Recombinant Mouse Protein FAM73B (Fam73b)

What is FAM73B and what is its primary function?

FAM73B (Family with sequence similarity 73 member B) is a mitochondrial outer membrane protein that functions as a crucial regulator of mitochondrial dynamics. Specifically, FAM73B has a pivotal role in Toll-like receptor (TLR)-regulated mitochondrial morphology switching from fusion to fission. Studies have demonstrated that FAM73B deficiency promotes mitochondrial fission, which subsequently enhances IL-12 production in macrophages and dendritic cells . This protein is encoded by the Fam73b gene (also known as mitoguardin 2) and has several gene aliases including R7476, AI790341, C9orf54, R74766, 5730472N09Rik, and Fam73 .

The molecular weight of recombinant mouse FAM73B protein is approximately 48,671 Da, and its function extends beyond mitochondrial dynamics to influence immune cell polarization and anti-tumor immunity . FAM73B deficiency has been shown to modify the immune landscape by promoting T-cell activation through enhanced IL-12 production and reduced IL-10 and IL-23 expression .

How does FAM73B differ from other mitochondrial dynamics regulators?

Unlike traditional mitochondrial fusion mediators such as Mitofusin 1 (MFN1) and Mitofusin 2 (MFN2), FAM73B appears to function through a distinct mechanism in regulating mitochondrial morphology. Research has shown that Mfns are dispensable for the mitochondrial morphology switch under polarization stress, indicating that FAM73B operates through a separate pathway .

FAM73B specifically affects the recruitment and accumulation of Parkin to mitochondria, which subsequently influences the CHIP-IRF1 axis through proteolysis. This distinctive mechanism differentiates FAM73B from other mitochondrial dynamics regulators that primarily act through direct membrane fusion or fission processes . While proteins like DRP1 require multiple post-translational modifications (phosphorylation, SUMOylation, ubiquitination) to regulate mitochondrial fission, FAM73B appears to function upstream of these processes by affecting the recruitment of key regulatory proteins to the mitochondria .

What are the optimal conditions for expressing recombinant FAM73B in E. coli?

Based on successful recombinant protein expression strategies, the following conditions can be adapted for FAM73B expression in E. coli:

• Growth medium: Use a medium containing 5 g/L yeast extract, 5 g/L tryptone, 10 g/L NaCl, and 1 g/L glucose with appropriate antibiotic selection (e.g., 30 μg/mL kanamycin) .

• Induction parameters: Grow bacterial culture to an optical density of 0.8 (measured at 600 nm), then induce protein expression with 0.1 mM IPTG .

• Expression conditions: Maintain culture at 25°C for approximately 4 hours post-induction to maximize soluble protein yield .

• Expected yield: With optimized conditions, yields of 250 mg/L of soluble protein have been achieved for other complex recombinant proteins and could serve as a target for FAM73B expression .

It's important to note that FAM73B, being a mitochondrial membrane protein, may require specialized conditions for optimal soluble expression. Fusion tags (such as MBP, GST, or SUMO) may enhance solubility, and inclusion of detergents during purification might be necessary to maintain native conformation.

What expression systems are available for producing recombinant FAM73B?

Multiple expression systems can be employed for recombinant FAM73B production, each with distinct advantages:

Commercial recombinant mouse FAM73B is available with greater than 85% purity as determined by SDS-PAGE, typically produced in one of these expression systems and supplied in either lyophilized or liquid format . The choice of expression system should align with the experimental requirements - E. coli may be sufficient for structural studies, while mammalian cells might be preferred for functional assays.

How can researchers assess the role of FAM73B in mitochondrial dynamics?

To investigate FAM73B's role in mitochondrial dynamics, researchers can employ several complementary approaches:

• Gene targeting approach: The "KO first" approach has been successfully used to delete the Fam73b gene in mice, allowing for in vivo assessment of FAM73B function in immune cells . This can be combined with specific cell type Cre lines to create conditional knockouts.

• Mitochondrial morphology analysis: Visualization of mitochondrial networks using fluorescent microscopy (MitoTracker staining or mitochondria-targeted fluorescent proteins) in wild-type versus FAM73B-deficient cells can reveal changes in fusion/fission dynamics .

• TLR stimulation assays: Since FAM73B functions in TLR-regulated mitochondrial morphology switching, treating macrophages with TLR agonists and monitoring changes in mitochondrial morphology provides insight into FAM73B's dynamic function .

• Parkin recruitment studies: Assessing Parkin expression and recruitment to mitochondria in the presence or absence of FAM73B can elucidate downstream mechanisms. Immunofluorescence or subcellular fractionation followed by Western blotting can be used to quantify Parkin localization .

• CHIP-IRF1 axis analysis: Monitoring the stability and activity of the CHIP-IRF1 axis through protein degradation assays and reporter gene assays can reveal how FAM73B influences this downstream signaling pathway .

What methodological approaches best detect the interaction between FAM73B and mitochondrial fission/fusion machinery?

