苷酸交换因子（GEF）及其相关的GTP结合蛋白（G蛋白）是信号传导机制中的关键调节元件，将来自细胞外环境的信息传递到特定的细胞内反应。其中，MAPK级联反应代表了普遍存在的下游效应途径。我们先前已经描述过，类似于Erk-1/2通路的Ras依赖性激活，当GTP被其相应的GEF加载时，小G蛋白的Rho家族的成员激活JNK级联。为了寻找JNK活性的新调节剂，我们已经确定Epac（cAMP激活的交换蛋白）是JNK-1的强激活剂。Epac是一个不断增长的GEF家族的成员，该家族特异性地显示Ras小G蛋白的Rap亚家族的交换活性。我们在这里报告，虽然Epac激活JNK几倍，Rap 1b的组成性活性（G12 V）突变体不，这表明Rap-GTP是不足以抑制Epac依赖的JNK激活。此外，内源性Rap的失活并没有阻断Epac向JNK的信号传导，这表明Rap的激活对于这种应答是不必要的。与这些观察结果一致，结构域缺失突变体分析表明，催化GEF结构域是Epac介导的JNK激活的关键。这些研究确定了与Ras交换基序结构域重叠的区域对JNK激活至关重要。与此一致，来自Epac的分离的Ras交换基序结构域足以激活JNK。我们的结论是，Epac通过一种新机制向JNK级联发出信号，该机制不涉及其典型的催化作用，即说唱特异性GDP/GTP交换。这不仅代表了一种激活JNK的新方法，也代表了Epac下游信号传导的一种尚未描述的机制。
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.