TAK1 Is Recruited to the Tumor Necrosis Factor-α (TNF-α) Receptor 1 Complex in a Receptor-interacting Protein (RIP)-dependent Manner and Cooperates with MEKK3 Leading to NF-κB Activation

Receptor-interacting protein (RIP) plays a critical role in tumor necrosis factor-alpha (TNF-alpha)-induced IkappaB kinase (IKK) activation and subsequent activation of transcription factor NF-kappaB. However, the molecular mechanism by which RIP mediates TNF-alpha-induced NF-kappaB activation is not completely defined. In this study, we have found that TAK1 is recruited to the TNF-alpha receptor complex in a RIP-dependent manner following the stimulation of TNF-alpha receptor 1 (TNF-R1). Moreover, a forced recruitment of TAK1 to TNF-R1 in the absence of RIP is sufficient to mediate TNF-alpha-induced NF-kappaB activation, indicating that the major function of RIP is to recruit its downstream kinases to the TNF-R1 complex. Interestingly, we also find that TAK1 and MEKK3 form a functional complex, in which TAK1 regulates autophosphorylation of MEKK3. The TAK1-mediated regulation of MEKK3 phosphorylation is dependent on the kinase activity of TAK1. Although TAK1-MEKK3 interaction is not affected by overexpressed TAB1, TAB1 is required for TAK1 activation and subsequent MEKK3 phosphorylation. Together, we conclude that TAK1 is recruited to the TNF-R1 complex via RIP and likely cooperates with MEKK3 to activate NF-kappaB in TNF-alpha signaling.