Phosphorylation-induced flexibility of proto-oncogenic Bcl3 regulates transcriptional activation by NF-κB p52 homodimer.

The overexpression of proto-oncogene Bcl3 is observed in various cancers. Bcl3 is extensively phosphorylated and associates with homodimers of NF-κB p50 and p52 to regulate transcription. Through cellular and biochemical assays, we observed that phospho-mimetic Glu substitution at Ser366 in addition to previously studied Ser33, 114 and 446 is necessary to switch Bcl3 from an IκB-like inhibitor to a transcriptional activator. To study interactive features of p52 and Bcl3, and phosphorylation- mediated changes in Bcl3 that regulate DNA-binding by p52, we performed HDX-MS of both Bcl3 and p52 within various complexes. Nature of interactions within Bcl3:(p52:p52) complex in presence and absence of DNA, differential flexibility of Bcl3, and allosteric changes in Bcl3 upon phospho-modifications revealed why a facile accommodation of DNA requires phosphorylation. The inhibitory nature of unphosphorylated Bcl3 on DNA binding by p52:p52 also relieved by a C-terminal deletion of Bcl3. Overall, this study revealed mechanistic bases of how Bcl3 phosphorylation regulates transcriptional potential of NF-κB and intricate cell physiology, a dysregulation of which can lead to cancers.

Cell free DNA in patients with pancreatic adenocarcinoma: clinicopathologic correlations.

Detection of circulating tumor DNA (ctDNA) from plasma cell free DNA (cfDNA) has shown promise for diagnosis, therapeutic targeting, and prognosis. This study explores ctDNA detection by next generation sequencing (NGS) and associated clinicopathologic factors in patients with pancreatic adenocarcinoma (PDAC). Patients undergoing surgical exploration or resection of pancreatic lesions were enrolled with informed consent. Plasma samples (4-6 ml) were collected prior to surgery and cfDNA was recovered from 95 plasma samples. Adequate cfDNA for NGS (20 ng) was obtained from 81 patients. NGS was performed using the Oncomine Lung cfDNA assay on the Ion Torrent S5 sequencing platform. Twenty-five patients (30.9%) had detectable mutations in KRAS and/or TP53 with allele frequencies ranging from 0.05 to 8.5%, while mutations in other genes were detected less frequently and always along with KRAS or TP53. Detectable ctDNA mutations were more frequent in patients with poorly differentiated tumors, and patients without detectable ctDNA mutations showed longer survival (medians of 10.5 months vs. 18 months, p = 0.019). The detection of circulating tumor DNA in pancreatic adenocarcinomas is correlated with worse survival outcomes.

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.

Molecular detection of bacteria, placental inflammation, and neonatal sepsis risk.

OBJECTIVE: To identify bacteria in umbilical cord tissue and investigate the association with placental inflammation and neonatal sepsis risk score. STUDY DESIGN: Retrospective cohort study from 2017-2019. RNA was extracted from umbilical cord tissue and NanoString nCounter used to identify seven bacteria genera. Sepsis risk score was calculated using the Kaiser sepsis calculator. Placental histopathology was abstracted from medical records. RESULTS: Detection of bacterial RNA in the umbilical cord (n = 96/287) was associated with high-stage maternal and fetal acute placental inflammation (maternal 35.4% vs 22.5%, p = 0.03 and fetal 34.4% vs 19.4%, p < 0.01) and maternal vascular malperfusion (36.5% vs 23.0%, p = 0.02). Detection of Ureaplasma spp. was also associated with increased sepsis risk score (1.5/1000 [0.6, 8.6] vs 0.9/1000 [0.2, 2.9], p = 0.04). CONCLUSION: Umbilical cord bacterial pathogens are linked to fetal and maternal placental inflammation and maternal vascular malperfusion during gestation and associated with increased sepsis risk score in the neonate.

Structures of NF-κB p52 homodimer-DNA complexes rationalize binding mechanisms and transcription activation.

