Role of Monoubiquitylation on the Control of I kappa B alpha Degradation and NF-kappa B Activity

9 p.The NF-κB pathway is regulated by multiple post-translational modifications including phosphorylation, ubiquitylation and SUMOylation. Many of these modifications act on the natural inhibitor IκBα modulating its capacity to control signal-mediated NF-κB activity. While the canonical pathway involving the phosphorylation and polyubiquitylation of IκBα has been well characterized, the role of these post-translational modifications in the control of basal NF-κB activity has not been deeply explored. Using the recently developed Tandem-repeated Ubiquitin Binding Entities (also known as ubiquitin traps) to capture ubiquitylated proteins, we identified monoubiquitylated forms of IκBα from multiple rat organs and cell types. The identification of these forms was demonstrated through different procedures such as immunoprecipitations with specific ubiquitin antibodies or His6-Ubiquitin pull downs. Monoubiquitylated forms of IκBα are resistant to TNFα-mediated degradation and can be captured using TUBEs, even after proteasome inhibitors treatment. As it occurs for monoSUMOylation, monoubiquitylation is not dependent of the phosphorylation of IκBα on the serines 32/36 and is not optimally degraded after TNFα stimulation. A ubiquitin-IκBα fusion exhibits phosphorylation defects and resistance to TNFα mediated degradation similar to the ones observed for endogenous monoubiquitylated IκBα. The N-terminal attachment of a single ubiquitin moiety on the IκBα fusion results in a deficient binding to the IKKβ kinase and recruitment of the SCF ligase component βTrCP, promoting a negative impact on the NF-κB activity. Altogether, our results suggest the existence of a reservoir of monoubiquitylated IκBα resistant to TNFα-induced proteolysis, which is able to interact and repress DNA binding and NF-κB transcriptional activity. Such pool of IκBα may play an important role in the control of basal and signal-mediated NF-κB activity.This work was funded by the Ramón y Cajal Program, Ministerio de Educación y Ciencia grant BFU2006-12991 and BFU2008-01108/BMC, Fondo de Investigaciones Sanitarias (FIS) CIBERhed, Government of the Autonomous Community of the Basque Country grant PI09-05, Department of Industry, Tourism and Trade of the Government of the Autonomous Community of the Basque Country (Etortek Research Programs 2008/2009) and from the Innovation Technology Department of the Bizkaia Country. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript