Recombinant Mouse E3 ubiquitin-protein ligase RNF128 (Rnf128)

What is the basic domain structure of RNF128 and how does it relate to its function?

RNF128 contains several functional domains that are critical to its ubiquitin ligase activity. The protein includes:

• A signal peptide at the N-terminus essential for correct targeting and transport

• A protease-associated (PA) domain that is evolutionarily conserved and captures target proteins for ubiquitination

• A transmembrane domain

• A RING finger domain at the C-terminus that exhibits E3 ligase activity

The RING domain is particularly crucial, as mutation or deletion of this domain disturbs ubiquitin ligase activity. Research has demonstrated that overexpression of wild-type RNF128 can rescue diminished lipid accretion in RNF128-deficient macrophages, whereas overexpression of RNF128 with a deleted RING domain (RNF128 ∆R) fails to restore this function .

What types of ubiquitination does RNF128 catalyze and how does this affect target protein fate?

RNF128 primarily catalyzes K63-linked polyubiquitination of target proteins rather than K48-linked chains. This distinction is significant for determining protein fate:

• K63-linked ubiquitination by RNF128 typically does not lead to proteasomal degradation but instead alters protein trafficking, localization, or function

• In macrophages, RNF128 mediates K63-linked ubiquitination of SRB1 at lysine 478, which promotes endosomal recycling to the cell membrane rather than lysosomal degradation

• For TBK1, RNF128-mediated K63-linked ubiquitination enhances its kinase activity without triggering degradation, promoting antiviral responses

These findings highlight that RNF128's ubiquitination activity modulates protein function rather than primarily targeting proteins for degradation.

How does RNF128 contribute to atherosclerosis development?

RNF128 accelerates atherogenesis through several mechanisms:

• It is specifically expressed in macrophages within atherosclerotic plaque lipid cores

• Persistent hyperlipidemia induces high expression of RNF128 in macrophages

• RNF128 promotes oxidized low-density lipoprotein (oxLDL) uptake, leading to foam cell formation

• It enhances inflammatory responses in macrophages

Experimental evidence demonstrates that RNF128 ablation in macrophages ameliorates atherosclerosis in both male and female mice under ApoE and LDLR deficiency backgrounds. Mice with macrophage-specific RNF128 conditional knockout (RNF128^fl/fl^Lyz2^cre) on an apolipoprotein E null (ApoE^−/−) background showed reduced development of atherosclerosis .

What is the molecular mechanism by which RNF128 regulates SRB1 and foam cell formation?

RNF128 regulates SRB1 through a sophisticated molecular mechanism:

• RNF128 directly binds to scavenger receptor B1 (SRB1) through its PA domain, interacting with the extracellular region of SRB1

• It catalyzes K63-linked polyubiquitination of SRB1 at lysine 478 on its cytoplasmic C-terminus

• This ubiquitination promotes endosomal SRB1 recycling to the cell membrane with assistance from Rab11, rather than entering lysosomes for degradation

• Increased membrane SRB1 enhances oxLDL uptake, leading to lipid accumulation and foam cell formation

• The process depends on RNF128's E3 ligase activity, as deletion of the RING domain abolishes this effect

This mechanism represents a novel pathway by which RNF128 promotes atherosclerosis through post-translational modification of a key lipid uptake receptor .

How can researchers experimentally verify RNF128's effect on oxLDL uptake in macrophages?

To verify RNF128's effect on oxLDL uptake, researchers can employ the following methodological approaches:

• Fluorescently labeled oxLDL uptake assay:

  • Use oxLDL labeled with a red fluorescent probe (Dil-oxLDL)

  • Compare fluorescence intensity between RNF128-silent and control macrophages

  • Include native LDL (Dil-nLDL) as a control to demonstrate specificity

• Genetic manipulation experiments:

  • Utilize macrophages from RNF128^fl/fl^Lyz2^cre mice

  • Perform rescue experiments with wild-type RNF128 and RING-domain deleted mutants

  • Compare lipid accumulation using Oil Red O staining

• Membrane protein biotinylation:

  • Treat macrophages with biotin after oxLDL exposure

  • Extract and quantify plasma membrane SRB1 proteins through Western blotting

  • Compare membrane SRB1 levels between RNF128-deficient and control cells

These approaches, as demonstrated in published research, can effectively measure the impact of RNF128 on oxLDL uptake and foam cell formation .

How does RNF128 influence innate antiviral immunity?

RNF128 functions as an essential positive regulator of innate antiviral immunity against both RNA and DNA viruses:

• It is induced by virus infection in mouse primary peritoneal macrophages and human monocytic cells

• RNF128 knockdown or deficiency impairs IRF3 activation and IFN-β signaling

• Rnf128^−/− mice show increased susceptibility to both RNA virus (vesicular stomatitis virus, VSV) and DNA virus (herpes simplex virus type 1, HSV-1) infections

Mechanistically, RNF128 associates with TBK1 and promotes its kinase activity through conjugation of K63-linked ubiquitin chains. This enhances antiviral signaling pathways that are essential for host defense against viral infections .

What is RNF128's role in T cell regulation and autoimmunity?

