Recombinant Bovine E3 ubiquitin-protein ligase RNF144B (RNF144B)

What is RNF144B and what is its primary function in cellular processes?

RNF144B is an E3 ubiquitin-protein ligase that plays significant roles in immune regulation, particularly in innate immune responses. It functions as a negative regulator of interferon production and antiviral immunity. RNF144B is induced by lipopolysaccharide (LPS) stimulation through MyD88-dependent NF-κB activation . It contains an IBR (In Between RING) domain that is crucial for its inhibitory functions through protein-protein interactions. As an E3 ligase, RNF144B facilitates the transfer of ubiquitin molecules to target proteins, marking them for various cellular fates including degradation via the proteasome or autophagy pathways .

What are the key structural domains of RNF144B and how do they contribute to its function?

RNF144B contains several crucial domains including RING finger domains and an IBR (In Between RING) domain. The IBR domain is particularly important as it mediates interaction with the scaffold/dimerization domain (SDD) of TANK binding kinase 1 (TBK1) . This interaction is central to RNF144B's role in regulating innate immune responses. The RING domains confer E3 ligase activity, enabling RNF144B to catalyze ubiquitin transfer to substrate proteins. These structural features allow RNF144B to regulate protein stability and activity through various types of ubiquitin linkages, notably K27- and K33-linked polyubiquitination of targets like MDA5 .

How is RNF144B expression regulated in different cell types and under various stimuli?

RNF144B expression is dynamically regulated in response to various stimuli:

The induction of RNF144B via the MyD88-NF-κB pathway can be inhibited by the NF-κB inhibitor BAY-11-7085, confirming the dependency on this signaling axis. MyD88 knockdown significantly reduces LPS-induced RNF144B expression . Additionally, RNF144B can be induced by p73 upon DNA damage, suggesting roles beyond immune regulation.

How does RNF144B regulate TLR4-mediated signaling in response to LPS?

RNF144B negatively regulates LPS-induced interferon production through a direct interaction with TANK binding kinase 1 (TBK1). Mechanistically, RNF144B:

• Is induced following LPS stimulation via MyD88-dependent NF-κB activation

• Interacts with the scaffold/dimerization domain (SDD) of TBK1 through its IBR domain

• Inhibits TBK1 phosphorylation and K63-linked polyubiquitination

• Leads to TBK1 inactivation, which prevents downstream IRF3 phosphorylation

• Ultimately reduces IFN-β production

This regulatory function is specific to the IFN-β pathway, as RNF144B knockdown increases IFNB1 expression without affecting TNFA expression . The interaction between RNF144B and TBK1 appears to be independent of RNF144B's E3 ligase activity in the context of LPS stimulation, suggesting a scaffold or competitive binding function in this particular pathway.

What role does RNF144B play in antiviral innate immune responses?

RNF144B functions as a negative regulator of antiviral immunity, particularly in the context of RNA virus infection. The regulatory mechanisms include:

• Targeting the CARDs (Caspase Activation and Recruitment Domains) of MDA5 (Melanoma Differentiation-Associated protein 5)

• Promoting K27- and K33-linked ubiquitination of MDA5

• Facilitating p62-mediated selective autophagic degradation of MDA5

• Inhibiting antiviral interferon production

This regulatory role has been demonstrated in both in vitro and in vivo experiments. Rnf144b knockout mice show enhanced resistance to EMCV (encephalomyocarditis virus) infection, with significantly higher survival rates compared to wild-type mice . The absence of RNF144B results in higher basal levels of MDA5, enhanced induction of type I interferons, and reduced viral replication in tissues such as heart and brain following infection.

How does the E3 ligase activity of RNF144B contribute to its immunoregulatory functions?

The E3 ligase activity of RNF144B is critical for some of its immunoregulatory functions but appears dispensable for others:

In the context of antiviral immunity, RNF144B's E3 ligase activity is essential for promoting the non-canonical K27- and K33-linked ubiquitination of MDA5, which serves as a signal for p62-mediated selective autophagy . This represents a novel mechanism for fine-tuning antiviral responses through post-translational modification of a pattern recognition receptor.

What are the most effective methods for studying RNF144B protein-protein interactions?

For investigating RNF144B protein-protein interactions, researchers can employ several complementary approaches:

• Co-immunoprecipitation (Co-IP): This has been successfully used to demonstrate interactions between RNF144B and TBK1 , as well as interactions with MDA5 . Use epitope-tagged versions (HA, Flag, Myc) of RNF144B and potential binding partners in overexpression systems, followed by pull-down with specific antibodies.

