P HOSPHORYLATION M EETS U BIQUITINATION : The Control of NF-jB Activity

Abstract

NF-jB (nuclear factor-jB) is a collective name for inducible dimeric transcription factors composed ofmembers ofthe Rel family ofDNA-binding proteins that recognize a common sequence motif. NF-jB is found in essentially all cell types and is involved in activation of an exceptionally large number of genes in response to infections, inflammation, and other stressful situations requiring rapid reprogram- ming of gene expression. NF-jB is normally sequestered in the cytoplasm of nonsti- mulated cells and consequently must be translocated into the nucleus to function. The subcellular location of NF-jB is controlled by a family of inhibitory proteins, IjBs, which bind NF-jB and mask its nuclear localization signal, thereby preventing nuclear uptake. Exposure of cells to a variety of extracellular stimuli leads to the rapid phos- phorylation, ubiquitination, and ultimately proteolytic degradation ofIjB, which frees NF-jB to translocate to the nucleus where it regulates gene transcription. NF-jB activation represents a paradigm for controlling the function of a regulatory protein via ubiquitination-dependent proteolysis, as an integral part of a phosphorylation- based signaling cascade. Recently, considerable progress has been made in under- standing the details ofthe signaling pathways that regulate NF-jB activity, particularly those responding to the proinflammatory cytokines tumor necrosis factor-␣ and inter- leukin-1. The multisubunit IjB kinase (IKK) responsible for inducible IjB phos- phorylation is the point of convergence for most NF-jB–activating stimuli. IKK contains two catalytic subunits, IKK␣ and IKKb, both of which are able to correctly phosphorylate IjB. Gene knockout studies have shed light on the very different physiological functions of IKK␣ and IKKb. After phosphorylation, the IKK phos- phoacceptor sites on IjB serve as an essential part of a specific recognition site for E3RSIjB/b-TrCP, an SCF-type E3 ubiquitin ligase, thereby explaining how IKK controls IjB ubiquitination and degradation. A variety of other signaling events, including phosphorylation ofNF-jB, hyperphosphorylation ofIKK, induction ofIjB synthesis, and the processing ofNF-jB precursors, provide additional mechanisms that modulate the level and duration of NF-jB activity.