The main NF-κB-activating pathways. On the left is the TNFα-dependent signalling pathway. The binding of TNFα to the TNF receptor TNFR1 triggers the sequential recruitment of the adaptors TRADD (TNFR1-associated death domain protein), RIP and TRAF2 (TNF receptor-associated factor 2) to the membrane. Then, TRAF2 mediates the recruitment of the IκB kinase (IKK) complex, composed of IKKα, IKKβ and NEMO (NF-kappa-B essential modulator), to the TNFR1 signalling complex. The scaffold proteins TAB2 and TAB3 subsequently bind to Lys63-polyubiquitylated substrates, such as receptor-interacting protein (RIP)1, resulting in TAK1 and then IKKβ activation. NEMO actually exerts its essential role in NF-κB activation by integrating upstream IKK-activating signals. Importantly, the linear ubiquitin (Ub) chain assembly complex (LUBAC), composed of two proteins, namely HOIL-1L (hemeoxidized IRP2 ubiquitin ligase-1) and HOIP (HOIL-1L interacting protein), binds NEMO in a TNFα-dependent manner, and generates and conjugates linear chains of ubiquitin on the scaffold protein of the IKK complex . The ubiquitin-binding motif of NEMO, referred to as the UBAN motif, is required to sense linear chains of ubiquitin. Activation of IKKβ leads to IκBα phosphorylation on specific residues, polyubiquitylation through binding of ubiquitin proteins and its degradation through the proteasome pathway. Then, the heretodimer p50-p65 binds to specific κB sites and activates a variety of NF-κB target genes coding for pro-inflammatory cytokines (IL-6) and chemokines. On the right is the alternative NF-κB-activating pathway. Binding of CD154 triggers the classical NEMO-dependent pathway (not illustrated) and the NEMO-independent cascade. This pathway relies on the recruitment of the heterodimer TRAF2-TRAF3 to the CD40 receptor. TRAF3 is required to connect the E3 ligases c-IAP1/2 (cellular inhibitor of apoptosis 1/2) to the kinase NIK (NF-κB-inducing kinase). NIK is activated by phosphorylation and is also subjected to a c-IAP1/2-dependent degradative polyubiquitination. IKKα homodimers are activated by NIK and phosphorylate the inhibitory molecule p100, the partial processing of which generates the NF-κB protein p52. This latter transcription factor moves into the nucleus as a heterodimer with RelB to regulate the expression of genes involved in lymphoid organogenesis or coding for chemokines (BLC (B lymphocyte chemokine)) or cytokines (BAFF (B-cell activating factor)).