• Domain mapping experiments: Construct truncation or deletion mutants of RNF144B (particularly focusing on the IBR domain) and potential interacting proteins to identify specific interaction domains. For example, experiments have shown that the IBR domain of RNF144B interacts with the SDD domain of TBK1 .

• Proximity ligation assays (PLA): This technique allows visualization of protein interactions in situ with high specificity and sensitivity, providing spatial information about where in the cell these interactions occur.

• Yeast two-hybrid screening: To identify novel interaction partners of RNF144B in an unbiased manner.

• Bimolecular fluorescence complementation (BiFC): To visualize and confirm direct protein-protein interactions in living cells.

When designing these experiments, it's crucial to include appropriate controls, such as domain deletion mutants and non-interacting protein partners, to validate the specificity of observed interactions.

What are the recommended approaches for analyzing RNF144B-mediated ubiquitination patterns?

To analyze RNF144B-mediated ubiquitination patterns, researchers should:

• Ubiquitination assays: Perform in vitro and in vivo ubiquitination assays using different ubiquitin lysine mutants (K27, K33, K48, K63, etc.) to determine the specific linkage types catalyzed by RNF144B. For example, studies have used K27O and K33O ubiquitin mutants (where only K27 or K33 is available for linkage) to demonstrate RNF144B's ability to promote these specific linkage types on MDA5 .

• Mass spectrometry analysis: Use proteomics approaches to identify ubiquitination sites and linkage types on target proteins. Tryptic digestion of ubiquitinated proteins leaves a characteristic diglycine remnant on modified lysines that can be detected by mass spectrometry.

• Linkage-specific antibodies: Utilize antibodies that recognize specific ubiquitin linkage types (K27, K33, K48, K63) to determine the predominant linkages catalyzed by RNF144B on different substrates.

• Ubiquitin chain restriction analysis: Use deubiquitinating enzymes with linkage specificity to cleave specific types of polyubiquitin chains, helping to determine chain composition.

• Site-directed mutagenesis: Mutate potential ubiquitination sites (lysine residues) on target proteins to identify the specific residues modified by RNF144B.

For analyzing the functional consequences of ubiquitination, combine these approaches with protein stability assays using cycloheximide chase experiments, as has been done to demonstrate RNF144B's role in promoting MDA5 degradation .

How can researchers effectively generate and validate RNF144B knockout models?

For generating and validating RNF144B knockout models, researchers should consider the following approach:

• CRISPR-Cas9 genome editing: Design guide RNAs targeting critical exons (such as exons 2 and 3) of the RNF144B gene, as demonstrated in previous studies . This approach can be used for both cell lines and for generating knockout mouse models.

• Validation strategies:

  • PCR genotyping of genomic DNA (for identifying deletions)

  • Western blot analysis to confirm absence of RNF144B protein expression

  • RT-qPCR to verify reduction in RNF144B mRNA levels

  • Functional validation by examining known RNF144B-regulated pathways (e.g., enhanced IFN-β production in response to stimuli)

• Control for off-target effects:

  • Use multiple guide RNA designs

  • Perform rescue experiments by re-expressing RNF144B in knockout cells

  • Compare multiple independent knockout clones

• Phenotypic characterization:

  • Analyze immune cell populations (lymphoid and myeloid cells) in knockout mice

  • Assess baseline cytokine expression

  • Challenge with relevant stimuli (LPS, viral infection) to evaluate physiological impact

  • Monitor survival rates following pathogen challenge

When establishing knockout mouse models, researchers should monitor for developmental abnormalities, though existing studies suggest Rnf144b knockout mice exhibit normal growth and development without discernible physical or behavioral abnormalities .

How can researchers distinguish between E3 ligase-dependent and -independent functions of RNF144B?

To distinguish between E3 ligase-dependent and -independent functions of RNF144B, researchers should implement a multi-faceted approach:

• Generate catalytically inactive mutants: Introduce point mutations in the RING domains (particularly catalytic cysteines or histidines) that abolish E3 ligase activity while preserving protein structure. Compare the effects of wild-type RNF144B versus these catalytically dead mutants in functional assays.

• Domain deletion constructs: Create constructs lacking specific domains (RING domains versus IBR domain) to separate ubiquitination functions from protein-protein interaction capabilities.

• Comparative pathway analysis:

  • For LPS-induced signaling, evidence indicates RNF144B functions independently of its E3 ligase activity, acting through protein-protein interactions with TBK1

  • For antiviral signaling, RNF144B's E3 ligase activity is required for MDA5 regulation through K27/K33 ubiquitination

• Rescue experiments in knockout systems: Reintroduce either wild-type or catalytically inactive RNF144B into knockout cells and assess restoration of function in various pathways.

