Recombinant Human E3 ubiquitin-protein ligase RNF128 (RNF128)

What is RNF128 and what is its basic function?

RNF128 is a membrane-localized E3 ubiquitin ligase with a canonical sequence of 428 amino acids (isoform 1). It possesses a cytosolic zinc-binding RING finger domain with catalytic activity and a luminal/extracellular protease-associated (PA) domain that captures transmembrane protein targets for ubiquitination . The primary function of RNF128 is to facilitate the attachment of ubiquitin molecules to substrate proteins, specifically promoting Lys63-linked polyubiquitination which regulates receptor internalization and sorting during endocytosis .

In the context of cardiovascular biology, RNF128 has been identified as a critical regulator of foam cell formation through its interaction with scavenger receptor B1 (SRB1). Recent research has demonstrated that RNF128 prevents SRB1 degradation through the lysosomal system and promotes oxidized low-density lipoprotein (oxLDL)-induced foam cell formation and inflammatory responses in macrophages .

Where is RNF128 primarily expressed in atherosclerotic lesions?

Based on single-cell RNA sequencing (scRNA-seq) data, RNF128 is specifically expressed in macrophages located in the lipid core of atherosclerotic plaques . This expression pattern is particularly significant as persistent hyperlipidemia induces high expression of RNF128 in macrophages . The macrophage-specific expression of RNF128 within atherosclerotic lesions suggests its specialized role in foam cell formation, which is a hallmark of atherosclerotic plaque development.

Experimental verification has confirmed that RNF128 is almost exclusively expressed in macrophages, and its expression is upregulated during atherogenesis . This localized expression pattern makes RNF128 a potential target for macrophage-specific interventions in atherosclerosis treatment.

What is the structural composition of RNF128?

RNF128 contains several distinct domains that are critical for its function:

• 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 cytosolic ubiquitination

• A transmembrane domain

• A C-terminal RING finger domain that exhibits E3 ligase activity

The RING finger domain contains highly conserved cysteine residues at positions 277 and 280 that are necessary for its E3 catalytic activity . Mutation in the RING domain disrupts the ubiquitin ligase activity, as demonstrated by experiments with catalytically inactive mutants lacking the RING domain (RNF128 ΔR) .

The PA domain of RNF128 is particularly important for its interaction with target proteins. Studies have shown that deletion of the PA domain causes RNF128 to lose its ability to interact with SRB1 protein, while a recombinant plasmid containing only the PA domain (RNF128-PA) efficiently binds with SRB1 .

What is the mechanism by which RNF128 promotes foam cell formation?

RNF128 promotes foam cell formation through a sophisticated mechanism involving SRB1 stabilization and enhanced oxLDL uptake:

• RNF128 directly binds to SRB1 through its PA domain, interacting with the extracellular region of SRB1

• RNF128 catalyzes Lys63-linked polyubiquitination on the cytoplasmic C-terminus of SRB1 at lysine 478

• This polyubiquitination promotes endosome SRB1 recycling to the cell membrane with the assistance of Rab11, instead of entering the lysosome for degradation

• The increased membrane presence of SRB1 enhances oxLDL uptake by macrophages, leading to lipid accumulation

• Consequently, macrophages accumulate lipids and augment inflammation, resulting in their transformation into foam cells

Experimental evidence supports this mechanism, as RNF128 ablation in macrophages reduces oxLDL uptake and lipid accumulation, while overexpression of wild-type RNF128 rescues the diminished lipid accretion observed in RNF128-deficient macrophages .

How does RNF128 interact with SRB1 and what domains are involved?

The interaction between RNF128 and SRB1 involves specific domains on both proteins:

• On RNF128: The PA domain in the N-terminus is essential for binding to SRB1. Experiments with truncated forms of RNF128 revealed that fragments containing the PA domain (RNF128-N and RNF128-ΔC) retained the ability to interact with SRB1, while fragments lacking this domain lost this capability .

• On SRB1: The extracellular region is crucial for interaction with RNF128. Co-immunoprecipitation experiments with truncated forms of SRB1 demonstrated that SRB1 lacking the extracellular domain (SRB1-ΔEXT) could not interact with RNF128 .

