Inhibition of NF-κB cellular function via specific targeting of the IκB-ubiquitin ligase

Activation of the transcription factor NF-κB is a paradigm for signal transduction through the ubiquitin-proteasome pathway: ubiquitin-dependent degradation of the transcriptional inhibitor IκB in response to cell stimulation. A major issue in this context is the nature of the recognition signal and the targeting enzyme involved in the proteolytic process. Here we show that following a stimulus-dependent phosphorylation, and while associated with NF-κB, IκB is targeted by a specific ubiquitin-ligase via direct recognition of the signal-dependent phosphorylation site; phosphopeptides corresponding to this site specifically inhibit ubiquitin conjugation of IκB and its subsequent degradation. The ligase recognition signal is functionally conserved between IκBα and IκBβ, and does not involve the nearby ubiquitination site. Microinjection of the inhibitory peptides into stimulated cells abolished NF-κB activation in response to TNFα and the consequent expression of E-selectin, an NF-κB-dependent cell-adhesion molecule. Inhibition of NF-κB function by specific blocking of ubiquitin ligase activity provides a novel approach for intervening in cellular processes via regulation of unique proteolytic events.