Recombinant Mouse E3 ubiquitin-protein ligase RNF144B (Rnf144b)

What is RNF144B and how does it function as an E3 ubiquitin ligase?

RNF144B (also known as p53-inducible RING-finger protein or p53RFP) belongs to the RBR (RING-in-between-RING) family of E3 ubiquitin ligases. It functions by facilitating the transfer of ubiquitin molecules to target proteins, marking them for proteasomal degradation or altering their function through non-degradative ubiquitination . RNF144B contains a characteristic N-terminal RING1-IBR-RING2 domain responsible for its catalytic activity and a C-terminal transmembrane domain that influences its subcellular localization . Unlike traditional RING-type E3 ligases, RBR family members like RNF144B function as RING/HECT hybrids, using the RING1 domain to bind E2 conjugating enzymes but forming a thioester intermediate through a conserved cysteine residue in the RING2 domain before transferring ubiquitin to substrate proteins .

How does mouse RNF144B differ from human RNF144B in expression and regulation?

One of the most striking characteristics of RNF144B is its species-specific regulation. While human RNF144B is strongly induced by lipopolysaccharide (LPS) in primary human macrophages and THP-1 cells, mouse Rnf144b is not LPS-inducible in multiple mouse cell types, including primary macrophages from C57BL/6 and BALB/c mice and RAW264.7 macrophage cell lines . This species-specific difference extends to in vivo models, where Rnf144b is not upregulated by Escherichia coli infection in C57BL/6 mice . Cap analysis of gene expression (CAGE) data confirms that while human and mouse RNF144B genes have conserved transcription start sites, the RNF144B promoter directs transcription in human but not mouse macrophages . This differential regulation appears related to subtle differences in transcription factor binding sites between the otherwise highly conserved TATA-containing promoters of the human and mouse genes .

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

RNF144B contains several critical domains that determine its function:

• RING1-IBR-RING2 (RBR) domain: Located at the N-terminus, this domain is responsible for the E3 ubiquitin ligase activity. The RING1 domain interacts with E2 conjugating enzymes, while the RING2 domain contains the catalytic cysteine residue that forms a thioester intermediate with ubiquitin .

• Transmembrane (TM) domain: Found at the C-terminus, this domain influences subcellular localization and may regulate RNF144B activity and substrate selectivity. The TM domain appears to be important for full E3 ligase activity in cellular contexts .

• In Between RING (IBR) domain: This region connects the two RING domains and has been shown to be important for protein-protein interactions. For example, RNF144B interacts with the scaffold/dimerization domain (SDD) of TBK1 through its IBR domain .

These domains work together to determine RNF144B's catalytic activity, substrate specificity, and subcellular localization, which collectively influence its diverse cellular functions.

What methods can effectively detect and measure RNF144B expression in different experimental systems?

For effective detection and measurement of RNF144B expression, researchers can employ:

• Quantitative Real-Time PCR (qRT-PCR): This technique allows precise quantification of RNF144B mRNA levels. Studies have used qRT-PCR with GAPDH as an internal control to detect and quantify RNF144B expression changes in response to various stimuli . Primer design is critical; for example, researchers have successfully used specific primer pairs that target conserved regions of RNF144B transcript.

• Western Blotting: For protein-level detection, western blotting using specific antibodies against RNF144B provides information about protein expression and potential post-translational modifications. This approach has been used to assess RNF144B induction in response to LPS in human macrophages .

• Cap Analysis of Gene Expression (CAGE): This technique has been successfully used to identify transcription start sites and promoter activity of RNF144B, revealing species-specific differences in promoter usage between humans and mice .

• Immunofluorescence: For studying subcellular localization, immunofluorescence staining with RNF144B-specific antibodies helps visualize protein distribution. This has been particularly useful for identifying RNF144B's membrane association and colocalization with target proteins .

How can researchers generate and validate RNF144B knockout models for functional studies?

To generate and validate RNF144B knockout models:

• CRISPR-Cas9 genome editing: This is the preferred method for generating RNF144B knockout cell lines and animal models. Studies have successfully used CRISPR-Cas9 to create Rnf144b-deficient mice and cell lines . When designing guide RNAs, targeting early exons (particularly those encoding the catalytic RING domains) is most effective for complete functional disruption.

• siRNA/shRNA-mediated knockdown: For transient suppression, specific siRNAs targeting RNF144B have been employed. Studies have demonstrated successful RNF144B knockdown in THP-1 cells using this approach . When designing siRNAs, researchers should target unique regions of RNF144B to avoid off-target effects on the homologous RNF144A.

