Critical Roles of Threonine 187 Phosphorylation in Cellular Stress-induced Rapid and Transient Activation of Transforming Growth Factor-β-activated Kinase 1 (TAK1) in a Signaling Complex Containing TAK1-binding Protein TAB1 and TAB2

Transforming growth factor-β-activated kinase 1 (TAK1) mitogen-activated protein kinase kinase kinase has been shown to be activated by cellular stresses including tumor necrosis factor-α (TNF-α). Here, we characterized the molecular mechanisms of cellular stress-induced TAK1 activation, focusing mainly on the phosphorylation of TAK1 at Thr-187 and Ser-192 in the activation loop. Thr-187 and Ser-192 are conserved among species from Caenorhabditis elegans to human, and their replacement with Ala resulted in inactivation of TAK1. Immunoblotting with a novel phospho-TAK1 antibody revealed that TNF-α significantly induced the phosphorylation of endogenous TAK1 at Thr-187, and subsequently the phosphorylated forms of TAK1 rapidly disappeared. Intermolecular autophosphorylation of Thr-187 was essential for TAK1 activation. RNA interference and overexpression experiments demonstrated that TAK1-binding protein TAB1 and TAB2 were involved in the phosphorylation of TAK1, but they regulated TAK1 phosphorylation differentially. Furthermore, SB203580 and p38α small interfering RNA enhanced TNF-α-induced Thr-187 phosphorylation as well as TAK1 kinase activity, indicating that the phosphorylation is affected by p38α/TAB1/TAB2-mediated feedback control of TAK1. These results indicate critical roles of Thr-187 phosphorylation in the stress-induced rapid and transient activation of TAK1 in a signaling complex containing TAB1 and TAB2.