Recombinant Bovine Protein FAM134B (FAM134B)

What is FAM134B and what is its primary function in cellular physiology?

FAM134B (Family with sequence similarity 134, member B) is the first identified endoplasmic reticulum (ER) autophagy receptor in mammals. It plays a critical role in ER homeostasis by facilitating ER-phagy, a selective autophagic process that degrades portions of the ER. The protein is primarily known for its function in fragmenting ER membranes to facilitate their subsequent degradation. FAM134B is a cis-Golgi transmembrane protein that is necessary for the long-term survival of nociceptive and autonomic ganglion neurons . The protein is highly conserved across species, from yeast to humans, indicating its evolutionary importance in cellular function .

What are the key structural domains of FAM134B and how do they contribute to its function?

The Reticulon domain (RTND) of FAM134B is indispensable for ER membrane fragmentation and ER-phagy. This domain mediates the oligomerization of FAM134B molecules, which is a key process in the fragmentation of ER membranes to facilitate ER-phagy . The human FAM134B protein consists of 497 amino acids with a calculated molecular weight of 55 kDa . The bovine FAM134B protein used in recombinant studies typically encompasses amino acids 1-356 and contains the critical functional domains required for its biological activity . The protein's ability to form oligomers through its RTND is crucial for inducing membrane curvature and subsequent fragmentation of ER membranes.

How evolutionarily conserved is FAM134B across species?

FAM134B demonstrates remarkable evolutionary conservation across numerous species, suggesting its fundamental importance in cellular function. The protein has been identified and characterized in numerous mammals including:

This high degree of conservation facilitates comparative studies and suggests that findings from bovine recombinant FAM134B research may have translational relevance to human physiology and pathology .

What expression systems are most effective for producing recombinant bovine FAM134B protein?

E. coli expression systems are predominantly used for producing recombinant bovine FAM134B protein. When expressing FAM134B in E. coli, researchers typically use a construct with an N-terminal His-tag to facilitate purification . For studies requiring post-translational modifications, mammalian expression systems such as HEK293T cells can be used, as demonstrated in studies examining acetylated FAM134B . When selecting an expression system, consider your specific research needs:

• For structural studies: E. coli-expressed protein may be sufficient

• For functional studies requiring post-translational modifications: Mammalian expression systems are preferable

• For large-scale production: Optimized E. coli systems with codon optimization for bovine sequences

Regardless of the system chosen, it's crucial to verify the proper folding and oligomerization capacity of the recombinant protein through techniques such as size exclusion chromatography and circular dichroism .

What are the optimal methods for purifying recombinant FAM134B while maintaining its functional integrity?

Purification of recombinant bovine FAM134B requires careful consideration of the protein's oligomerization tendency and membrane-associated nature. A recommended purification protocol includes:

• Affinity chromatography: Using Nickel-NTA resin for His-tagged FAM134B

• Size exclusion chromatography: To separate monomeric from oligomeric forms and remove aggregates

• Quality control: Verification through circular dichroism and native gel electrophoresis to ensure proper folding and absence of protein aggregation

When purifying FAM134B, it's essential to avoid harsh detergents that might disrupt the Reticulon domain's structure. Temperature control during purification is also critical, as FAM134B stability can be temperature-sensitive. Researchers should verify that the purified recombinant FAM134B retains its membrane fragmentation activity through liposome fragmentation assays, as demonstrated in previous studies .

How can researchers verify that recombinant FAM134B is properly folded and functionally active?

Verifying the proper folding and functional activity of recombinant FAM134B involves multiple complementary approaches:

• Biophysical characterization:

  • Size exclusion chromatography to assess oligomerization state

  • Circular dichroism to evaluate secondary structure

  • Native gel electrophoresis to confirm absence of aggregation

• Functional assays:

  • In vitro liposome fragmentation assay to test membrane remodeling capacity

  • ER fragmentation assay in cells expressing recombinant FAM134B

  • Co-immunoprecipitation with known interaction partners like LC3 or GABARAP proteins

• Post-translational modification assessment:

  • Western blotting with antibodies specific to acetylated lysine residues

  • Mass spectrometry to confirm specific modifications at sites like K160

It's crucial to include appropriate controls in these assays, such as known inactive mutants (e.g., mutations in the Reticulon domain) to benchmark the activity of your recombinant protein preparation.

What molecular events trigger FAM134B-mediated ER-phagy?