This interaction has been verified through multiple methods:

• Co-immunoprecipitation with both tagged recombinant proteins and endogenous proteins

• In vitro binding assays with purified recombinant proteins

• Confocal microscopy showing colocalization of RNF128 and SRB1 in cells

Interestingly, the level of immunoprecipitated protein declined in a time-dependent manner in response to various durations of oxLDL treatment, suggesting that the interaction dynamics between RNF128 and SRB1 are influenced by the lipid environment .

What is the significance of K63-linked polyubiquitination in RNF128's function?

K63-linked polyubiquitination catalyzed by RNF128 plays a crucial role in protein trafficking and recycling rather than degradation:

• Unlike K48-linked ubiquitination which typically targets proteins for proteasomal degradation, K63-linked ubiquitination regulates receptor internalization and sorting during endocytosis

• RNF128 specifically catalyzes K63-linked polyubiquitination of SRB1 at lysine 478 on its cytoplasmic C-terminus, which is highly conserved in humans and mice

• This modification serves as a signal that promotes Rab11-mediated recycling of SRB1 to the plasma membrane instead of lysosomal degradation

• The E3 ligase activity of RNF128 is essential for this function, as demonstrated by experiments showing that both the RING domain deletion mutant (RNF128 ΔR) and point mutant RNF128 C2A (C277A and C280A) lose the ability to catalyze polyubiquitination of SRB1

The significance of this specific ubiquitination is evident in experiments showing that mutation of the K478 residue on SRB1 (SRB1 K478R) abolishes RNF128-mediated K63-linked ubiquitination and prevents the interaction between SRB1 and Rab11, thereby disrupting the recycling pathway .

How can researchers generate RNF128 knockout models for atherosclerosis studies?

To study the role of RNF128 in atherosclerosis, researchers have successfully employed several genetic approaches:

• Macrophage-specific conditional knockout:

  • Cross RNF128ᶠˡ/ᶠˡ mice with Lyz2-Cre mice to generate macrophage-specific RNF128 conditional knockout (RNF128-CKO) mice

  • The resulting RNF128ᶠˡ/ᶠˡLyz2ᶜʳᵉ mice can then be crossed with ApoE⁻/⁻ or placed on an LDLR-deficient background to study atherosclerosis

• LDLR-deficient models:

  • Induce LDLR deficiency using PCSK9 in RNF128-CKO mice as an alternative model

  • This approach phenocopies the effects observed in ApoE⁻/⁻ background mice

• Cell-specific knockout validation:

  • Verify knockout efficiency by isolating macrophages from RNF128-CKO mice and analyzing RNF128 expression at both mRNA and protein levels

  • Use Western blotting, qPCR, and immunohistochemistry to confirm the absence of RNF128 in targeted cells

These genetic models have demonstrated that macrophage-specific deletion of RNF128 ameliorates diet-induced atherosclerosis by reducing lipid accumulation and lesion inflammation in both male and female mice .

What techniques are effective for studying RNF128-mediated ubiquitination?

Several complementary techniques have proven effective for investigating RNF128-mediated ubiquitination:

• In vitro ubiquitination assays:

  • Purify recombinant RNF128, SRB1, and ubiquitin proteins

  • Set up reactions containing E1, E2, recombinant RNF128 (E3), SRB1 (substrate), ATP, and different types of ubiquitin (wild-type, K48-only, or K63-only)

  • Analyze ubiquitination patterns by immunoblotting

• Cell-based ubiquitination assays:

  • Transfect cells with tagged constructs (e.g., Myc-SRB1, Flag-RNF128, and HA-ubiquitin variants)

  • Immunoprecipitate the substrate protein and detect ubiquitination by Western blotting using tag-specific antibodies

  • Compare wild-type ubiquitin with mutant ubiquitin (K48 and K63) to determine linkage specificity

• Endogenous ubiquitination analysis:

  • Isolate macrophages from control and RNF128-deficient mice

  • Treat cells with oxLDL to stimulate the process

  • Immunoprecipitate endogenous SRB1 and detect ubiquitination using linkage-specific antibodies (anti-K48-Ub and anti-K63-Ub)

• Ubiquitination site identification:

  • Generate point mutants of the substrate protein, replacing individual lysine residues with arginine

  • Perform ubiquitination assays with these mutants to identify the specific residue(s) modified by RNF128

  • Create "lysine-less" mutants and reintroduce single lysines to confirm the ubiquitination sites

Using these techniques, researchers identified that RNF128 specifically catalyzes K63-linked polyubiquitination of SRB1 at lysine 478, which is critical for SRB1 recycling and membrane localization .