The mammalian NF-κB p52:p52 homodimer together with its cofactor Bcl3 activates transcription of κB sites with a central G/C base pair (bp), while it is inactive toward κB sites with a central A/T bp. To understand the molecular basis for this unique property of p52, we have determined the crystal structures of recombinant human p52 protein in complex with a P-selectin(PSel)-κB DNA (5'-GGGGTGACCCC-3') (central bp is underlined) and variants changing the central bp to A/T or swapping the flanking bp. The structures reveal a nearly two-fold widened minor groove in the central region of the DNA as compared to all other currently available NF-κB-DNA complex structures, which have a central A/T bp. Microsecond molecular dynamics (MD) simulations of free DNAs and p52 bound complexes reveal that free DNAs exhibit distinct preferred conformations, and p52:p52 homodimer induces the least amount of DNA conformational changes when bound to the more transcriptionally active natural G/C-centric PSel-κB, but adopts closed conformation when bound to the mutant A/T and swap DNAs due to their narrowed minor grooves. Our binding assays further demonstrate that the fast kinetics favored by entropy is correlated with higher transcriptional activity. Overall, our studies have revealed a novel conformation for κB DNA in complex with NF-κB and pinpoint the importance of binding kinetics, dictated by DNA conformational and dynamic states, in controlling transcriptional activation for NF-κB.

Inhibition and structure-activity relationship of dietary flavones against three Loop 1-type human gut microbial ß-glucuronidases.

Gut microbial ß-glucuronidases (GUSs) inhibition is a new approach for managing some diseases and medication therapy. However, the structural and functional complexity of GUSs have posed tremendous challenges to discover specific or broad-spectrum GUSs inhibitors using Escherichia coli GUS (EcoGUS) alone. This study first assessed the effects of twenty-one dietary flavones employing three Loop 1-type GUSs of different taxonomic origins, which were considered to be the main GUSs involved in deglucuronidation of small molecules, on p-nitrophenyl-ß-D-glucuronide hydrolysis and a structure-activity relationship is preliminarily proposed based on both in vitro assays and a docking study with representative compounds. EcoGUS and Staphylococcus pasteuri GUS showed largely similar inhibition propensities with potencies positively correlating with the total hydroxyl groups and those at ring B of flavones, while docking results revealed strong interactions developed via ring A and/or C. Streptococcus agalactiae GUS (SagaGUS) exhibited distinct inhibition propensities, displaying late-onset inhibition and steep dose-response profiles with most tested compounds. The α-helix in loop 1 region of SagaGUS which causes spatial hindrance but offers a hydrophobic surface for contacting with the carbonyl group on ring C of flavones is believed to be essential for the allosteric inhibition of SagaGUS. Taken together, the study with a series of flavones revealed varied preferences for GUSs belonging to the same Loop 1-type, highlighting the necessity of adopting multi-GUSs instead of EcoGUS alone for screening broad-spectrum GUSs inhibitors or tailoring the inhibition based on specific GUS structure.

Atypical IκB Bcl3 enhances the generation of the NF-κB p52 homodimer.

The NF-κB family of dimeric transcription factors regulate diverse biological functions. Their cellular expression profiles differ, which lead to different concentrations in different cell/tissue types. Although the activation mechanisms of different NF-κB dimers have been widely investigated, there is limited information on specific NF-κB dimers' formation. The NF-κB p52:p52 homodimer regulates an important subset of target genes in cancer cells; however, the molecular mechanism of the generation of this specific homodimer remains unclear. Our study has revealed that the atypical IκB protein, Bcl3, plays an essential role in enhancing the p52:p52 homodimer population which is a unique mechanism to p52 within the NF-κB family. p52 was shown to heterodimerize with four other NF-κB subunits (RelA, RelB, cRel, and p50); all heterodimers, except p52:p50, are significantly more stable than the p52:p52 homodimer. Bcl3 is able to compete with all other NF-κB subunits in cells for efficient p52:p52 homodimer formation which consequently leads to the upregulation of target genes that are involved in cell proliferation, migration, and inflammation, which explain why aberrant activation of Bcl3 and p52 leads to cancer.

Structural basis of SARS-CoV-2 and its variants binding to intermediate horseshoe bat ACE2.

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has caused a global pandemic. Intermediate horseshoe bats (Rhinolophus affinis) are hosts of RaTG13, the second most phylogenetically related viruses to SARS-CoV-2. We report the binding between intermediate horseshoe bat ACE2 (bACE2-Ra) and SARS-CoV-2 receptor-binding domain (RBD), supporting the pseudotyped SARS-CoV-2 viral infection. A 3.3 Å resolution crystal structure of the bACE2-Ra/SARS-CoV-2 RBD complex was determined. The interaction networks of Patch 1 showed differences in R34 and E35 of bACE2-Ra compared to hACE2 and big-eared horseshoe bat ACE2 (bACE2-Rm). The E35K substitution, existing in other species, significantly enhanced the binding affinity owing to its electrostatic attraction with E484 of SARS-CoV-2 RBD. Furthermore, bACE2-Ra showed extensive support for the SARS-CoV-2 variants. These results broaden our knowledge of the ACE2/RBD interaction mechanism and emphasize the importance of continued surveillance of intermediate horseshoe bats to prevent spillover risk.