• Validation techniques:

  • Western blotting: Confirm protein ablation

  • qRT-PCR: Verify transcript reduction

  • Functional assays: Assess phenotypic changes expected from RNF144B deficiency, such as enhanced IFN-β production in response to viral infection or altered cell cycle progression

• Controls: Include wild-type cells/animals and, when possible, rescue experiments with re-introduced RNF144B to confirm phenotype specificity.

For mouse models, researchers have successfully used Rnf144b knockout mice to study antiviral responses against EMCV infection, demonstrating increased survival rates compared to wild-type mice .

What are the optimal methods for identifying and validating RNF144B substrates?

Identifying and validating RNF144B substrates requires multiple complementary approaches:

• Mass spectrometry-based proteomics:

  • Stable Isotope Labeling with Amino acids in Cell culture (SILAC) coupled with mass spectrometry has been successfully employed to identify proteins that accumulate upon RNF144B depletion or are reduced upon RNF144B overexpression .

  • Immunoprecipitation followed by mass spectrometry can identify proteins that physically interact with RNF144B.

• Ubiquitination assays:

  • In vitro ubiquitination assays using purified components (E1, E2, RNF144B, and potential substrates) to demonstrate direct ubiquitination.

  • Cell-based ubiquitination assays where cells are co-transfected with tagged versions of RNF144B, ubiquitin, and candidate substrates, followed by immunoprecipitation and western blotting to detect ubiquitinated species.

• Validation experiments:

  • Co-immunoprecipitation to confirm physical interaction between RNF144B and candidate substrates.

  • Domain mapping to identify specific interaction regions, as demonstrated for the interaction between RNF144B's IBR domain and TBK1's scaffold/dimerization domain .

  • Subcellular colocalization studies using fluorescently tagged proteins or immunofluorescence.

  • Half-life analysis of candidate substrates in the presence or absence of RNF144B.

• Functional confirmation:

  • Rescue experiments where phenotypes caused by RNF144B deficiency are assessed in the presence of wild-type or mutant versions of the putative substrate.

  • Ubiquitination site mapping using mass spectrometry to identify the specific lysine residues modified by RNF144B.

These approaches have successfully identified several RNF144B substrates, including MDA5 in antiviral immunity and proteins involved in cell cycle regulation and genomic stability .

How does RNF144B regulate inflammasome responses in human macrophages?

RNF144B plays a critical role in inflammasome priming in human macrophages through selective regulation of IL-1β expression:

• Inflammasome priming mechanism: RNF144B specifically promotes lipopolysaccharide (LPS)-inducible IL-1β mRNA expression in human macrophages . This represents a species-specific regulatory mechanism, as mouse Rnf144b does not exhibit this function.

• Selectivity in cytokine regulation: Importantly, RNF144B does not regulate the expression of several other LPS-inducible cytokines (such as IL-10 and IFN-γ) or affect the expression of inflammasome components or substrates (including procaspase-1 and pro-IL-18) . This selective regulation suggests a specialized role in inflammasome priming.

• Experimental validation: Gene silencing experiments have confirmed that RNF144B is necessary for effective inflammasome priming in primary human macrophages . When RNF144B is depleted using siRNA, IL-1β induction by LPS is significantly reduced, demonstrating its critical role in this process.

• Clinical implications: The human-specific nature of this regulatory mechanism suggests potentially important differences in inflammasome regulation between humans and mice, which could impact the translation of findings from mouse models to human inflammatory diseases.

This regulatory function appears to be distinct from RNF144B's role in inhibiting TBK1 activation (discussed in 3.2), suggesting context-specific functions of this E3 ligase in different aspects of immune regulation.

What role does RNF144B play in LPS-induced TBK1 signaling pathways?

RNF144B exhibits complex, sometimes seemingly contradictory roles in TBK1 signaling:

• Negative regulation of TBK1 activation: RNF144B has been shown to negatively regulate LPS-induced IFN production by interacting directly with TBK1 . Specifically, RNF144B:

  • Binds to the scaffold/dimerization domain (SDD) of TBK1 through its IBR domain

  • Inhibits TBK1 phosphorylation and K63-linked polyubiquitination

  • Leads to TBK1 inactivation, IRF3 dephosphorylation, and reduced IFN-β production

• Experimental evidence:

  • RNF144B knockdown with siRNA increases IRF3 activation and IFN-β production in response to LPS stimulation

  • Overexpression of RNF144B decreases TBK1 phosphorylation and IRF3 activation

  • Interestingly, this inhibitory function appears to be independent of RNF144B's E3 ligase activity

• Induction mechanism: RNF144B itself is induced by LPS through a MyD88-dependent NF-κB activation pathway . This suggests a negative feedback loop where LPS stimulation induces RNF144B, which then limits TBK1-dependent responses.