How can researchers analyze the effect of RNF128 on protein trafficking and recycling?

To investigate RNF128's role in protein trafficking and recycling, researchers can employ the following methodological approaches:

• Protein stability assays:

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

  • Monitor protein degradation over time using Western blotting

  • Compare degradation rates between control and RNF128-deficient cells

• Degradation pathway identification:

  • Use specific inhibitors of the proteasome (MG132) or lysosome (chloroquine, bafilomycin A1, ammonium chloride)

  • Determine which pathway is responsible for protein degradation by observing protein accumulation

  • Analyze whether RNF128 affects degradation through these pathways

• Subcellular localization studies:

  • Use confocal microscopy with fluorescently tagged proteins

  • Track protein movement between cellular compartments

  • Co-stain with markers for different cellular compartments (e.g., Rab11 for recycling endosomes)

• Membrane protein isolation:

  • Extract membrane proteins using specialized protocols

  • Quantify the amount of protein (e.g., SRB1) present at the membrane versus intracellular compartments

  • Compare membrane localization between control and RNF128-deficient cells

• Co-immunoprecipitation with trafficking regulators:

  • Assess interactions between the target protein (SRB1) and trafficking regulators (e.g., Rab11)

  • Determine how RNF128 affects these interactions

  • Use domain mutants to identify regions required for these interactions

By employing these techniques, researchers discovered that RNF128 promotes the recycling of SRB1 to the plasma membrane in a Rab11-dependent manner, thereby preventing its lysosomal degradation .

What is the impact of RNF128 on atherosclerosis development?

RNF128 has been demonstrated to accelerate atherogenesis through several mechanisms:

• Enhanced foam cell formation:

  • By stabilizing SRB1 and promoting its recycling to the plasma membrane, RNF128 increases oxLDL uptake by macrophages

  • This leads to excessive lipid accumulation and transformation of macrophages into foam cells

• Increased inflammatory response:

  • RNF128-mediated lipid accumulation in macrophages augments inflammatory responses

  • This contributes to the progression of atherosclerotic lesions

• In vivo evidence from multiple models:

  • Macrophage-specific RNF128 knockout in ApoE⁻/⁻ mice ameliorates the development of atherosclerosis in both males and females

  • Similar protective effects are observed in LDLR-deficient mice induced by PCSK9

These findings suggest that pharmacological inhibition of RNF128 may provide a potential therapeutic strategy for atherogenesis. Future research should explore the relationship between RNF128 variants in humans and atherosclerosis progression .

How does RNF128 expression change during atherosclerosis progression?

Based on experimental evidence:

• RNF128 expression is upregulated during atherogenesis:

  • Single-cell RNA sequencing data shows that RNF128 is specifically expressed in macrophages of the lipid core in atherosclerotic plaques

  • Persistent hyperlipidemia induces high expression of RNF128 in macrophages

• Temporal pattern:

  • RNF128 expression increases as atherosclerosis progresses

  • This correlates with increased foam cell formation and plaque development

Understanding the dynamic expression pattern of RNF128 during disease progression provides insights into its potential role as a biomarker and therapeutic target for atherosclerosis.

What are the potential therapeutic applications targeting RNF128?

Based on current understanding of RNF128's role in atherosclerosis, several therapeutic applications can be envisioned:

• Small molecule inhibitors:

  • Developing specific inhibitors targeting the RING domain of RNF128 to block its E3 ligase activity

  • This approach could reduce foam cell formation by preventing SRB1 recycling and oxLDL uptake

• Peptide-based interventions:

  • Designing peptides that disrupt the interaction between RNF128's PA domain and SRB1

  • This would prevent RNF128-mediated stabilization of SRB1 and reduce foam cell formation

• Macrophage-targeted delivery systems:

  • Since RNF128 is specifically expressed in macrophages, developing macrophage-targeted delivery systems for RNF128 inhibitors could enhance therapeutic efficacy while minimizing off-target effects

• Gene therapy approaches:

  • Using RNA interference or CRISPR-Cas9 technology to downregulate RNF128 expression in atherosclerotic plaques

The discovery that macrophage-specific deletion of RNF128 ameliorates diet-induced atherosclerosis provides strong evidence that pharmacological inhibition of RNF128 may represent a novel therapeutic strategy for atherosclerosis treatment .