Origin of the Functional Distinctiveness of NF-κB/p52.

The transcription regulators of the NF-κB family have emerged as a critical factor affecting the function of various adult tissues. The NF-κB family transcription factors are homo- and heterodimers made up of five monomers (p50, p52, RelA, cRel and RelB). The family is distinguished by sequence homology in their DNA binding and dimerization domains, which enables them to bind similar DNA response elements and participate in similar biological programs through transcriptional activation and repression of hundreds of genes. Even though the family members are closely related in terms of sequence and function, they all display distinct activities. In this review, we discuss the sequence characteristics, protein and DNA interactions, and pathogenic involvement of one member of family, NF-κB/p52, relative to that of other members. We pinpoint the small sequence variations within the conserved region that are mostly responsible for its distinct functional properties.

Umbilical cord miRNAs to predict neonatal early onset sepsis.

OBJECTIVE: To determine if miRNA (miR) expression in umbilical cord blood and umbilical cord tissue differs between neonates with early onset sepsis (EOS) versus neonates without true infection. METHODS: Retrospective case-control study design of human patients with EOS (n = 8), presumed sepsis (N = 12) and non-infected control patients (N = 21). Differential expression of >300 miRs was examined using the MIHS-3001ZE-miScript miRNA PCR Array Human miFinder 384HC. Expression levels of miRs were normalized using the global Ct mean of expressed miR and compared between groups. Data analysis was performed using GeneGlobe data analysis software. Ratios of over and under-expressed miRs were calculated and compared between groups using receiver operating characteristic (ROC) curves. RESULTS: Both umbilical cord plasma and umbilical cord tissue revealed several miRs with differential expression with little overlap between the two specimen types. The most overexpressed miR in plasma of EOS patients was miR-211-5p and the most overexpressed in EOS cord tissue was miR-223-5p. ROC curves comparing the ratios of over and under-expressed miRs for EOS patients and controls resulted in an area under the curve of 0.787 for cord plasma (miR-211-5p/miR-142-3p) and 0.988 for umbilical cord tissue (miR-223-5p/miR-22-3p), indicating good discrimination. CONCLUSIONS: miRs show differential expression in EOS versus non-infected controls and presumed sepsis. A ratio of over and under-expressed miRs can provide a potentially sensitive and specific diagnostic test for EOS.

Chronic villitis of unknown etiology: Investigations into viral pathogenesis.

INTRODUCTION: Chronic villitis of unknown etiology (VUE) is a chronic inflammatory lesion of third trimester placenta, which contributes to major adverse obstetric outcomes. However, the inciting factors and mechanisms by which VUE contributes to adverse outcomes are poorly understood. This limits our ability to develop preventions or interventions. Our goals were to determine whether viruses can be detected in placental tissues with VUE and to determine whether gene expression profiles support an antiviral response. METHODS: We extracted RNA and DNA from 20 placentas with high-grade chronic villitis and 20 control placentas without inflammation. Viruses were assessed using ViroCap viral nucleic acid enrichment coupled with metagenomic sequencing. RNA sequencing was used to evaluate the inflammatory gene expression profiles in each placenta. RESULTS: We detected at least 1 virus in 50% of the samples tested. We found that herpesviruses, were found more frequently in cases compared with controls (P = 0.01). Antiviral pathways, including defense response to virus, interferon gamma response, and IFN alpha/beta response, were upregulated in cases. We observed two clusters of gene expression profiles in the VUE cases, suggesting multiple inflammatory profiles are associated with VUE. DISCUSSION: These data support a viral etiology for some cases of VUE. Furthermore, gene expression profiles suggest the possibility of more than one cause or manifestation of VUE. Viral mechanisms should be explored as potential targets for prevention or intervention in VUE.

Genetic risk of extranodal natural killer T-cell lymphoma: a genome-wide association study in multiple populations.

