We demonstrate that POSH, a scaffold for the JNK signaling pathway, binds to Akt2. A POSH mutant that is unable to bind Akt2 (POSH W489A) exhibits enhanced-binding to MLK3, and this increase in binding is accompanied by increased activation of the JNK signaling pathway. In addition, we show that the association of MLK3 with POSH is increased upon inhibition of the endogenous phosphatidylinositol 3-kinase/Akt signaling pathway. Thus, the assembly of an active JNK signaling complex by POSH is negatively regulated by Akt2. Further, the level of Akt-phosphorylated MLK3 is reduced in cells expressing the Akt2 binding domain of POSH, which acts as a dominant interfering protein. Taken together, our results support a model in which Akt2 binds to a POSH-MLK-MKK-JNK complex and phosphorylates MLK3; phosphorylation of MLK3 by Akt2 results in the disassembly of the JNK complex bound to POSH and down-regulation of the JNK signaling pathway.
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