• Temporal dynamics: RNF144B responds dynamically to LPS stimulation, with expression patterns that suggest it helps limit the duration and magnitude of inflammatory responses .

This regulatory function highlights RNF144B's importance in preventing excessive inflammation and maintaining immune homeostasis, particularly in human cells where it is strongly LPS-inducible.

How does RNF144B influence antiviral immune responses?

RNF144B functions as a negative regulator of antiviral immunity, particularly against RNA viruses, through several mechanisms:

• Targeting of MDA5: RNF144B specifically targets melanoma differentiation-associated protein 5 (MDA5), a critical RNA sensor that detects viral RNA and initiates antiviral responses . RNF144B:

  • Binds to the CARD domains of MDA5

  • Promotes K27- and K33-linked ubiquitination of MDA5 (non-degradative ubiquitination)

  • Facilitates p62-mediated selective autophagic degradation of MDA5

• Impact on antiviral signaling:

  • Rnf144b knockout cells show enhanced responses to high molecular weight poly(I:C) (a MDA5 ligand)

  • Rnf144b deficiency leads to increased expression of type I interferons and proinflammatory cytokines upon viral infection

  • RNA virus replication (including EMCV and VSV) is suppressed in Rnf144b knockout cells

• In vivo significance:

  • Rnf144b knockout mice exhibit significantly higher survival rates than wild-type mice when infected with EMCV

  • These mice show elevated serum levels of IFN-β following viral infection

  • The protective effect appears specific to RNA viruses, as HSV (a DNA virus) replication is not affected by Rnf144b status

• Experimental methodology:

  • EMCV infection model: Mice were injected intraperitoneally with 6×10^7 PFU/mouse of EMCV and monitored for survival

  • IFN-β levels were measured by ELISA in serum collected 12 hours post-infection

  • Viral RNA loads were assessed in heart and brain tissues by qRT-PCR

This function of RNF144B in regulating MDA5-mediated antiviral responses provides important insights into the fine-tuning of innate immunity and potential therapeutic opportunities for enhancing antiviral responses.

What evidence supports RNF144B's role as a tumor suppressor?

Multiple lines of evidence establish RNF144B as a tumor suppressor:

• Growth inhibitory properties:

  • RNF144B deficiency increases cellular proliferation and transformation in both human and mouse oncogene-expressing cells

  • Overexpression of RNF144B in cancer cell lines suppresses growth and colony formation

• p53 connection:

  • RNF144B is a TP53-activated gene, linking it to the well-established tumor suppressor network

  • It is induced in a p53-dependent manner during DNA damage response

  • The gene appears to be a direct transcriptional target of p53, containing p53 response elements

• Clinical associations:

  • RNF144B deficiency correlates with worse prognosis in human tumors

  • Loss of RNF144B is associated with increased aneuploidy in cancers

  • Genomic analyses show deletion or downregulation of RNF144B in various cancer types

• Functional mechanisms:

  • RNF144B regulates protein degradation associated with cell cycle progression, DNA damage response, and genomic stability

  • Its loss leads to chromosomal instability and mitotic defects

  • Cancer cells with RNF144B deficiency show resistance to cell cycle inhibitors that induce chromosomal instability

• Experimental validation:

  • In lung adenocarcinoma models, RNF144B deficiency enhances tumor formation and progression

  • Proteomic and transcriptomic analyses reveal RNF144B's role in targeting proteins involved in maintaining genomic stability

These findings collectively establish RNF144B as an important tumor suppressor that functions primarily by maintaining genomic stability and proper cell cycle progression.

How does RNF144B function in maintaining genomic stability?

RNF144B plays a critical role in maintaining genomic stability through several mechanisms:

• Regulation of cell cycle proteins:

  • Proteomic analyses have identified RNF144B targets involved in cell cycle progression

  • RNF144B mediates the degradation of specific proteins that, when dysregulated, can lead to aberrant mitosis

• Prevention of chromosomal instability:

  • RNF144B deficiency induces chromosomal instability and mitotic defects

  • Cells lacking RNF144B show increased frequency of chromosome mis-segregation, micronuclei formation, and aneuploidy

  • These defects appear directly linked to RNF144B's E3 ligase activity targeting specific substrates involved in chromosome segregation

• DNA damage response regulation:

  • RNF144B is induced during DNA damage in a p53-dependent manner

  • It participates in pathways that ensure proper DNA repair and prevent replication of damaged DNA