Several methodological approaches can effectively detect interactions between FAM73B and mitochondrial dynamics proteins:

• Co-immunoprecipitation (Co-IP): This technique can identify protein-protein interactions between FAM73B and mitochondrial fusion/fission proteins such as MFN1, MFN2, and DRP1. Cell lysates can be immunoprecipitated with anti-FAM73B antibodies and probed for interacting partners .

• Proximity ligation assay (PLA): This microscopy-based technique can visualize protein interactions at endogenous levels with high specificity and sensitivity, making it suitable for detecting interactions between FAM73B and components of the mitochondrial dynamics machinery.

• Mitochondrial subfraction analysis: Isolation of mitochondrial outer membrane fractions followed by Western blot analysis can determine the relative abundance of FAM73B and its potential binding partners within the same mitochondrial compartment .

• Live-cell imaging with FRET/BRET: Tagging FAM73B and potential interacting partners with appropriate fluorophores allows real-time monitoring of protein interactions in living cells.

• Functional rescue experiments: In FAM73B-deficient cells, expression of wild-type or mutant FAM73B can determine which domains are essential for interaction with mitochondrial dynamics proteins and subsequent functional outcomes .

How does FAM73B deficiency impact anti-tumor immunity in mouse models?

FAM73B deficiency profoundly enhances anti-tumor immunity in multiple mouse cancer models through several mechanisms:

• Melanoma model: FAM73B knockout mice show significantly suppressed B16 melanoma tumor growth and increased survival rates compared to wild-type mice . Specifically, tumor-bearing FAM73B KO mice demonstrated:

  • Enhanced IL-12 serum levels

  • Reduced IL-10 serum levels

  • Upregulated IFN-γ production

  • Increased frequency of IFN-γ-positive CD4+ and CD8+ effector T cells in draining lymph nodes and spleen

• MCA-induced fibrosarcoma model: FAM73B KO mice developed fibrosarcoma at a significantly lower incidence rate when treated with 800 μg of methylcholantrene (MCA) and monitored for up to 150 days . These mice also showed:

  • Profound increase in IL-12 serum levels

  • Elevated IFN-γ serum levels

The enhanced anti-tumor immunity results from FAM73B deficiency promoting mitochondrial fission in macrophages and dendritic cells, which leads to increased IL-12 production. This cytokine shift subsequently activates T cells to produce IFN-γ, creating a more favorable immune environment for tumor control .

What molecular mechanisms link FAM73B to macrophage polarization and cytokine production?

The molecular mechanisms connecting FAM73B to macrophage polarization and cytokine production involve several interconnected pathways:

• Mitochondrial morphology regulation: FAM73B functions as a critical regulator of mitochondrial dynamics during macrophage polarization. Its deletion leads to severe mitochondrial fragmentation .

• Parkin accumulation and recruitment: Mitochondrial fission caused by FAM73B deficiency promotes accumulation and recruitment of Parkin to mitochondria .

• CHIP degradation pathway: Parkin directly induces monoubiquitinated CHIP degradation, which affects downstream signaling pathways .

• IRF1 stabilization: The degradation of monoubiquitinated CHIP results in stabilization of IRF1, a crucial transcription factor for IL-12 production .

• Cytokine expression profile: This signaling cascade leads to promoted TLR-induced IL-12 expression and inhibited IL-10 and IL-23 expression in macrophages and dendritic cells .

• T cell activation: The altered cytokine profile, particularly increased IL-12, enhances T-cell activation and IFN-γ production, further reinforcing anti-tumor immune responses .

Interestingly, this mechanism appears independent of certain classical pathways, as FAM73B deficiency caused a significant reduction in inducible Ifnb expression without affecting TBK1 and IRF3 phosphorylation .

How do post-translational modifications regulate mitochondrial dynamics proteins in relation to FAM73B function?

Post-translational modifications (PTMs) of mitochondrial dynamics proteins represent a complex regulatory layer that may intersect with FAM73B function:

• Phosphorylation: DRP1, a key mitochondrial fission protein, undergoes extensive phosphorylation at multiple sites that can either promote or inhibit fission. For instance:

  • Phosphorylation at S616 by CDK1, CDK5, ERK1/2, and PKCδ typically activates DRP1 and promotes fission

  • Phosphorylation at S637 by PKA inhibits DRP1 activity, while phosphorylation at the same site by CaMK1α and ROCK1 can increase division activity

• SUMOylation: DRP1 is regulated by SUMOylation status, with SUMO-1 modification preventing lysosomal degradation and promoting mitochondrial division. Conversely, deSUMOylation of SUMO-2/3 from DRP1 by SENP3 reinforces mitochondrial fission .

• Ubiquitination: E3 ubiquitin ligases like MARCH can modify DRP1, with context-dependent effects on mitochondrial dynamics. This modification has shown both activation and inhibition of DRP1 in different cell types .