RNF128 (also known as GRAIL) plays a critical role in T cell tolerance and regulatory T cell function:

• It regulates T cell activation to avoid autoimmunity

• RNF128-deficient (Rnf128^−/−) mice show markedly higher scores of experimental autoimmune encephalomyelitis (EAE) compared to wild-type mice

• Even heterozygous Rnf128^+/− mice develop marked symptoms of EAE

This suggests that RNF128 is a key regulator of T cell tolerance, and its deficiency can lead to exacerbated autoimmune responses. The mechanism involves RNF128's ability to regulate T cell activation thresholds and potentially affects regulatory T cell function .

How does RNF128 regulate neutrophil infiltration and inflammatory responses in acute lung injury?

RNF128 negatively regulates acute lung injury (ALI) through two main mechanisms:

• In neutrophils:

  • RNF128 binds to myeloperoxidase (MPO)

  • It reduces MPO levels and activity, inhibiting neutrophil activation

• In alveolar macrophages:

  • RNF128 interacts with TLR4, targeting it for degradation

  • It inhibits NF-κB activation, decreasing pro-inflammatory cytokine production

  • This reduces neutrophil infiltration and inflammatory responses

Experimental evidence shows that RNF128-deficient mice develop more severe lung damage and increased immune cell infiltration during LPS-induced ALI. Conversely, AAV9-mediated RNF128 overexpression alleviates lung tissue damage and reduces inflammatory cell infiltration, suggesting RNF128 as a promising therapeutic target for ALI .

How is RNF128 implicated in colorectal cancer progression?

Research on RNF128's role in colorectal cancer (CRC) has yielded conflicting results:

Pro-tumorigenic evidence:

• RNF128 is highly expressed in CRC tissues compared to adjacent normal tissues

• RNF128 knockdown reduces proliferation, migration, and invasion of colorectal cancer cells

• RNF128 inhibits the Hippo signaling pathway by binding to and ubiquitinating MST, promoting its degradation

• In xenograft models, RNF128 knockdown suppresses tumor growth

Anti-tumorigenic evidence:

• Some studies report decreased RNF128 expression in CRC tissues

• RNF128 has been shown to inhibit CRC progression via the Wnt/β-catenin pathway

• Overexpression of RNF128 suppresses CRC cell proliferation, migration, and invasion through β-catenin ubiquitination

These contradictory findings highlight the complexity of RNF128's role in cancer and suggest context-dependent functions that require further investigation.

What is the prognostic significance of RNF128 expression in lung adenocarcinoma?

RNF128 expression serves as a favorable prognostic factor in lung adenocarcinoma:

These findings suggest that RNF128 expression may be an independent predictor of favorable outcomes in patients with untreated lung adenocarcinoma who undergo surgical resection. The mechanism may involve modulation of the tumor immune microenvironment, particularly by affecting TAM populations .

What are the recommended approaches for generating RNF128 knockout mouse models?

Based on published research, recommended approaches for generating RNF128 knockout mice include:

• CRISPR/Cas9 technology:

  • Target exon 2 of the Rnf128-201 transcript, which contains a 248 bp coding sequence

  • Design sgRNA and donor vector for microinjection into fertilized eggs

  • Verify knockout by PCR and sequencing

  • Establish stable F1 generation by mating positive F0 mice with wild-type mice

• Cell-specific conditional knockout:

  • Cross RNF128^fl/fl^ mice with cell-specific Cre recombinase expressing mice (e.g., Lyz2^cre^ for macrophage-specific deletion)

  • Further cross with disease model mice (e.g., ApoE^−/−^ mice for atherosclerosis studies)

For studying RNF128 in specific disease contexts, researchers have successfully used macrophage-specific conditional RNF128 knockout (RNF128^fl/fl^Lyz2^cre^) mice crossed with ApoE^−/−^ mice to investigate atherosclerosis .

What methods can be used to study RNF128-mediated protein ubiquitination?

To study RNF128-mediated ubiquitination, researchers can employ the following methods:

• Co-immunoprecipitation (Co-IP) with ubiquitin analysis:

  • Transfect cells with RNF128 and target protein (e.g., SRB1, TBK1)

  • Add proteasome inhibitors (e.g., MG132) to prevent degradation

  • Immunoprecipitate the target protein and detect ubiquitination by Western blotting with ubiquitin antibodies

  • Use specific antibodies against K48-linked or K63-linked ubiquitin chains to determine ubiquitination type

• In vitro ubiquitination assay:

  • Purify recombinant RNF128, E1, E2, ubiquitin, and substrate proteins

  • Combine components in reaction buffer with ATP

  • Detect ubiquitination by Western blotting

  • Compare wild-type ubiquitin with K48R or K63R mutants to determine linkage specificity

• Mutational analysis:

  • Generate lysine-to-arginine mutations in potential ubiquitination sites of target proteins

  • Compare ubiquitination patterns between wild-type and mutant proteins

  • For RNF128 itself, create RING domain deletions to verify E3 ligase activity

These approaches have been successfully used to demonstrate that RNF128 catalyzes K63-linked polyubiquitination of targets like SRB1 at specific lysine residues (e.g., K478) .