  • Cells lacking RNF144B show altered responses to DNA damaging agents

• Experimental observations:

  • RNF144B-deficient cells exhibit increased aneuploidy

  • These cells show resistance to cell cycle inhibitors that typically induce chromosomal instability

  • Clinical data correlate RNF144B deficiency with higher levels of aneuploidy in human tumors

• Methodological approaches:

  • Chromosome mis-segregation can be assessed in RNF144B-deficient cells using fluorescence microscopy with DNA staining

  • Aneuploidy can be measured by karyotype analysis or flow cytometry

  • Proteomics approaches have identified specific substrates and pathways affected by RNF144B loss

These functions highlight RNF144B's importance in cancer prevention through maintenance of genomic integrity, a critical barrier against malignant transformation.

How is RNF144B being investigated as a potential biomarker in cancer research?

RNF144B is emerging as a potential biomarker in cancer research through various approaches:

• Expression analysis in clinical samples:

  • Studies have shown that RNF144B is significantly overexpressed in 50% of primary tumors from patients with high-grade serous ovarian cancer (HGSOC) compared to normal ovary tissue

  • Interestingly, RNF144B expression is reduced in tumors after chemotherapy, suggesting a potential role in treatment response

• Prognostic value:

  • RNF144B deficiency correlates with worse prognosis in human tumors

  • The connection between RNF144B levels and patient outcomes suggests its potential utility as a prognostic biomarker

• Genomic profiling approaches:

  • Amplification and overexpression of RNF144B occurs in 16% of HGSOC according to TCGA data

  • Deletion and downregulation occurs in 38% of HGSOCs for the related gene PPP2R2A, suggesting these may be used as complementary biomarkers

• Integration with other biomarkers:

  • Novel scoring systems that integrate information from different genetic alterations have been developed to identify cancer-driver genes, including RNF144B

  • These approaches combine copy number alterations, expression data, and functional impacts

• Methodological considerations for biomarker development:

  • Immunohistochemistry of tumor samples can assess RNF144B protein levels

  • qRT-PCR provides a quantitative measure of RNF144B mRNA expression

  • Next-generation sequencing approaches can identify mutations or copy number alterations affecting RNF144B

• Potential clinical applications:

  • Treatment stratification based on RNF144B status

  • Monitoring of RNF144B levels during therapy as a response indicator

  • Identification of patients who might benefit from therapies targeting genomic instability

These investigations highlight RNF144B's potential as a clinically relevant biomarker, particularly in cancers where genomic instability plays a key role in pathogenesis.

How can researchers reconcile seemingly contradictory findings about RNF144B function in different contexts?

Reconciling contradictory findings about RNF144B requires careful consideration of several factors:

• Context-dependent activity:

  • RNF144B shows distinct functions in different cellular contexts. For example, it promotes IL-1β expression in inflammasome priming while inhibiting IFN-β production through TBK1 inhibition .

  • Researchers should explicitly define cellular context (cell type, stimulation conditions, species) when comparing findings.

• Species-specific differences:

  • The stark difference in LPS-inducibility between human and mouse RNF144B highlights the importance of species considerations .

  • When comparing studies, researchers should always clarify which species is being investigated and avoid direct extrapolation between species.

• Methodology standardization:

  • Different experimental approaches (overexpression vs. knockdown, acute vs. chronic manipulation) can yield seemingly contradictory results.

  • Standardized protocols for RNF144B manipulation, including appropriate controls and validation methods, should be established.

• Substrate specificity:

  • RNF144B targets different substrates in different pathways. For example, it regulates MDA5 in antiviral immunity and potentially different targets in tumor suppression .

  • Comprehensive substrate identification using proteomics approaches can help map the full range of RNF144B functions.

• Integration of findings:

  • A unified model of RNF144B function should account for:

    • Its regulation by multiple stimuli (LPS, viral infection, DNA damage)

    • Its different subcellular localizations

    • Its diverse target repertoire

    • Its differential effects on various signaling pathways

• Experimental approach:

  • Use complementary methods (genetic knockouts, siRNA, domain mutants)

  • Employ both gain-of-function and loss-of-function approaches

  • Validate findings across multiple cell types when possible

  • Consider temporal dynamics of RNF144B activity

By systematically addressing these factors, researchers can develop a more comprehensive understanding of RNF144B's multifaceted roles in cellular physiology and disease.

What are the emerging techniques for studying RNF144B-dependent ubiquitination in physiological contexts?

