Nuclear factor-kappa B (NF-kB) is a transcription factor that plays critical functional roles in inflammation, immunity, cell proliferation, differentiation, and survival. It exists in an inactive state in the cytosol and can be stimulated by molecules such as TNFα, and other cell stressors.
NF-kB is found in almost all cellular cell types and is identified as a regulator of kB light chain expression in mature B and plasma cells.
Because NF-kB has the ability to influence the expression of numerous genes, its activity is tightly regulated at multiple levels. The primary mechanism for regulating NF-kB is through inhibitory IkB proteins and the IKK complex, which phosphorylates IkBs.
Nuclear Factor-kappa B (NF-kB) Activation
NF-kB activation is initiated by TNFα. When TNFα binds to TNF receptors and activate them, IkB kinase (IKK) is ultimately triggered, which leads to the phosphorylation of IkB. The phosphorylation of IkB results in ubiquitination and degradation of IkB. When this happens, the remaining NF-kB dimmer (e.g., p65/p50 or p50/p50 subunits) translocates to the nucleus, where it then binds to a DNA consensus sequence of target genes.
Nuclear Factor-kappa B (NF-kB) Structure-Function relationship
NF-kB is a multiple-gene family of proteins that can form stable homo- and heterodimeric complexes, which vary in their DNA binding specificity and transcriptional activation potential.
There are five proteins of the NF-kB family in mammalian cells; they are RelA (P65), C-Rel, RelB, and NF-kB1 (p50 and its precursor p105), and NF-kB2 (p52 and its precursor p100). NF-kB and Rel proteins share a highly conserved 300 amino acid N-terminal Rel homology domain (RHD), which is responsible for DNA binding, dimerization, and association with IkB inhibitory proteins.
The p50/p65 complex shows strong transcriptional activation, whereas p50/p50 and p52/p52 homodimers suppress transcription of NF-kB target genes.
Nuclear Factor-kappa B (NF-kB) Is Inhibited By IkB Family Proteins
NF-kB is inhibited by IkB family proteins through NF-kB/IkB complex formation. There are seven inhibitory protein members of IkB family. They are IkBα, IkBβ, IkBε, IkBϒ, Bcl3, NF-kB1 precursor and NFkB2 precursor.
The IkB family members have a common ankyrin repeat domain. They regulate the subcellular localization, and hence, the DNA binding and transcriptional activity of NF-kB proteins.
IkB Regulates NF-kB Translocation to the Nucleus.
NF-kB is localized in the cytoplasm in an NF-kB/IkB complex, which is inactive. The inactive NF-kB/IkB co2mplex is a result of masking of the nuclear localization signals (NLS) on the NF-kB subunits by the IkB proteins. Hence, the degradation of IkB would lead to unmasking of the NLS, allowing NF-kB to undergo translocation to the nucleus.
The IkB proteins show a preference for specific NF-kB/Rel complexes, which provides a means to regulate the activation of distinct Rel/NF-kB complexes.
After the binding and transcriptional activity of NF-kB on DNA, it induces the expression of IkBα, which enters the nucleus and remove NF-kB from DNA by forming NF-kB/IkB complex with the released NF-kB. The complex is then expelled from the nucleus back to the cytoplasm as a result of potent nuclear export signals on IkB and p65.
IkB Degradation Is Mediated By Ubiquiti/Proteasome System
The activation of NF-kB is achieved through the signal induced proteolytic degradation of IkB. This degradation is initiated by the stimuli dependent phosphorylation of IkB at specific N-terminal residues (S32/S36 for IkBα, S19/S23 for IkBβ), and is mediated by the ubiquitin/proteasome system.
Phosphorylation of IkB however, is not enough to initiate degradation of IkB. Ubiquitination and subsequent degradation depends on the recognition of phosphorylated IkB by the β-TrCp, an F-box/WD containing component of the Skp1-cullin-F-box (SCF) class of E3 ubiquitin ligases.
IKKs Mediates The Phosphorylation Of IkB On Serine 32 and 36
Phosphorylation of IkBa on the Serine 32 and 36 is mediated by IkB kinases (IKKs), whose activity is induced by activators of the NF-kB pathway. IKK contains two subunits, IKK1 (IKKα) and IKK2 (IKKβ). It also contains a regulatory subunit, NEMO ( IKKy, IKKAPI, FIP3).
IKK1 and IKK2 are homologous kinases, and both contain an N-terminal kinase domain and a C-terminal region with two protein interaction motifs, a leucine zipper (LZ), and ahelix-loop-helix (HLH) motif.
The LZ domain is responsible for demerization of IKK1 and IKK2. It is also essential for IKK complex activity. The IKK1/2 complex associates with NEMO through a short interaction motif located at the C-terminus of either catalytic subunit. Short peptides derived from the interaction motif can be used to disrupt the IKK complex and prevent its activation. NEMO connects the IKK complex to upstream activators through its C-terminus, which contains a zinc finger motif. NEMO also undergoes stimulus dependent interaction with components of TNF receptor complex.
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