What experimental designs best evaluate RNF128's impact on protein recycling versus degradation?

To differentiate between RNF128's effects on protein recycling versus degradation, researchers should consider:

• Protein stability assays:

  • Treat cells with cycloheximide (CHX) to inhibit protein synthesis

  • Monitor protein degradation over time in RNF128-expressing versus deficient cells

  • Compare degradation rates with specific inhibitors of proteasomes (MG132) or lysosomes (chloroquine, bafilomycin A1)

• Subcellular localization tracking:

  • Use fluorescently tagged proteins to monitor trafficking

  • Perform colocalization studies with markers for different cellular compartments:

    • Rab11 for recycling endosomes

    • LAMP2 for lysosomes

    • Calnexin for endoplasmic reticulum

• Cell surface biotinylation assays:

  • Biotinylate cell surface proteins

  • Track internalization and recycling of biotinylated proteins

  • Compare recycling rates between RNF128-expressing and deficient cells

• Combination approaches with lysosomal inhibitors:

  • Block lysosomal degradation using inhibitors (chloroquine, 3-MA, leupeptin)

  • Maintain constant total protein levels

  • Specifically monitor membrane-associated protein levels through extraction of membrane proteins

  • Evaluate protein-protein interactions (e.g., SRB1-Rab11) in the presence of lysosomal inhibitors

This experimental design allows separation of RNF128's effects on protein recycling from its effects on degradation, as demonstrated in studies of SRB1 trafficking .

How can researchers address the conflicting reports on RNF128's role in colorectal cancer?

To address conflicting reports on RNF128 in colorectal cancer, researchers should:

• Perform comprehensive expression analysis:

  • Analyze multiple public databases (GEPIA, TCGA) alongside in-house samples

  • Stratify analysis by cancer stage, molecular subtype, and patient characteristics

  • Use multiple techniques (IHC, qRT-PCR, Western blot) to verify expression patterns

• Investigate pathway-specific effects:

  • Simultaneously examine both Wnt/β-catenin and Hippo signaling pathways

  • Determine if RNF128 differentially affects these pathways in different contexts

  • Consider genetic background differences that may influence pathway dominance

• Design clear rescue experiments:

  • In RNF128 knockdown cells showing reduced tumor capabilities, re-express RNF128

  • Test both wild-type RNF128 and catalytically inactive mutants

  • Evaluate if rescue effects are dependent on specific downstream pathways

• Examine cell-type specificity:

  • Test multiple CRC cell lines with different genetic backgrounds

  • Determine if RNF128 effects are consistent across cell types or context-dependent

These approaches can help resolve whether RNF128 has dual roles in CRC depending on context, or if methodological differences account for the contradictory findings .

What are the key considerations for studying RNF128 across different cellular contexts?

When studying RNF128 across different cellular contexts, researchers should consider:

• Cell-type specific expression patterns:

  • RNF128 is expressed in specific cell populations (e.g., macrophages in atherosclerotic plaques)

  • Expression may be induced by specific stimuli (e.g., hyperlipidemia, viral infection)

  • Verify expression in each cell type before functional studies

• Target protein availability:

  • RNF128 interacts with different proteins in different cell types:

    • SRB1 in macrophages

    • TBK1 in antiviral responses

    • MST in cancer cells

    • TLR4 and MPO in lung injury models

  • Confirm expression of potential targets in the studied cell type

• Stimulus-dependent functions:

  • RNF128 may have different functions depending on cellular activation state

  • Include appropriate stimuli (e.g., oxLDL for foam cells, viral infection for immune cells)

  • Measure RNF128 activity under both basal and stimulated conditions

• Domain-specific functions:

  • The PA domain is critical for target recognition

  • The RING domain is essential for E3 ligase activity

  • Include domain-specific mutants in functional studies

By carefully considering these factors, researchers can better understand RNF128's context-dependent functions and resolve apparent contradictions in the literature.

What emerging technologies might advance our understanding of RNF128 biology?

Several emerging technologies hold promise for advancing RNF128 research:

• Proximity labeling techniques:

  • BioID or TurboID fusion proteins to identify novel RNF128 interaction partners

  • APEX2 for temporal mapping of RNF128 interactions following stimulation

  • These approaches could reveal context-specific RNF128 interactomes

• Single-cell approaches:

  • Single-cell RNA-seq to identify cell populations expressing RNF128 in complex tissues

  • Single-cell proteomics to correlate RNF128 protein levels with cellular phenotypes

  • Spatial transcriptomics to map RNF128 expression within tissue microenvironments

• Live-cell ubiquitination sensors:

  • FRET-based sensors to monitor RNF128-mediated ubiquitination in real-time

  • Optogenetic control of RNF128 activity to examine temporal aspects of signaling

  • These tools could help understand dynamics of RNF128 function

• CRISPR-based screening:

  • Genome-wide CRISPR screens to identify genes that modify RNF128 function

  • CRISPR activation/inhibition screens to map RNF128-dependent pathways

  • Base editing to introduce specific mutations in RNF128 or its targets