The tumor suppressor Sef is a scaffold for the classical NF-κB/RELA:P50 signaling module.

The classical NF-κB transcription factor (RelA:p50) and the tumor suppressor Sef axis constitute a negative regulatory loop in which Sef, a target of NF-κB/RelA:p50, fine-tunes NF-κB/RelA:p50 transcriptional-activation in response to inflammatory stimuli trough binding to p50. Similar to the inhibitor IκBα, Sef sequesters NF-κB/RelA:p50 in the cytoplasm of unstimulated cells. Despite its key roles in regulating multiple cellular processes and its potential role as mediator between inflammation and cancer, Sef structural domains required to fulfill its tasks are poorly characterized, and how Sef specificity towards RelA:p50 is achieved is unknown. In-vitro binding assays using bacterially expressed Sef and Co-IP experiments, revealed that in addition to p50, Sef directly interacts with IκBα, and the IKKβ subunit of the IKK complex which mediates RelA:p50 induction by inflammatory stimuli. These interactions are ligand-independent and do not require Sef post-translational modifications. Deletion mutagenesis mapped binding site to IKKβ in a 74- residue segment juxtaposing Sef transmembrane domain, whereas several Sef regions seem to interact with IκBα. Moreover, we identified two new sites which together with the previously identified conserved tyrosine constitute three discontinuous Sef regions each indispensable for Sef binding to RelA:p50 and inhibiting its cytokine induced transcriptional activation. Contrary to IκBα, endogenous Sef is not degraded upon cytokine-stimulation, and its targeting in different cell types markedly enhances cytokine-induced NF-κB nuclear translocation. These results reveal Sef as the first scaffold that brings together the components of NF-κB/RelA:p50 signaling-module. Sef scaffolding function explains the basis for Sef specificity towards inhibiting inflammatory cytokine-induction of NF-κB/RelA:p50.