BACKGROUND: Extranodal natural killer T-cell lymphoma (NKTCL; nasal type) is an aggressive malignancy with a particularly high prevalence in Asian and Latin American populations. Epstein-Barr virus infection has a role in the pathogenesis of NKTCL, and HLA-DPB1 variants are risk factors for the disease. We aimed to identify additional novel genetic variants affecting risk of NKTCL. METHODS: We did a genome-wide association study of NKTCL in multiple populations from east Asia. We recruited a discovery cohort of 700 cases with NKTCL and 7752 controls without NKTCL of Han Chinese ancestry from 19 centres in southern, central, and northern regions of China, and four independent replication samples including 717 cases and 12 650 controls. Three of these independent samples (451 cases and 5301 controls) were from eight centres in the same regions of southern, central, and northern China, and the fourth (266 cases and 7349 controls) was from 11 centres in Hong Kong, Taiwan, Singapore, and South Korea. All cases had primary NKTCL that was confirmed histopathologically, and matching with controls was based on geographical region and self-reported ancestry. Logistic regression analysis was done independently by geographical regions, followed by fixed-effect meta-analyses, to identify susceptibility loci. Bioinformatic approaches, including expression quantitative trait loci, binding motif and transcriptome analyses, and biological experiments were done to fine-map and explore the functional relevance of genome-wide association loci to the development of NKTCL. FINDINGS: Genetic data were gathered between Jan 1, 2008, and Jan 23, 2019. Meta-analysis of all samples (a total of 1417 cases and 20 402 controls) identified two novel loci significantly associated with NKTCL: IL18RAP on 2q12.1 (rs13015714; p=2·83 × 10-16; odds ratio 1·39 [95% CI 1·28-1·50]) and HLA-DRB1 on 6p21.3 (rs9271588; 9·35 × 10-26 1·53 [1·41-1·65]). Fine-mapping and experimental analyses showed that rs1420106 at the promoter of IL18RAP was highly correlated with rs13015714, and the rs1420106-A risk variant had an upregulatory effect on IL18RAP expression. Cell growth assays in two NKTCL cell lines (YT and SNK-6 cells) showed that knockdown of IL18RAP inhibited cell proliferation by cell cycle arrest in NKTCL cells. Haplotype association analysis showed that haplotype 47F-67I was associated with reduced risk of NKTCL, whereas 47Y-67L was associated with increased risk of NKTCL. These two positions are component parts of the peptide-binding pocket 7 (P7) of the HLA-DR heterodimer, suggesting that these alterations might account for the association at HLA-DRB1, independent of the previously reported HLA-DPB1 variants. INTERPRETATION: Our findings provide new insights into the development of NKTCL by showing the importance of inflammation and immune regulation through the IL18-IL18RAP axis and antigen presentation involving HLA-DRB1, which might help to identify potential therapeutic targets. Taken in combination with additional genetic and other risk factors, our results could potentially be used to stratify people at high risk of NKTCL for targeted prevention. FUNDING: Guangdong Innovative and Entrepreneurial Research Team Program, National Natural Science Foundation of China, National Program for Support of Top-Notch Young Professionals, Chang Jiang Scholars Program, Singapore Ministry of Health's National Medical Research Council, Tanoto Foundation, National Research Foundation Singapore, Chang Gung Memorial Hospital, Recruitment Program for Young Professionals of China, First Affiliated Hospital and Army Medical University, US National Institutes of Health, and US National Cancer Institute.

Genome reading by the NF-κB transcription factors.

The NF-κB family of dimeric transcription factors regulates transcription by selectively binding to DNA response elements present within promoters or enhancers of target genes. The DNA response elements, collectively known as κB sites or κB DNA, share the consensus 5'-GGGRNNNYCC-3' (where R, Y and N are purine, pyrimidine and any nucleotide base, respectively). In addition, several DNA sequences that deviate significantly from the consensus have been shown to accommodate binding by NF-κB dimers. X-ray crystal structures of NF-κB in complex with diverse κB DNA have helped elucidate the chemical principles that underlie target selection in vitro. However, NF-κB dimers encounter additional impediments to selective DNA binding in vivo. Work carried out during the past decades has identified some of the barriers to sequence selective DNA target binding within the context of chromatin and suggests possible mechanisms by which NF-κB might overcome these obstacles. In this review, we first highlight structural features of NF-κB:DNA complexes and how distinctive features of NF-κB proteins and DNA sequences contribute to specific complex formation. We then discuss how native NF-κB dimers identify DNA binding targets in the nucleus with support from additional factors and how post-translational modifications enable NF-κB to selectively bind κB sites in vivo.

Arabidopsis DXO1 links RNA turnover and chloroplast function independently of its enzymatic activity.

