TAK1, a member of the MAP3K family, plays an essential role in activation of JNK/p38 MAPKs and IKK in the IL-1 beta and TNF alpha signaling pathway. Upon stimulation, TAK1 is rapidly and transiently activated. While the activation mechanism of TAK1 in these signaling pathways is well characterized, how its activity is terminated still remains unclear. To identify the molecule(s) involved in TAK1 regulation, we performed tandem affinity purification (TAP) in HeLa cells stably expressing TAP-tagged TAK1. FBXW5, an F-box family protein, was identified as a previously unknown component of the IL-1 beta-induced TAK1 complex. FBXW5 associated with endogenous TAK1 in an IL-1 beta-dependent manner. Overexpression of FBXW5 inhibited IL-1 beta-induced activation of JNK/p38 MAPKs and NF-kappa B as well as phosphorylation of TAK1 on Thr187. Conversely, knockdown of FBXW5 resulted in the prolonged activation of TAK1 upon IL-1 beta stimulation. These results suggest that FBXW5 negatively regulates TAK1 in the IL-1 beta signaling pathway. (C) 2009 Elsevier Inc. All rights reserved.