Emerging techniques for studying RNF144B-dependent ubiquitination in physiological contexts include:

• Proximity-based labeling approaches:

  • BioID or TurboID: Fusing a biotin ligase to RNF144B allows biotinylation of proteins in close proximity, helping identify potential interactors and substrates in living cells

  • APEX2: Provides spatial and temporal resolution of RNF144B interactions in different subcellular compartments

• Advanced ubiquitination profiling:

  • Ubiquitin remnant profiling: Utilizing antibodies against the di-glycine remnant left on ubiquitinated lysines after trypsin digestion, coupled with mass spectrometry

  • UbiSite and similar technologies: Enable site-specific identification of ubiquitination with greater sensitivity

  • Ubiquitin chain-specific antibodies: Help distinguish between different ubiquitin linkage types (K27, K33, K48, K63) that RNF144B might catalyze

• Live-cell imaging of ubiquitination:

  • FRET/FLIM-based ubiquitin sensors: Allow real-time monitoring of ubiquitination events

  • Split fluorescent protein-based ubiquitin reconstitution: Enables visualization of specific ubiquitination events

• Single-cell analysis:

  • Single-cell proteomics: Reveals cell-to-cell variability in RNF144B activity

  • Multiparameter flow cytometry: Combines ubiquitination status with other cellular markers

• In situ approaches:

  • Proximity ligation assay (PLA): Detects endogenous RNF144B-substrate interactions in fixed cells or tissues

  • CODEX multiplexed imaging: Allows visualization of multiple proteins in tissue context

• Physiological models:

  • Organoids: Provide more physiologically relevant systems for studying RNF144B function

  • Conditional and tissue-specific knockout models: Enable precise temporal and spatial control of RNF144B expression

  • CRISPR-mediated endogenous tagging: Allows tracking of RNF144B and its substrates at physiological levels

• Computational approaches:

  • Machine learning algorithms: Predict potential RNF144B substrates based on sequence and structural features

  • Systems biology modeling: Integrates RNF144B into broader signaling networks

These advanced techniques are helping researchers move beyond simplified overexpression systems to understand RNF144B function in more physiologically relevant contexts, providing deeper insights into its roles in health and disease.

How might therapeutic strategies targeting RNF144B be developed for inflammatory diseases or cancer?

Developing therapeutic strategies targeting RNF144B requires careful consideration of its context-specific functions:

• Target validation approaches:

  • Genetic proof-of-concept: Using conditional knockout models to establish disease-modifying effects of RNF144B modulation

  • Small molecule probe development: Creating tool compounds that inhibit or activate RNF144B to validate therapeutic potential

  • Target engagement biomarkers: Developing assays to measure RNF144B activity in patient samples

• Potential therapeutic approaches for inflammatory conditions:

  • Inhibition strategies for conditions where RNF144B promotes inflammation:

    • Small molecule inhibitors targeting RNF144B's catalytic domain

    • Disruption of protein-protein interactions between RNF144B and specific inflammatory substrates

  • Activation strategies for conditions where RNF144B negatively regulates inflammation:

    • Stabilizers of RNF144B protein

    • Inducers of RNF144B expression

• Cancer therapeutic strategies:

  • Restoration approaches for tumors with RNF144B deficiency:

    • Gene therapy to reintroduce functional RNF144B

    • Small molecules that mimic RNF144B function

  • Synthetic lethality approaches:

    • Identifying and targeting vulnerabilities created by RNF144B loss

    • RNF144B-deficient cancer cells show resistance to cell cycle inhibitors that induce chromosomal instability, suggesting alternative therapeutic strategies are needed

• Antiviral applications:

  • Inhibition of RNF144B could enhance antiviral immunity:

    • RNA virus infections might be particularly amenable to this approach

    • Temporary suppression of RNF144B could boost innate immune responses during acute viral infections

• Delivery challenges and solutions:

  • Tissue-specific targeting: Developing delivery systems that target specific tissues where RNF144B modulation is therapeutically beneficial

  • Timing considerations: Creating controllable systems for transient vs. sustained RNF144B modulation

• Biomarker strategies:

  • Patient stratification: Identifying patients most likely to benefit from RNF144B-targeted therapies

  • Pharmacodynamic markers: Developing assays to monitor target engagement and efficacy

• Combination approaches:

  • Synergistic combinations: Identifying drugs that work synergistically with RNF144B modulation

  • Resistance management: Developing strategies to overcome potential resistance to RNF144B-targeted therapies

The development of RNF144B-targeted therapeutics represents an emerging opportunity, particularly given its roles in cancer biology and immune regulation, but requires careful consideration of its context-dependent functions to maximize efficacy while minimizing side effects.