Transient interactions modulate the affinity of NF-κB transcription factors for DNA.

The dimeric nuclear factor kappa B (NF-κB) transcription factors (TFs) regulate gene expression by binding to a variety of κB DNA elements with conserved G:C-rich flanking sequences enclosing a degenerate central region. Toward defining mechanistic principles of affinity regulated by degeneracy, we observed an unusual dependence of the affinity of RelA on the identity of the central base pair, which appears to be noncontacted in the complex crystal structures. The affinity of κB sites with A or T at the central position is ~10-fold higher than with G or C. The crystal structures of neither the complexes nor the free κB DNAs could explain the differences in affinity. Interestingly, differential dynamics of several residues were revealed in molecular dynamics simulation studies, where simulation replicates totaling 148 µs were performed on NF-κB:DNA complexes and free κB DNAs. Notably, Arg187 and Arg124 exhibited selectivity in transient interactions that orchestrated a complex interplay among several DNA-interacting residues in the central region. Binding and simulation studies with mutants supported these observations of transient interactions dictating specificity. In combination with published reports, this work provides insights into the nuanced mechanisms governing the discriminatory binding of NF-κB family TFs to κB DNA elements and sheds light on cancer pathogenesis of cRel, a close homolog of RelA.

Euonymus alatus and its monomers alleviate liver fibrosis both in mice and LX2 cells by blocking TßR1-Smad2/3 and TNF-α-NF-κB pathways.

This study aimed to investigate the protective effects, effective constituents and preliminary mechanisms of Euonymus alatus on liver fibrosis and screen new high-efficacy drug for fibrosis. 112 male C57BL/6 mice were randomly divided into 14 groups: control group (CG), CCL4 group (CTG), low/medium/high dose of Euonymus alatus ethanol extracts (EAE), catechin (CA), dihydroquercetin (DHQ) and kaempferol (KA) groups. The study lasted for 30 days by injecting CCL4 in peritoneal cavity to make fibrosis model, all mice were sacrificed to observe morphological changes and collagenous fiber by HE and Masson staining, to test liver index, ALT, AST, to measure the expression of α-SMA and collagen I by immunohistochemistry and western blotting, to discuss the pathways of TßR1-Smad2/3 and TNF-α-NF-κB by WB and Elisa; after being evaluated the efficacy, anti-fibrosis drug of highest efficacy was chosen to repeat these indexes in human hepatic stellate cells-LX2. Results showed that EAE/CA/DHQ/KA prevented increases in liver index, ALT, AST, α-SMA, collagen I, TßR1, Smad2/3, TNF-α and p-NF-κB caused by CCL4 in dose-dependence, they also improved the liver morphology, decreased inflammatory cell infiltration and collagenous fiber in dose-dependence, CA' efficacy was best in mice; in LX-2, CA also decreased the expression of α-SMA, collagen I, TGF-ß, Smad2/3. All findings suggested that Euonymus alatus could alleviate liver inflammation and fibrosis by inhibiting TßR1-Smad2/3 and TNF-α-NF-κB pathways, flavonoid were effective constituents and catechin was screened as a new star for its best performance.

A Guide to Production, Crystallization, and Structure Determination of Human IKK1/α.

A class of extracellular stimuli requires activation of IKK1/α to induce generation of an NF-κB subunit, p52, through processing of its precursor p100. p52 functions as a homodimer or heterodimer with another NF-κB subunit, RelB. These dimers in turn regulate the expression of hundreds of genes involved in inflammation, cell survival, and cell cycle. IKK1/α primarily remains associated with IKK2/ß and NEMO as a ternary complex. However, a small pool of it is also observed as a low molecular weight complex(es). It is unknown if the p100 processing activity is triggered by activation of IKK1/α within the larger or the smaller complex pool. Constitutive activity of IKK1/α has been detected in several cancers and inflammatory diseases. To understand the mechanism of activation of IKK1/α, and enable its use as a drug target, we expressed recombinant IKK1/α in different host systems, such as E. coli, insect, and mammalian cells. We succeeded in expressing soluble IKK1/α in baculovirus infected insect cells, obtaining mg quantities of highly pure protein, crystallizing it in the presence of inhibitors, and determining its X-ray crystal structure. Here, we describe the detailed steps to produce the recombinant protein, its crystallization, and its X-ray crystal structure determination.

[Effects of Coriaria Sinica Maxim's extract on microcirculation and oxidative stress of wounds in rats with deep second-degree burn].

OBJECTIVE: To investigate the effects of Coriaria Sinica Maxim's extract(CSME) on microcirculation and oxidative stress of wounds in rats with deep second-degree burn. METHODS: One hundred and eighty rats were randomly divided into normal saline group(NS), white petroleum group(WPL), silver sulfadiazine group (SSD), Coriariasinica Maxim's extract group which were divided into low dose(CSME-L),middle dose(CSME-M) and high dose(CSME-H). After anesthesia with burn instrument to burn the hair removal area of rats, these wounds were confirmed by pathological results with deep second degree burns.And then,those drugs were applied respectively on the wounds,such as NS、WPL、SSD and different concentrations of CSME. After injury at 48 h, 7 d, 14 d and 21 d,the healing rate(HR) of wound was measured, and the microvessel density (MVD), tissue moisture (TM), vascular endothelial growth factor (VEGF), model driven architecture (MDA), superoxide dismutase(SOD) and hydroxyproline(HYP) were detected, too. All pathological sections of the wound tissue were observed. RESULTS: The HR of CSME groups were obviously increased with a dose-dependent manner, which was significantly higher than that of NS and WPL (P<0.05); On the 21st day, the diameter, number, distribution of the vessels and and the TM were less than other groups with a dose-dependent manner; On the 7th and 14th day after injury, CSME groups were significantly higher than the NS, WPL and SSD with a dose-dependent manner (P<0.05), but, on the 21st day after injury, they were lower than NS, WPL and SSD with a dose-dependent (P<0.05) manner. The levels of SOD, HYP, NO and ET in CSME groups were higher than those in other groups with dose-dependent on SOD activity, HYP, NO and ET content (P<0.05), while MDA activity was weaker than other groups (P<0.05). Similarly, pathological findings were also shown that CSME groups were better than other groups with a dose-dependent manner in decrease decreasing of wound repair time and hyperplasia of scar tissue. CONCLUSIONS: CSME can relieve tissue edema, promote wound contraction, speed up the formation of eschar and accelerate the proliferation of granulation tissue, which are beneficial to the wound healing in the early stages. But, it can inhibit the hyperplasia of granulation tissue to prevent the excessive scar hyperplasia of burn wound in the later stages. Its mechanism is related to regulation what microcirculation, oxidativestress, NO and VEGF.

Protein Cofactors Are Essential for High-Affinity DNA Binding by the Nuclear Factor κB RelA Subunit.

Transcription activator proteins typically contain two functional domains: a DNA binding domain (DBD) that binds to DNA with sequence specificity and an activation domain (AD) whose established function is to recruit RNA polymerase. In this report, we show that purified recombinant nuclear factor κB (NF-κB) RelA dimers bind specific κB DNA sites with an affinity significantly lower than that of the same dimers from nuclear extracts of activated cells, suggesting that additional nuclear cofactors might facilitate DNA binding by the RelA dimers. Additionally, recombinant RelA binds DNA with relatively low affinity at a physiological salt concentration in vitro. The addition of p53 or RPS3 (ribosomal protein S3) increases RelA:DNA binding affinity 2- to >50-fold depending on the protein and ionic conditions. These cofactor proteins do not form stable ternary complexes, suggesting that they stabilize the RelA:DNA complex through dynamic interactions. Surprisingly, the RelA-DBD alone fails to bind DNA under the same solution conditions even in the presence of cofactors, suggesting an important role of the RelA-AD in DNA binding. Reduced RelA:DNA binding at a physiological ionic strength suggests that multiple cofactors might be acting simultaneously to mitigate the electrolyte effect and stabilize the RelA:DNA complex in vivo. Overall, our observations suggest that the RelA-AD and multiple cofactor proteins function cooperatively to prime the RelA-DBD and stabilize the RelA:DNA complex in cells. Our study provides a mechanism for nuclear cofactor proteins in NF-κB-dependent gene regulation.

DNA-binding affinity and transcriptional activity of the RelA homodimer of nuclear factor κB are not correlated.

The nuclear factor κB (NF-κB) transcription factor family regulates genes involved in cell proliferation and inflammation. The promoters of these genes often contain NF-κB-binding sites (κB sites) arranged in tandem. How NF-κB activates transcription through these multiple sites is incompletely understood. We report here an X-ray crystal structure of homodimers comprising the RelA DNA-binding domain containing the Rel homology region (RHR) in NF-κB bound to an E-selectin promoter fragment with tandem κB sites. This structure revealed that two dimers bind asymmetrically to the symmetrically arranged κB sites at which multiple cognate contacts between one dimer to the corresponding DNA are broken. Because simultaneous RelA-RHR dimer binding to tandem sites in solution was anti-cooperative, we inferred that asymmetric RelA-RHR binding with fewer contacts likely indicates a dissociative binding mode. We found that both κB sites are essential for reporter gene activation by full-length RelA homodimer, suggesting that dimers facilitate DNA binding to each other even though their stable co-occupation is not promoted. Promoter variants with altered spacing and orientation of tandem κB sites displayed unexpected reporter activities that were not explained by the solution-binding pattern of RelA-RHR. Remarkably, full-length RelA bound all DNAs with a weaker affinity and specificity. Moreover, the transactivation domain played a negative role in DNA binding. These observations suggest that other nuclear factors influence full-length RelA binding to DNA by neutralizing the transactivation domain negative effect. We propose that DNA binding by NF-κB dimers is highly complex and modulated by facilitated association-dissociation processes.

Structural Basis for the Activation of IKK1/α.

Distinct signaling pathways activate the NF-κB family of transcription factors. The canonical NF-κB-signaling pathway is mediated by IκB kinase 2/ß (IKK2/ß), while the non-canonical pathway depends on IKK1/α. The structural and biochemical bases for distinct signaling by these otherwise highly similar IKKs are unclear. We report single-particle cryoelectron microscopy (cryo-EM) and X-ray crystal structures of human IKK1 in dimeric (∼150 kDa) and hexameric (∼450 kDa) forms. The hexamer, which is the representative form in the crystal but comprises only ∼2% of the particles in solution by cryo-EM, is a trimer of IKK1 dimers. While IKK1 hexamers are not detectable in cells, the surface that supports hexamer formation is critical for IKK1-dependent cellular processing of p100 to p52, the hallmark of non-canonical NF-κB signaling. Comparison of this surface to that in IKK2 indicates significant divergence, and it suggests a fundamental role for this surface in signaling by these kinases through distinct pathways.

p100/IκBδ sequesters and inhibits NF-κB through kappaBsome formation.

Degradation of I kappaB (κB) inhibitors is critical to activation of dimeric transcription factors of the NF-κB family. There are two types of IκB inhibitors: the prototypical IκBs (IκBα, IκBß, and IκBε), which form low-molecular-weight (MW) IκB:NF-κB complexes that are highly stable, and the precursor IκBs (p105/IκBγ and p100/IκBδ), which form high-MW assemblies, thereby suppressing the activity of nearly half the cellular NF-κB [Savinova OV, Hoffmann A, Ghosh G (2009) Mol Cell 34(5):591-602]. The identity of these larger assemblies and their distinct roles in NF-κB inhibition are unknown. Using the X-ray crystal structure of the C-terminal domain of p100/IκBδ and functional analysis of structure-guided mutants, we show that p100/IκBδ forms high-MW (IκBδ)4:(NF-κB)4 complexes, referred to as kappaBsomes. These IκBδ-centric "kappaBsomes" are distinct from the 2:2 complexes formed by IκBγ. The stability of the IκBδ tetramer is enhanced upon association with NF-κB, and hence the high-MW assembly is essential for NF-κB inhibition. Furthermore, weakening of the IκBδ tetramer impairs both its association with NF-κB subunits and stimulus-dependent processing into p52. The unique ability of p100/IκBδ to stably interact with all NF-κB subunits by forming kappaBsomes demonstrates its importance in sequestering NF-κB subunits and releasing them as dictated by specific stimuli for developmental programs.

Probing kinase activation and substrate specificity with an engineered monomeric IKK2.

Catalytic subunits of the IκB kinase (IKK), IKK1/IKKα, and IKK2/IKKß function in vivo as dimers in association with the necessary scaffolding subunit NEMO/IKKγ. Recent X-ray crystal structures of IKK2 suggested that dimerization might be mediated by a smaller protein-protein interaction than previously thought. Here, we report that removal of a portion of the scaffold dimerization domain (SDD) of human IKK2 yields a kinase subunit that remains monomeric in solution. Expression in baculovirus-infected Sf9 insect cells and purification of this engineered monomeric human IKK2 enzyme allows for in vitro analysis of its substrate specificity and mechanism of activation. We find that the monomeric enzyme, which contains all of the amino-terminal kinase and ubiquitin-like domains as well as the more proximal portions of the SDD, functions in vitro to direct phosphorylation exclusively to residues S32 and S36 of its IκBα substrate. Thus, the NF-κB-inducing potential of IKK2 is preserved in the engineered monomer. Furthermore, we observe that our engineered IKK2 monomer readily autophosphorylates activation loop serines 177 and 181 in trans. However, when residues that were previously observed to interfere with IKK2 trans autophosphorylation in transfected cells are mutated within the context of the monomer, the resulting Sf9 cell expressed and purified proteins were significantly impaired in their trans autophosphorylation activity in vitro. This study further defines the determinants of substrate specificity and provides additional evidence in support of a model in which activation via trans autophosphorylation of activation loop serines in IKK2 requires transient assembly of higher-order oligomers.

PRMT5 dimethylates R30 of the p65 subunit to activate NF-κB.

The ubiquitous inducible transcription factor NF-κB plays central roles in immune and inflammatory responses and in tumorigenesis. Complex posttranslational modifications of the p65 subunit (RelA) are a major aspect of the extremely flexible regulation of NF-κB activity. Although phosphorylation, acetylation, ubiquitination, and lysine methylation of NF-κB have been well described, arginine methylation has not yet been found. We now report that, in response to IL-1ß, the p65 subunit of NF-κB is dimethylated on arginine 30 (R30) by protein-arginine methyltransferase 5 (PRMT5). Expression of the R30A and R30K mutants of p65 substantially decreased the ability of NF-κB to bind to κB elements and to drive gene expression. A model in which dimethyl R30 is placed into the crystal structure of p65 predicts new van der Waals contacts that stabilize intraprotein interactions and indirectly increase the affinity of p65 for DNA. PRMT5 was the only arginine methyltransferase that coprecipitated with p65, and its overexpression increased NF-κB activity, whereas PRMT5 knockdown had the opposite effect. Microarray analysis revealed that ∼85% of the NF-κB-inducible genes that are down-regulated by the R30A mutation are similarly down-regulated by knocking PRMT5 down. Many cytokine and chemokine genes are among these, and conditioned media from cells expressing the R30A mutant of p65 had much less NF-κB-inducing activity than media from cells expressing the wild-type protein. PRMT5 is overexpressed in many types of cancer, often to a striking degree, indicating that high levels of this enzyme may promote tumorigenesis, at least in part by facilitating NF-κB-induced gene expression.

Role of lysine methylation of NF-κB in differential gene regulation.

Lysine methylation of the p65 subunit of nuclear factor κB (NF-κB) on K218 and K221 together or K37 alone strongly enhances gene expression in response to cytokines. We analyzed the effects of K-to-Q mutations in the REL homology domain of p65 on the response to IL-1ß in 293 cells with low levels of p65. The K218/221Q mutation greatly reduced the expression of 39 of 82 genes, whereas the K37Q mutation reduced the expression of 23 different genes. Enhanced expression of the lysine demethylase FBXL11, which catalyzes the demethylation of K218 and K221 specifically, inhibited the expression of most of the genes that were inhibited by the DKQ mutation. CHIP-Seq analysis showed that the K218/221Q mutation greatly reduces the affinity of p65 for many promoters and that the K37Q mutation does not. Structural modeling showed that the newly introduced methyl groups of K218 and K221 interact directly with DNA to increase the affinity of p65 for specific κB sites. Thus, the K218/221Q and K37Q mutations have dramatically different effects because methylations of these residues affect different genes by distinct mechanisms.

A structural basis for IκB kinase 2 activation via oligomerization-dependent trans auto-phosphorylation.

Activation of the IκB kinase (IKK) is central to NF-κB signaling. However, the precise activation mechanism by which catalytic IKK subunits gain the ability to induce NF-κB transcriptional activity is not well understood. Here we report a 4 Å x-ray crystal structure of human IKK2 (hIKK2) in its catalytically active conformation. The hIKK2 domain architecture closely resembles that of Xenopus IKK2 (xIKK2). However, whereas inactivated xIKK2 displays a closed dimeric structure, hIKK2 dimers adopt open conformations that permit higher order oligomerization within the crystal. Reversible oligomerization of hIKK2 dimers is observed in solution. Mutagenesis confirms that two of the surfaces that mediate oligomerization within the crystal are also critical for the process of hIKK2 activation in cells. We propose that IKK2 dimers transiently associate with one another through these interaction surfaces to promote trans auto-phosphorylation as part of their mechanism of activation. This structure-based model supports recently published structural data that implicate strand exchange as part of a mechanism for IKK2 activation via trans auto-phosphorylation. Moreover, oligomerization through the interfaces identified in this study and subsequent trans auto-phosphorylation account for the rapid amplification of IKK2 phosphorylation observed even in the absence of any upstream kinase.