• Proteomic approaches: Compare ubiquitinome profiles in systems with wild-type versus catalytically inactive RNF144B to identify E3 ligase-dependent targets.

This multi-level approach will help delineate which cellular functions require RNF144B's enzymatic activity versus those dependent on its scaffold or adaptor properties.

What are the physiological implications of the interaction between RNF144B and MDA5 in viral infections?

The RNF144B-MDA5 interaction has significant physiological implications in viral infections:

• Regulation of antiviral immunity: RNF144B negatively regulates antiviral responses by promoting MDA5 degradation through K27/K33-linked ubiquitination and subsequent autophagy . This represents a mechanism to prevent excessive or prolonged inflammation.

• Virus-specific effects: RNF144B deficiency enhances resistance specifically to RNA viruses like EMCV and VSV, but not to DNA viruses like HSV . This specificity aligns with MDA5's role as a sensor for double-stranded RNA generated during RNA virus replication.

• Survival advantage: Rnf144b knockout mice exhibit significantly higher survival rates following EMCV infection compared to wild-type mice , demonstrating the physiological relevance of this regulatory pathway.

• Tissue-specific protection: In Rnf144b knockout mice, EMCV replication is significantly reduced in critical tissues such as heart and brain, accompanied by enhanced production of type I interferons and pro-inflammatory cytokines .

• Impact on IFN signaling: RNF144B not only regulates virus-induced IFN production but also moderately suppresses tonic IFN signaling, as evidenced by slightly elevated basal levels of Ifnb1 and interferon-stimulated genes (ISGs) in Rnf144b knockout cells .

These findings suggest that targeting the RNF144B-MDA5 interaction might represent a potential therapeutic strategy for enhancing antiviral immunity against RNA viruses.

How do the dual roles of RNF144B in bacterial (LPS) and viral (MDA5) signaling pathways interact?

RNF144B exerts negative regulatory effects on both bacterial (LPS-induced) and viral (MDA5-mediated) signaling pathways, but through distinct mechanisms:

The convergence and divergence of these pathways raises several important considerations:

• Temporal regulation: RNF144B is induced by LPS stimulation, suggesting its role as a negative feedback regulator in bacterial sensing. It's unclear whether similar induction occurs during viral infection.

• Pathway cross-talk: Both pathways converge on TBK1 activation, but RNF144B appears to regulate TBK1 differently in each context. In viral infection, RNF144B's effect on TBK1 phosphorylation is dependent on MDA5 , suggesting an indirect mechanism through MDA5 regulation.

• Evolutionary significance: The dual regulatory roles may represent an evolved mechanism to prevent excessive inflammation while maintaining appropriate pathogen-specific responses.

• Experimental design implications: When studying RNF144B in infection models, researchers must carefully consider the stimulus-specific mechanisms and potentially different outcomes depending on the pathogen type.

• Therapeutic targeting considerations: Attempts to modulate RNF144B activity for therapeutic purposes would need to account for potentially divergent effects on bacterial versus viral defense mechanisms.

What are the potential research applications of RNF144B in understanding inflammatory disorders?

RNF144B's roles in regulating innate immune responses suggest several potential research applications for understanding inflammatory disorders:

• Autoimmune diseases: Given RNF144B's negative regulation of type I interferon production, its dysfunction could contribute to interferon-driven autoimmune diseases like systemic lupus erythematosus (SLE) or type I interferonopathies. Researchers could investigate RNF144B expression and polymorphisms in patient cohorts.

• Viral myocarditis models: Since RNF144B regulates EMCV replication and Rnf144b knockout mice show reduced viral load in heart tissue , this model could be valuable for studying mechanisms of viral myocarditis and developing potential interventions.

• Sepsis research: RNF144B's regulation of LPS-induced signaling suggests potential involvement in sepsis pathophysiology. Researchers could investigate whether modulating RNF144B activity affects outcomes in experimental sepsis models.

• Chronic viral infections: The role of RNF144B in regulating antiviral responses warrants investigation in contexts of chronic viral infections, where persistent interferon signaling may be either beneficial or detrimental.

• Neuroinflammation: Given the observed protection of brain tissue in Rnf144b knockout mice during EMCV infection , researchers could explore RNF144B's role in neuroinflammatory conditions, including viral encephalitis and potentially other neuroinflammatory disorders.

• Biomarker development: Changes in RNF144B expression or activity could potentially serve as biomarkers for inflammatory conditions or predict response to therapies targeting innate immune pathways.