The DXO family of proteins participates in eukaryotic mRNA 5'-end quality control, removal of non-canonical NAD+ cap and maturation of fungal rRNA precursors. In this work, we characterize the Arabidopsis thaliana DXO homolog, DXO1. We demonstrate that the plant-specific modification within the active site negatively affects 5'-end capping surveillance properties of DXO1, but has only a minor impact on its strong deNADding activity. Unexpectedly, catalytic activity does not contribute to striking morphological and molecular aberrations observed upon DXO1 knockout in plants, which include growth and pigmentation deficiency, global transcriptomic changes and accumulation of RNA quality control siRNAs. Conversely, these phenotypes depend on the plant-specific N-terminal extension of DXO1. Pale-green coloration of DXO1-deficient plants and our RNA-seq data reveal that DXO1 affects chloroplast-localized processes. We propose that DXO1 mediates the connection between RNA turnover and retrograde chloroplast-to-nucleus signaling independently of its deNADding properties.

Nicotinamide Promotes Cell Survival and Differentiation as Kinase Inhibitor in Human Pluripotent Stem Cells.

Nicotinamide, the amide form of vitamin B3, is widely used in disease treatments and stem cell applications. However, nicotinamide's impact often cannot be attributed to its nutritional functions. In a vitamin screen, we find that nicotinamide promotes cell survival and differentiation in human pluripotent stem cells. Nicotinamide inhibits the phosphorylation of myosin light chain, suppresses actomyosin contraction, and leads to improved cell survival after individualization. Further analysis demonstrates that nicotinamide is an inhibitor of multiple kinases, including ROCK and casein kinase 1. We demonstrate that nicotinamide affects human embryonic stem cell pluripotency and differentiation as a selective kinase inhibitor. The findings in this report may help researchers design better strategies to develop nicotinamide-related stem cell applications and disease treatments.

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.

Bcl3 Phosphorylation by Akt, Erk2, and IKK Is Required for Its Transcriptional Activity.

Unlike prototypical IκB proteins, which are inhibitors of NF-κB RelA, cRel, and RelB dimers, the atypical IκB protein Bcl3 is primarily a transcriptional coregulator of p52 and p50 homodimers. Bcl3 exists as phospho-protein in many cancer cells. Unphosphorylated Bcl3 acts as a classical IκB-like inhibitor and removes p50 and p52 from bound DNA. Neither the phosphorylation site(s) nor the kinase(s) phosphorylating Bcl3 is known. Here we show that Akt, Erk2, and IKK1/2 phosphorylate Bcl3. Phosphorylation of Ser33 by Akt induces switching of K48 ubiquitination to K63 ubiquitination and thus promotes nuclear localization and stabilization of Bcl3. Phosphorylation by Erk2 and IKK1/2 of Ser114 and Ser446 converts Bcl3 into a transcriptional coregulator by facilitating its recruitment to DNA. Cells expressing the S114A/S446A mutant have cellular proliferation and migration defects. This work links Akt and MAPK pathways to NF-κB through Bcl3 and provides mechanistic insight into how Bcl3 functions as an oncoprotein through collaboration with IKK1/2, Akt, and Erk2.

The NF-κB subunit RelB controls p100 processing by competing with the kinases NIK and IKK1 for binding to p100.

The heterodimer formed by the nuclear factor κB (NF-κB) subunits p52 and RelB is the product of noncanonical signaling in which the key event is the proteolytic processing of p100 to generate p52. The kinases NF-κB-inducing kinase (NIK) and inhibitor of κB kinase 1 (IKK1; also known as IKKα) are activated during noncanonical signaling and play essential roles in p100 processing. In resting cells, RelB remains associated with unprocessed p100 as a transcriptionally inert p100:RelB complex, which is part of a larger assembly with other NF-κB factors known as the "kappaBsome." We investigated how these two different RelB-containing complexes with opposing effects on target gene transcription are formed. We found that RelB controls the extent of both p100 processing and kappaBsome formation during noncanonical signaling. Within an apparently "transitional" complex that contains RelB, NIK, IKK1, and p100, RelB and the NIK:IKK1 complex competed with each other for binding to a region of p100. A fraction of p100 in the transitional complex was refractory to processing, which resulted in the formation of the kappaBsome. However, another fraction of p100 protein underwent NIK:IKK1-mediated phosphorylation and processing while remaining bound to RelB, thus forming the p52:RelB heterodimer. Our results suggest that changes in the relative concentrations of RelB, NIK:IKK1, and p100 during noncanonical signaling modulate this transitional complex and are critical for maintaining the fine balance between the processing and protection of p100.