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Remaining members include RelB, c-Rel and cleaved proteins NF-κB1 (p50) and NF-κB2 (p52). p50 and p65 form the most common heterodimer in the NF-κB signaling pathway, present in most cell types. In contrast, other members, for example c-Rel, are predominantly found only in hematopoietic cells (Napetschnig and Wu, 2013; Bassères and Baldwin, 2006).
p65, RelB and c-Rel are characterized by a Rel homology domain located in the N-terminal, which is combined with a transactivating domain in the C-terminal. In contrast, NF-κB1 and NF-κB2 are processed from large precursors (p105 and p100, respectively) into p50 and p52 subunits. This is a ubiquitin-mediated process, involving selective degradation from the C-terminal to remove ankyrin repeats (Napetschnig and Wu, 2013; Bassères and Baldwin, 2006).
As part of the NF-κB signaling pathway, p65 is typically involved in the body’s inflammatory response. This pathway can be induced by stressful stimuli including: free radicals, ultraviolet irradiation (UV), tumor necrosis factor α (TNFα), interleukin 1-beta (IL-1β), pathogen-associated molecular patterns (PAMPs) or bacterial lipopolysaccharides (LPS). NF-κB has been implicated in memory and synaptic plasticity while aberrant protein levels have been linked to cancer (Courtois et al., 2006; Gutierrez and Davies, 2011).
In unstimulated cells, NF-κB subunits are restricted to the cytoplasm due to the inhibitory effects of the inhibitor of κB ( IκB) family. IκBα or IκBβ selectively bind to the p50/p65 heterodimer and masks their nuclear localization signal (NLS), preventing their nuclear translocation.
Stressful stimuli induce NF-κB signaling through several cell membrane-bound receptors that converge on the activation of IκB kinase (IKK) complex. The activated IKK complex phosphorylates IκBα, priming it for subsequent ubiquitylation and proteasome-mediated degradation. Disassociation of IκBα induces NF-κB heterodimers to located to the nucleus and bind to specific gene promoters to modulate the expression of pro- and anti-inflammatory proteins. As a negative feedback loop, the expression of IκBα and IκBβ are up-regulated by the NF-κB heterodimers, in order to terminate the signaling pathway.
NF-κB subunits have been reported to be extensively modified, including: O-linked N-acetylglucosylation, ubiquitylation, nitrosylation, acetylation, prolyl isomerization, methylation as well as phosphorylation. The best studied family member is p65 (reviewed by Napetschnig and Wu, 2013; Gilmore, 2006).
NF-κB signaling plays a crucial role to fight infections, however some pathogens have developed ways to alter NF-κB activity to their advantage. For example, the Human immunodeficiency virus (HIV) contains binding sites within its genome for NF-κB subunits, which drive viral gene expression as well as boosting its replication (Hiscott et al., 2011).