What are the common challenges in producing recombinant RNF144B protein and how can they be addressed?

Producing functional recombinant RNF144B presents several challenges:

• Maintaining E3 ligase activity: E3 ubiquitin ligases often lose activity during purification due to oxidation of critical cysteine residues in RING domains.

  • Solution: Include reducing agents (DTT, β-mercaptoethanol) throughout purification and storage.

  • Solution: Express and purify from anaerobic environments when possible.

• Solubility issues: Membrane-associated proteins like RNF144B (which can translocate to mitochondria) may have solubility problems.

  • Solution: Use detergents compatible with downstream applications (e.g., CHAPS, NP-40).

  • Solution: Express truncated versions containing only soluble domains for specific interaction studies.

• Protein stability: E3 ligases may exhibit auto-ubiquitination, leading to heterogeneity and degradation.

  • Solution: Use catalytically inactive mutants for structural studies.

  • Solution: Include deubiquitinating enzyme inhibitors during purification.

• Expression systems: Bacterial systems may not provide appropriate post-translational modifications.

  • Solution: Consider insect cell or mammalian expression systems that better support proper folding and modifications.

  • Solution: Co-express with chaperones to improve folding.

• Maintaining native conformation: Ensuring that recombinant RNF144B retains its ability to interact with partners like TBK1 or MDA5.

  • Solution: Validate functionality through in vitro ubiquitination assays.

  • Solution: Confirm protein-protein interactions with known partners using pull-down assays.

How can researchers accurately measure and interpret changes in RNF144B-mediated regulation following various stimuli?

To accurately measure and interpret changes in RNF144B-mediated regulation:

• Kinetic analysis: Perform time-course experiments to capture the dynamic nature of RNF144B induction and activity.

  • LPS induces RNF144B with expression peaking around 6 hours

  • Follow downstream effects on target proteins (TBK1, MDA5) across multiple timepoints

• Quantification methods:

  • RT-qPCR for mRNA expression changes

  • Western blotting with densitometry for protein level changes

  • Phospho-specific antibodies for measuring TBK1 and IRF3 activation

  • ELISA for secreted cytokines like IFN-β

• Pathway dissection:

  • Use specific inhibitors (BAY-11-7085 for NF-κB)

  • Employ siRNA/shRNA knockdown of pathway components (MyD88, TBK1)

  • Compare responses across multiple stimuli (LPS, poly(I:C), viral infection)

• Subcellular localization: Track RNF144B localization changes using:

  • Immunofluorescence microscopy

  • Subcellular fractionation and Western blotting

  • Live-cell imaging with fluorescently tagged RNF144B

• Genetic models: Compare results across:

  • Wild-type cells/animals

  • RNF144B knockdown/knockout models

  • Rescue experiments with wild-type or mutant RNF144B

• Controls for specificity:

  • Measure effects on both regulated pathways (IFNB1) and non-regulated pathways (TNFA)

  • Include catalytically inactive RNF144B mutants to distinguish enzymatic from non-enzymatic functions

What are the best approaches for studying the role of RNF144B in different tissue and cell types during pathogenic challenges?

To comprehensively study RNF144B's role across different tissues and cell types during pathogenic challenges, researchers should:

• Tissue-specific expression analysis:

  • RT-qPCR and Western blot analysis of RNF144B expression across tissues

  • Immunohistochemistry to visualize expression patterns within complex tissues

  • Single-cell RNA sequencing to identify cell populations with high or regulated RNF144B expression

• Conditional knockout strategies:

  • Generate tissue-specific or cell-type-specific Rnf144b knockout mice using Cre-lox technology

  • Compare phenotypes to global knockouts to identify tissue-specific contributions

• Ex vivo primary cell systems:

  • Isolate and culture primary cells from different tissues (e.g., BMDMs, BMDCs, MEFs, primary hepatocytes)

  • Challenge with relevant pathogens or PAMPs to assess RNF144B function

• Infection models for different pathogens:

  • RNA viruses (EMCV, VSV) that are regulated by MDA5-dependent pathways

  • DNA viruses (HSV) that are less affected by RNF144B

  • Bacterial challenges (using LPS or live bacteria) to assess TLR4-mediated responses

• Pathology assessments:

  • Histological analysis of infected tissues

  • Viral load quantification in different organs

  • Cytokine profiling in tissue homogenates and serum

  • Survival and morbidity monitoring

• Adoptive transfer experiments:

  • Transfer Rnf144b-deficient immune cells to wild-type recipients (or vice versa)

  • Determine contribution of specific cell types to observed phenotypes

This comprehensive approach will help delineate the tissue-specific and cell-type-specific roles of RNF144B in the context of different pathogenic challenges.