FAM134B-mediated ER-phagy is triggered by several molecular events that occur in response to cellular stress:

• ER stress activation: Chemical inducers like thapsigargin (Tg) trigger ER stress, which activates CAMKII-mediated phosphorylation of FAM134B at its Reticulon domain

• Post-translational modifications cascade:

  • CAMKII-mediated phosphorylation of FAM134B

  • CBP-mediated acetylation at K160

  • These modifications enhance FAM134B oligomerization capacity

• Oligomerization: Modified FAM134B forms oligomers that induce ER membrane curvature and fragmentation

• Autophagosome recruitment: FAM134B interacts with LC3/GABARAP proteins to recruit autophagic machinery to fragmented ER segments

This sequence of events represents a highly regulated process that maintains ER homeostasis under stress conditions. Understanding these triggers is essential for designing experiments to study FAM134B function in different cellular contexts.

How does FAM134B oligomerization contribute to ER membrane fragmentation?

FAM134B oligomerization is a critical mechanistic step in ER membrane fragmentation that precedes ER-phagy. The process involves:

• Reticulon domain-mediated oligomerization: The Reticulon domain (RTND) of FAM134B mediates the self-association of FAM134B molecules

• Enhanced oligomerization through acetylation: Acetylation at K160, mediated by CBP, dramatically enhances FAM134B oligomerization capacity

• Membrane curvature induction: Oligomerized FAM134B inserts into the ER membrane and induces membrane curvature

• Fragmentation of ER network: The induced curvature leads to scission of ER tubules, creating isolated ER fragments

• Autophagic targeting: The fragmented ER membranes are then recognized by the autophagy machinery through FAM134B's interaction with LC3/GABARAP proteins

This oligomerization-dependent mechanism can be assessed experimentally using techniques such as liposome fragmentation assays, where recombinant FAM134B is incubated with artificial liposomes, and the resulting membrane structures are visualized by electron microscopy .

What is the regulatory relationship between FAM134B and other ER-phagy factors?

FAM134B functions within a complex regulatory network of ER-phagy factors, with several key relationships:

• CBP acetylation regulatory axis:

  • CBP (CREB-binding protein) directly acetylates FAM134B at K160

  • This acetylation enhances FAM134B oligomerization and ER fragmentation activity

• SIRT7 deacetylation counterbalance:

  • SIRT7 deacetylates FAM134B at K160

  • This creates a dynamic regulatory circuit that can fine-tune ER-phagy rates

  • Inhibition of SIRT7 with compound 97491 maintains K160 acetylation levels

• ER stress-related degradation factors:

  • FAM134B inhibits the expression of ER stress-related degradation factors including DERL2, EDEM1, SEL1L, and HRD1

  • This relationship is particularly relevant in hepatocellular carcinoma contexts

• Autophagy machinery interaction:

  • FAM134B directly interacts with LC3 and GABARAP proteins through its LC3-interacting region

  • This interaction is essential for recruiting the autophagy machinery to fragmented ER

Understanding these regulatory relationships is crucial for interpreting experimental results and designing targeted interventions in FAM134B-dependent pathways.

How does acetylation affect FAM134B function and what enzymes regulate this modification?

Acetylation is a critical post-translational modification that significantly enhances FAM134B function:

• Site of acetylation: The primary acetylation site is lysine 160 (K160) in FAM134B

• Effect on function:

  • K160 acetylation dramatically enhances FAM134B oligomerization

  • Acetylated FAM134B shows increased ER membrane fragmentation activity

  • This modification is essential for efficient ER-phagy induction

• Regulatory enzymes:

  • Acetyltransferase: CBP (CREB-binding protein) directly acetylates FAM134B at K160

  • Deacetylase: SIRT7 specifically deacetylates acetyl-K160 of FAM134B

• Dynamic regulation:

  • Under ER stress conditions, CBP-mediated acetylation increases

  • SIRT7 activity creates a counterbalance, allowing for precise control

To study FAM134B acetylation experimentally, researchers can use recombinant FAM134B with CBP in in vitro acetylation reactions, followed by functional assays like liposome fragmentation tests or cell-based ER fragmentation assays .

What methodologies are most effective for studying FAM134B post-translational modifications in vitro?

Studying FAM134B post-translational modifications requires a multi-faceted approach:

• In vitro modification assays:

  • For acetylation: Recombinant FAM134B is incubated with purified CBP and acetyl-CoA

  • For deacetylation: Acetylated FAM134B is treated with recombinant SIRT7 in the presence of NAD+

  • Controls should include specific inhibitors like SIRT7 inhibitor 97491

• Detection methods:

  • Western blotting using modification-specific antibodies (anti-acetyl-lysine)

  • Mass spectrometry for precise site identification and quantification

  • Biochemical assays measuring FAM134B oligomerization (native PAGE, size exclusion chromatography)

• Functional consequences:

  • Liposome fragmentation assays to measure membrane remodeling activity

  • In vitro oligomerization assays

  • Structure-function studies comparing wild-type and non-modifiable mutants (e.g., K160R)

• Simulation of cellular conditions:

  • Recreating physiological pH, ion concentrations, and redox conditions

  • Including relevant binding partners that might affect modification states

These methodologies provide complementary information about FAM134B modifications and their functional consequences, allowing for comprehensive characterization of this regulatory mechanism.