A structural basis for selective dimerization by NF-κB RelB.

Transcription factors of the nuclear factor kappaB (NF-κB) family arise through the combinatorial association of five distinct Rel subunits into functional dimers. However, not every dimer combination is observed in cells. The RelB subunit, for example, does not appear as a homodimer and forms heterodimers exclusively in combination with p50 or p52 subunits. We previously reported that the RelB homodimer could be forced to assemble through domain swapping in vitro. In order to understand the mechanism of selective dimerization among Rel subunits, we have determined the x-ray crystal structures of five RelB dimers. We find that RelB forms canonical side-by-side heterodimers with p50 and p52. We observe that, although mutation of four surface hydrophobic residues that are unique to RelB does not affect its propensity to form homodimers via domain swapping, alteration of two interfacial residues converts RelB to a side-by-side homodimer. Surprisingly, these mutant RelB homodimers remain distinct from canonical side-by-side NF-κB dimers in that the two monomers move away from one another along the 2-fold axis to avoid non-complementary interactions at the interface. The presence of distinct residues buried within the hydrophobic core of the RelB dimerization domain appears to influence the conformations of the surface residues that mediate the dimer interface. This conclusion is consistent with prior observations that alterations of domain core residues change dimerization propensity in the NF-κB family transcription factors. We suggest that RelB has evolved into a specialized NF-κB subunit with unique amino acids optimized for selective formation of heterodimers with p50 and p52.

NF-κB regulation: lessons from structures.

The signaling module that specifies nuclear factor-κΒ (NF-κB) activation is a three-component system: NF-κB, inhibitor of NF-κΒ (IκΒ), and IκΒ kinase complex (IKK). IKK receives upstream signals from the surface or inside the cell and converts itself into a catalytically active form, leading to the destruction of IκB in the inhibited IκB:NF-κB complex, leaving active NF-κB free to regulate target genes. Hidden within this simple module are family members that all can undergo various modifications resulting in expansion of functional spectrum. Three-dimensional structures representing all three components are now available. These structures have allowed us to interpret cellular observations in molecular terms and at the same time helped us to bring forward new concepts focused towards understanding the specificity in the NF-κB activation pathway.

New guaianolides from Artemisia anomala.

Two new guaianolides artemanomalides A and B were isolated from the aerial parts of Artemisia anomala S. Moore. Their structures were characterized as 2-oxo-5α, 10α-dihydroxy-guaia-3-en-1α, 6ß, 7α, 11ß H-12, 6-olide (1) and 8α-acetoxy-2-oxo-5α, 10α-dihydroxy-guaia-3, 11(13)-dien-1α, 6ß, 7αH-12, 6-olide (2) on the basis of extensive spectroscopic analyses. Compounds 1 and 2 showed inhibitory activities against COX-2 enzyme with IC(50) values of 8.8 and 3.6 µM.

The SRSF1 linker induces semi-conservative ESE binding by cooperating with the RRMs.

SR proteins promote spliceosome formation by recognizing exonic splicing enhancers (ESEs) during pre-mRNA splicing. Each SR protein binds diverse ESEs using strategies that are yet to be elucidated. Here, we show that the RNA-binding domain (RBD) of SRSF1 optimally binds to decameric purine rich ESE sequences although locations of purines are not stringently specified. The presence of uracils either within or outside of the recognition site is detrimental for binding with SRSF1. The entire RBD, comprised of two RRMs and a glycine-rich linker, is essential for ESE binding. Mutation within each segment reduced or nearly abolished binding, suggesting that these segments mediate cooperative binding. The linker plays a decisive role in organizing ESE binding. The flanking basic regions of the linker appear to communicate with each other in bringing the two RRMs close together to form the complex with RNA. Our study thus suggests semi-conservative adaptable interaction between ESE and SRSF1, and such binding mode is not only essential for the recognition of plethora of physiological ESE sequences but may also be essential for the interaction with various factors during the spliceosome assembly.

The specificity of innate immune responses is enforced by repression of interferon response elements by NF-κB p50.