What are promising areas for future research on RNF144B beyond its currently known functions?

Several promising areas for future RNF144B research include:

• Expanded substrate identification: Beyond TBK1 and MDA5, RNF144B likely has additional substrates that remain unidentified. Unbiased proteomic approaches such as BioID, proximity labeling, or quantitative ubiquitinome analysis could reveal novel targets across different cellular contexts.

• Roles in additional pattern recognition receptor pathways: While RNF144B's roles in TLR4 and MDA5 pathways are established , its potential involvement in other innate immune receptors (NOD-like receptors, C-type lectin receptors) remains unexplored.

• Intersection with metabolism: Many immune regulatory proteins have secondary roles in cellular metabolism. Given RNF144B's mitochondrial localization upon certain stimuli, investigating its potential role in metabolic regulation could be fruitful.

• Cancer biology: RNF144B has known roles in regulating p73 and Bax , suggesting functions in cell death and cancer. Exploring RNF144B expression and function across different cancer types could reveal novel therapeutic opportunities.

• Regulation of RNF144B itself: While LPS-induced regulation via MyD88-NF-κB is known , other regulatory mechanisms controlling RNF144B expression, localization, and activity remain largely unexplored. Post-translational modifications of RNF144B itself could provide another layer of regulation.

• Structural biology: Obtaining crystal structures of RNF144B, particularly in complex with its binding partners or substrates, would provide valuable insights into its mechanism of action and potentially facilitate drug development efforts.

How might targeting RNF144B be exploited therapeutically for viral infections or inflammatory diseases?

Therapeutic targeting of RNF144B presents several potential strategies:

• Enhancing antiviral immunity: Inhibition of RNF144B could enhance antiviral responses, particularly against RNA viruses like EMCV and VSV. This approach is supported by the improved survival of Rnf144b knockout mice during EMCV infection . Potential approaches include:

  • Small molecule inhibitors of RNF144B's E3 ligase activity

  • Peptide-based disruptors of the RNF144B-MDA5 interaction

  • Antisense oligonucleotides or siRNAs to reduce RNF144B expression

• Modulating inflammatory responses: In contexts where excessive inflammation is detrimental, enhancing RNF144B activity might dampen inflammatory responses by reducing interferon production. This could be relevant for:

  • Autoimmune disorders with type I interferon signatures

  • Chronic inflammatory conditions

  • Cytokine storm syndromes

• Cell type-specific targeting: Developing delivery systems to target RNF144B modulators to specific cell types (e.g., macrophages, dendritic cells) could provide precision in therapeutic applications while minimizing off-target effects.

• Combination approaches: Combining RNF144B inhibition with existing antivirals might enhance efficacy, particularly for RNA virus infections where RNF144B plays a significant regulatory role.

• Diagnostic applications: Monitoring RNF144B expression levels or activity could serve as a biomarker for predicting immune responses or disease progression in certain contexts.

The dual roles of RNF144B in different pathways necessitate careful consideration of context-specific effects when developing therapeutic strategies.

What are the most important unresolved questions regarding RNF144B's role in immune regulation?

Several critical unresolved questions about RNF144B in immune regulation warrant investigation:

• Species-specific differences: Current research has focused primarily on mouse and human systems. Are there significant differences in RNF144B function across species that might impact translational research?

• Regulation of RNF144B expression in different contexts: While LPS-induced expression is well-characterized , how is RNF144B regulated during viral infections or in different disease states? Are there epigenetic mechanisms controlling its expression?

• Integration of bacterial and viral sensing pathways: How does RNF144B coordinate responses across different pathogen types? Does prior exposure to one type of pathogen, with consequent RNF144B induction, impact responses to subsequent challenges with different pathogens?

• Crosstalk with adaptive immunity: Does RNF144B's regulation of innate immune responses impact subsequent adaptive immune responses to pathogens or vaccines?

• Physiological significance of differential mechanisms: Why does RNF144B use distinct mechanisms (E3 ligase-dependent versus independent) to regulate different innate immune pathways? Does this reflect evolutionary adaptation to different types of threats?

• Relationship with inflammasome activation: Given RNF144B's regulation of innate immune signaling, does it also impact inflammasome activation and IL-1β/IL-18 production?

• Genetic variations in human populations: Do polymorphisms in the RNF144B gene correlate with susceptibility to infectious diseases or inflammatory disorders in humans?

Addressing these questions will provide a more comprehensive understanding of RNF144B's roles in immune regulation and potentially reveal new therapeutic opportunities.