How do phosphorylation and acetylation interact to regulate FAM134B activity?

FAM134B activity is regulated by a coordinated interplay between phosphorylation and acetylation:

• Sequential modification:

  • ER stress triggers CAMKII-mediated phosphorylation at the Reticulon domain of FAM134B

  • This phosphorylation appears to prime FAM134B for subsequent acetylation at K160 by CBP

• Synergistic effects:

  • Phosphorylation enhances FAM134B's membrane association

  • Acetylation dramatically increases oligomerization capacity

  • Together, these modifications create a highly active form of FAM134B capable of efficient ER fragmentation

• Regulatory circuit:

  • SIRT7-mediated deacetylation provides a counterbalance to CBP acetylation

  • This creates a dynamic, reversible regulatory system that can fine-tune ER-phagy rates

  • The balance between these modifications likely determines the extent of ER-phagy under different stress conditions

• Experimental approach:

  • To study this interplay, researchers can use phosphomimetic mutants (e.g., serine to aspartate) in combination with acetylation-deficient mutants (K160R)

  • This allows dissection of the individual and combined contributions of these modifications

Understanding this interconnected regulatory system is essential for developing interventions that target specific steps in the FAM134B activation pathway.

What is the role of FAM134B in hereditary sensory and autonomic neuropathy (HSAN II)?

FAM134B dysfunction is directly linked to hereditary sensory and autonomic neuropathy type II (HSAN II), providing critical insights into the physiological importance of this protein:

• Genetic basis:

  • Mutations in the FAM134B gene cause HSAN II

  • These mutations typically lead to loss of FAM134B function

• Neuronal implications:

  • FAM134B is necessary for the long-term survival of nociceptive and autonomic ganglion neurons

  • Its dysfunction leads to progressive degeneration of sensory and autonomic neurons

• Cellular mechanism:

  • Impaired ER-phagy leads to accumulation of abnormal ER structures

  • This accumulation creates cellular stress that ultimately triggers neuronal death

  • The Reticulon domain mutations particularly affect FAM134B's ability to fragment ER membranes

• Research models:

  • Patient-derived cells carrying FAM134B mutations

  • Animal models with targeted FAM134B disruption

  • In vitro systems using recombinant mutant FAM134B proteins

Studying recombinant bovine FAM134B and its mutations can provide valuable insights into the molecular mechanisms underlying HSAN II, potentially leading to therapeutic approaches for this currently untreatable condition.

How does FAM134B contribute to cancer progression, particularly in hepatocellular carcinoma?

Recent research has uncovered a complex role for FAM134B in cancer, particularly in hepatocellular carcinoma (HCC):

• Expression pattern:

  • FAM134B is significantly upregulated in human liver cancer tissue

  • Higher expression is observed in HCC cell lines Hep3B and Huh7 compared to normal liver cells

• Cancer-promoting mechanism:

  • FAM134B inhibits HCC cell autophagy

  • It promotes liver cancer progression by inhibiting the expression of ER stress-related degradation factors

  • Key downregulated factors include DERL2, EDEM1, SEL1L, and HRD1

• Experimental evidence:

  • Knockdown of FAM134B (sh-FAM134B) effectively inhibits HCC cell proliferation

  • FAM134B knockdown promotes HCC cell apoptosis

  • sh-FAM134B induces autophagy in HCC cells

  • These effects are mediated through induction of ER stress

• Therapeutic implications:

  • FAM134B may represent a novel therapeutic target in HCC

  • Modulators of FAM134B function or expression could potentially alter HCC progression

  • Understanding the structural basis of FAM134B function could aid in rational drug design

This emerging role of FAM134B in cancer biology highlights the importance of studying its structure-function relationships and regulatory mechanisms in different cellular contexts.

What approaches can be used to modulate FAM134B activity for potential therapeutic applications?

Based on current understanding of FAM134B biology, several approaches could be developed to modulate its activity for therapeutic purposes:

• Post-translational modification targeting:

  • CBP inhibitors to reduce FAM134B acetylation in cancer contexts where it's overactive

  • SIRT7 inhibitors (like compound 97491) to enhance FAM134B acetylation in contexts where boosting ER-phagy would be beneficial

• Oligomerization modulation:

  • Small molecules that either promote or interfere with FAM134B oligomerization

  • Peptide-based interventions targeting the Reticulon domain interface

• Genetic approaches:

  • siRNA or shRNA targeting FAM134B in cancer contexts

  • Gene therapy to restore functional FAM134B in HSAN II

• Recombinant protein delivery:

  • Cell-penetrating versions of functional FAM134B domains

  • Pre-acetylated recombinant FAM134B to bypass regulatory limitations

• Screening methodology:

  • High-throughput screens using recombinant bovine FAM134B in liposome fragmentation assays

  • Cell-based screens measuring ER fragmentation

  • Structure-based virtual screening based on the Reticulon domain

Developing these therapeutic approaches requires detailed understanding of FAM134B structure-function relationships and regulatory mechanisms, highlighting the importance of basic research with recombinant proteins.