• Acetylation: Acetylation at K642 of DRP1 has been observed to activate the protein in mouse cardiomyocytes .

The interaction between these PTMs and FAM73B function remains an area for further investigation. Since FAM73B affects Parkin recruitment to mitochondria, and Parkin functions as an E3 ubiquitin ligase, there may be a complex interplay between FAM73B-regulated processes and the ubiquitination status of mitochondrial dynamics proteins .

What experimental approaches can distinguish the specific effects of FAM73B from other mitochondrial dynamics regulators?

Distinguishing FAM73B-specific effects from those of other mitochondrial dynamics regulators requires sophisticated experimental approaches:

• Conditional and tissue-specific knockout models: Using Cre-lox systems to delete FAM73B specifically in myeloid cells while preserving expression in other tissues helps isolate cell-type-specific functions. This approach has successfully demonstrated that myeloid cell-specific FAM73B deficiency enhances Th1 responses, while T cell-specific knockout does not produce the same effect .

• Comparative knockout studies: Parallel analysis of FAM73B knockout alongside knockouts of other mitochondrial dynamics proteins (e.g., MFN1/MFN2) can identify unique versus overlapping phenotypes. For example, both FAM73B and MFN1/MFN2 depletion cause severe mitochondrial fragmentation, but their downstream effects may differ .

• Rescue experiments with domain-specific mutants: Reintroducing wild-type versus mutant versions of FAM73B into knockout cells can identify which protein domains are responsible for specific functions.

• Proximity-dependent labeling: Techniques like BioID or APEX2 fused to FAM73B can identify unique proximal proteins in the mitochondrial membrane, distinguishing FAM73B-specific interaction partners from those of other mitochondrial dynamics proteins.

• Single-cell analysis: Examining the effects of FAM73B deficiency at the single-cell level can reveal cell-to-cell variability and potential compensatory mechanisms that may be obscured in population-level analyses.

• Metabolic flux analysis: Since mitochondrial dynamics affect cellular metabolism, comparing metabolic profiles between FAM73B-deficient cells and cells lacking other mitochondrial dynamics proteins can reveal unique metabolic signatures associated with FAM73B function.

How might targeting FAM73B function lead to novel cancer immunotherapies?

The research findings on FAM73B suggest several promising avenues for cancer immunotherapy development:

• Macrophage reprogramming: Since FAM73B deficiency promotes anti-tumor macrophage polarization (increased IL-12, decreased IL-10), developing small molecule inhibitors of FAM73B could potentially reprogram tumor-associated macrophages toward an anti-tumor phenotype .

• Combination with checkpoint inhibitors: The enhanced T-cell activation and IFN-γ production observed in FAM73B-deficient models could potentiate the effects of existing checkpoint inhibitor therapies that rely on functional T-cell responses .

• Ex vivo modification strategies: Autologous macrophages or dendritic cells could be modified ex vivo to downregulate FAM73B expression before reinfusion into patients, potentially enhancing anti-tumor immune responses.

• Mitochondrial dynamics modifiers: Developing compounds that mimic the effect of FAM73B deficiency on mitochondrial dynamics could provide a more targeted approach than direct protein inhibition.

• Targeting downstream pathways: The identified CHIP-IRF1 axis downstream of FAM73B could offer additional therapeutic targets that might be more accessible than the mitochondrial membrane-bound FAM73B itself .

The research findings that FAM73B deficiency suppresses tumor growth in both melanoma and MCA-induced fibrosarcoma models suggest that targeting this pathway could have broad applications across multiple cancer types .

What are the current limitations in studying FAM73B function and how might they be overcome?

Current research on FAM73B faces several methodological and conceptual challenges:

• Membrane protein expression difficulties: As a mitochondrial membrane protein, recombinant FAM73B can be challenging to express in soluble, functional form. This limitation could be addressed through:

  • Optimized expression protocols combining factorial design approaches for buffer compositions, induction conditions, and expression temperatures

  • Use of detergent screening to identify conditions that maintain native conformation

  • Expression of soluble domains for structural studies

• Distinguishing direct vs. indirect effects: Determining whether FAM73B directly interacts with mitochondrial dynamics machinery or exerts its effects indirectly remains challenging. Advanced approaches such as:

  • Cryo-electron microscopy of FAM73B in mitochondrial membranes

  • Chemical crosslinking followed by mass spectrometry

  • In vitro reconstitution systems with purified components
    could help resolve these questions.

• Translating mouse findings to humans: While mouse studies show promising anti-tumor effects of FAM73B deficiency, translation to human systems requires:

  • Validation in human primary cells and organoids

  • Development of humanized mouse models

  • Correlation studies in human tumor samples examining FAM73B expression and immune infiltration patterns

• Potential off-target effects: Complete FAM73B inhibition may have unintended consequences in non-immune tissues. Development of:

  • Tissue-specific delivery systems

  • Partial inhibitors that modulate rather than abolish function

  • Careful toxicology studies in diverse tissue types would be crucial for therapeutic development.