The specific binding of transcription factors to cognate sequence elements is thought to be critical for the generation of specific gene expression programs. Members of the nuclear factor κB (NF-κB) and interferon (IFN) regulatory factor (IRF) transcription factor families bind to the κB site and the IFN response element (IRE), respectively, of target genes, and they are activated in macrophages after exposure to pathogens. However, how these factors produce pathogen-specific inflammatory and immune responses remains poorly understood. Combining top-down and bottom-up systems biology approaches, we have identified the NF-κB p50 homodimer as a regulator of IRF responses. Unbiased genome-wide expression and biochemical and structural analyses revealed that the p50 homodimer repressed a subset of IFN-inducible genes through a previously uncharacterized subclass of guanine-rich IRE (G-IRE) sequences. Mathematical modeling predicted that the p50 homodimer might enforce the stimulus specificity of composite promoters. Indeed, the production of the antiviral regulator IFN-ß was rendered stimulus-specific by the binding of the p50 homodimer to the G-IRE-containing IFNß enhancer to suppress cytotoxic IFN signaling. Specifically, a deficiency in p50 resulted in the inappropriate production of IFN-ß in response to bacterial DNA sensed by Toll-like receptor 9. This role for the NF-κB p50 homodimer in enforcing the specificity of the cellular response to pathogens by binding to a subset of IRE sequences alters our understanding of how the NF-κB and IRF signaling systems cooperate to regulate antimicrobial immunity.

NF-kappaB p52:RelB heterodimer recognizes two classes of kappaB sites with two distinct modes.

The X-ray structure of the nuclear factor-kappaB (NF-kappaB) p52:RelB:kappaB DNA complex reveals a new recognition feature not previously seen in other NF-kappaB:kappaB DNA complexes. Arg 125 of RelB is in contact with an additional DNA base pair. Surprisingly, the p52:RelB R125A mutant heterodimer shows defects both in DNA binding and in transcriptional activity only to a subclass of kappaB sites. We found that the Arg 125-sensitive kappaB sites contain more contiguous and centrally located A:T base pairs than do the insensitive sites. A protein-induced kink observed in this complex, which used an AT-rich kappaB site, might allow the DNA contact by Arg 125; such a kink might not be possible in complexes with non-AT-rich kappaB sites. Furthermore, we show that the p52:RelB heterodimer binds to a broader spectrum of kappaB sites when compared with the p50:RelA heterodimer. We suggest that the p52:RelB heterodimer is more adaptable to complement sequence and structural variations in kappaB sites when compared with other NF-kappaB dimers.

X-ray structure of a NF-kappaB p50/RelB/DNA complex reveals assembly of multiple dimers on tandem kappaB sites.

We describe here the X-ray crystal structure of NF-kappaB p50/RelB heterodimer bound to a kappaB DNA. Although the global modes of subunit association and kappaB DNA recognition are similar to other NF-kappaB/DNA complexes, this complex reveals distinctive features not observed for non-RelB complexes. For example, Lys274 of RelB is removed from the protein-DNA interface whereas the corresponding residues in all other subunits make base-specific contacts. This mode of binding suggests that RelB may allow the recognition of more diverse kappaB sequences. Complementary surfaces on RelB and p50, as revealed by the crystal contacts, are highly suggestive of assembly of multiple p50/RelB heterodimers on tandem kappaB sites in solution. Consistent with this model our in vitro binding experiments reveal optimal assembly of two wild-type p50/RelB heterodimers on tandem HIV kappaB DNA with 2 bp spacing but not by a mutant heterodimer where one of the RelB packing surface is altered. We suggest that multiple NF-kappaB dimers assemble at diverse kappaB promoters through direct interactions utilizing unique protein-protein interaction surfaces.

NF-kappaB RelB forms an intertwined homodimer.

The X-ray structure of the RelB dimerization domain (DD) reveals that the RelBDD assumes an unexpected intertwined fold topology atypical of other NF-kappaB dimers. All typical NF-kappaB dimers are formed by the association of two independently folded immunoglobulin (Ig) domains. In RelBDD, two polypeptides reconstruct both Ig domains in the dimer with an extra beta sheet connecting the two domains. Residues most critical to NF-kappaB dimer formation are invariant in RelB, and Y300 plays a positive role in RelBDD dimer formation. The presence of RelB-specific nonpolar residues at the surface removes several intradomain surface hydrogen bonds that may render the domain fold unstable. Intertwining may stabilize the RelBDD homodimer by forming the extra beta sheet. We show that, as in the crystal, RelB forms an intertwined homodimer in solution. We suggest that the transiently stable RelB homodimer might prevent its rapid degradation, allowing for heterodimer formation with p50 and p52.