What are the most effective assays for measuring FAM134B-mediated ER membrane fragmentation?

Several complementary assays can be used to measure FAM134B-mediated ER membrane fragmentation:

• In vitro liposome fragmentation assay:

  • Recombinant FAM134B protein is incubated with artificial liposomes

  • Resulting membrane structures are visualized by electron microscopy

  • This assay directly measures FAM134B's membrane remodeling capacity

• Cellular ER fragmentation assay:

  • Cells expressing EGFP-FAM134B are examined by fluorescence microscopy

  • Fragmentation is quantified by counting EGFP-FAM134B-positive puncta

  • This can be done in both unstimulated and ER stress-induced conditions (e.g., with thapsigargin)

• Correlative light and electron microscopy (CLEM):

  • Combines fluorescence microscopy with electron microscopy

  • Allows precise localization of FAM134B in relation to ER fragments and autophagic structures

• Immunoelectron microscopy (IEM):

  • Directly visualizes FAM134B localization at the ultrastructural level

  • Can confirm that FAM134B-labeled structures are authentic ER fragments

• Co-localization analysis:

  • FAM134B co-localization with ER markers (e.g., BAP31)

  • Co-localization with autophagy markers (LAMP2, LC3)

  • Confirms that FAM134B-positive structures are indeed fragmented ER undergoing autophagy

These assays provide complementary information about FAM134B's function and should be selected based on specific research questions.

How can researchers effectively study the interaction between FAM134B and autophagy machinery?

Studying the interaction between FAM134B and the autophagy machinery requires multiple approaches:

• Co-immunoprecipitation (co-IP):

  • Pull-down of FAM134B followed by western blotting for autophagy proteins (LC3, GABARAP)

  • Reciprocal co-IP pulling down autophagy proteins and blotting for FAM134B

  • This confirms direct protein-protein interactions

• Fluorescence co-localization:

  • Co-expression of fluorescently tagged FAM134B and autophagy proteins

  • Live-cell imaging to track dynamic interactions

  • Quantification of co-localization coefficients

• Proximity ligation assay (PLA):

  • Detects protein interactions that occur within 40 nm

  • Provides spatial resolution of interactions within cells

  • Can detect endogenous protein interactions

• Biochemical binding assays:

  • Surface plasmon resonance (SPR) or isothermal titration calorimetry (ITC) with purified recombinant proteins

  • Determine binding affinities and kinetics

  • Assess how modifications (acetylation) affect binding parameters

• Functional autophagy assays:

  • LC3-II conversion assays in the presence/absence of FAM134B

  • Autophagic flux measurements using tandem fluorescent-tagged LC3

  • ER-specific autophagy cargo degradation assays

• Structure-function studies:

  • Mutational analysis of the LC3-interacting region (LIR) in FAM134B

  • Assess how modifications affect LIR-mediated interactions

These methodologies together provide a comprehensive understanding of how FAM134B engages with the autophagy machinery to facilitate ER-phagy.

What are the considerations for using recombinant bovine FAM134B as a model for human FAM134B function?

When using recombinant bovine FAM134B as a model for human FAM134B function, several considerations are important:

• Sequence homology and conservation:

  • Bovine and human FAM134B share significant sequence homology

  • Key functional domains (Reticulon domain, LC3-interacting region) are highly conserved

  • Post-translational modification sites may differ and should be verified

• Structural considerations:

  • Recombinant bovine FAM134B (amino acids 1-356) includes the critical functional domains

  • Protein folding and oligomerization properties should be compared to human FAM134B

  • Bovine protein may have species-specific structural features

• Functional validation:

  • Parallel experiments with human and bovine proteins to confirm conserved functions

  • Cross-species complementation experiments (e.g., rescue of human FAM134B knockout cells with bovine FAM134B)

  • Verification that key interactions with partner proteins are conserved

• Post-translational modifications:

  • Confirm that regulatory modifications occur at equivalent sites

  • Verify that modifying enzymes (CBP, SIRT7) recognize both species' proteins

  • Assess whether modification-dependent functions are conserved

• Expression system considerations:

  • E. coli-expressed bovine FAM134B lacks mammalian post-translational modifications

  • Consider mammalian expression systems for studies focusing on regulated function

  • Codon optimization may be necessary for efficient bacterial expression

By addressing these considerations, researchers can maximize the translational relevance of findings from studies using recombinant bovine FAM134B to human physiology and pathology.