Phosphorylation Meets Ubiquitination: The Control of NF-κB Activity

Abstract

NF-kappaB (nuclear factor-kappaB) is a collective name for inducible dimeric transcription factors composed of members of the Rel family of DNA-binding proteins that recognize a common sequence motif. NF-kappaB 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 reprogramming of gene expression. NF-kappaB is normally sequestered in the cytoplasm of nonstimulated cells and consequently must be translocated into the nucleus to function. The subcellular location of NF-kappaB is controlled by a family of inhibitory proteins, IkappaBs, which bind NF-kappaB and mask its nuclear localization signal, thereby preventing nuclear uptake. Exposure of cells to a variety of extracellular stimuli leads to the rapid phosphorylation, ubiquitination, and ultimately proteolytic degradation of IkappaB, which frees NF-kappaB to translocate to the nucleus where it regulates gene transcription. NF-kappaB activation represents a paradigm for controlling the function of a regulatory protein via ubiquitination-dependent proteolysis, as an integral part of a phosphorylationbased signaling cascade. Recently, considerable progress has been made in understanding the details of the signaling pathways that regulate NF-kappaB activity, particularly those responding to the proinflammatory cytokines tumor necrosis factor-alpha and interleukin-1. The multisubunit Ika…