Recombinant Human Ion channel TACAN (TMEM120A)

What is TACAN and what is its relationship to TMEM120A?

TACAN (Transmembrane Activity-regulated Channel of Nociceptors) and TMEM120A refer to the same protein. TMEM120A was initially identified as a nuclear envelope transmembrane protein (NET) and was originally named NET29 . Later research proposed this protein functions as a mechanosensitive ion channel involved in sensing mechanical pain, leading to the alternative name TACAN . The protein is highly conserved in vertebrates and has been implicated in adipocyte differentiation in earlier studies .

What is the basic structural organization of TACAN/TMEM120A?

The cryo-EM structure of human TMEM120A shows it forms a tightly packed homodimer with extensive interactions. Each TMEM120A subunit can be divided into two domains:

• N-terminal coiled-coil domain (CCD) containing CC1 and CC2 helices

• C-terminal transmembrane domain (TMD) containing six membrane-spanning helices (H1-H6) that form an α-helical barrel

These domains are connected by a membrane-penetrating re-entrant loop with a short helix. Both the N-terminus and C-terminus are located on the intracellular side of the membrane . When viewed from the cytosolic side, the long axes of CCD and TMD form an angle of approximately 50° .

Why is there disagreement about TACAN's role as a mechanosensitive ion channel?

The function of TACAN has become a subject of significant scientific controversy. The contrasting findings include:

Evidence supporting mechanosensitive channel role:

• TACAN is expressed in a subset of nociceptors

• Heterologous expression increases mechanically evoked currents in cell lines

• Purification and reconstitution in synthetic lipids generates a functional ion channel

• Nociceptor-specific inducible knockout decreases mechanosensitivity and reduces behavioral responses to painful mechanical stimuli

Evidence challenging mechanosensitive channel role:

• Cellular patch-recording methods failed to identify mechanosensitive ion channel activity

• At high protein concentrations in membrane reconstitution, TACAN produces heterogeneous conduction levels that are not mechanosensitive

• These conduction properties are most consistent with disruptions of the lipid bilayer rather than ion channel activity

• The structure lacks a discernible ion conduction pathway in the TMD

• Structural features resemble a fatty acid elongase (ELOVL7), suggesting a potential role in fatty acid metabolism

This controversy highlights the importance of using multiple complementary approaches when investigating ion channel properties and functions.

What methodological factors might explain the divergent findings on TACAN's function?

The discrepancies in findings may stem from several methodological considerations:

• Protein concentration effects: At high protein-to-lipid ratios (≥1:100, m:m), TACAN could disrupt membrane integrity, producing transient currents that might be misinterpreted as channel activity

• Recording conditions: Different electrophysiological approaches (patch clamp vs. bilayer recordings) may yield different results

• Expression systems: Heterologous expression may produce different results than studying the native protein

• Protein preparation: Differences in purification methods could affect protein conformation and function

• Knockout specificity: The effects of TACAN knockout on mechanosensitivity could potentially involve indirect mechanisms

Researchers should carefully consider these factors when designing experiments to investigate TACAN's function.

What is the significance of the coenzyme A binding in TACAN?

The cryo-EM structure of TACAN revealed a bound coenzyme A (CoA) molecule within each subunit . CoA is a critical cofactor for many metabolic processes, particularly those involving fatty acid metabolism. Key details about this binding include:

• The CoA molecule is bound in a deep pocket formed by the six transmembrane helices

• This pocket is only open to the inside (cytosolic side) and completely sealed off from the outside

• The presence of CoA was confirmed by mass spectrometry

• The CoA binding, combined with structural similarities to ELOVL7, suggests TACAN may function as an enzyme in fatty acid metabolism rather than as an ion channel

This finding has significantly influenced the ongoing debate about TACAN's primary biological function.

How does TACAN's structure compare to known ion channels and other membrane proteins?

TACAN's structure differs significantly from canonical ion channels:

• No clear channel pore: The TMD of each TMEM120A subunit contains six transmembrane helices (TMs) but has no clear structural feature of a channel protein

• Structural similarity to ELOVL: TACAN's protomer is related in three-dimensional structure to a fatty acid elongase (ELOVL7), despite low sequence homology

• Dimeric organization: TACAN forms a tightly packed dimer with extensive interactions mediated by the N-terminal coiled coil domain, the C-terminal transmembrane domain, and the re-entrant loop

• α-barrel structure: The six TMs form an α-barrel with a deep pocket where a CoA molecule is bound

These structural features align more closely with enzymatic membrane proteins than with typical ion channels, which generally have a clear aqueous pore for ion conduction.

What are the optimal methods for expressing and purifying recombinant TACAN for structural and functional studies?

Based on successful studies, an effective protocol for TACAN expression and purification includes:

• Expression system: Human TMEM120A can be expressed in HEK293F cells using the BacMam system

• Solubilization: The protein should be solubilized in lauryl maltose neopentyl glycol (LMNG) detergent

• Purification: Final purification in digitonin detergent preserves the dimeric state of the protein

• Quality control: Size-exclusion chromatography can verify protein homogeneity and dimeric state

• Functional verification: Reconstitution into liposomes or nanodiscs for functional assays

This approach has yielded sufficient quantities of homogeneous protein for both structural and functional studies.

What techniques are most appropriate for studying TACAN's potential mechanosensitive properties?

To rigorously investigate TACAN's potential mechanosensitive properties, researchers should consider multiple complementary approaches:

• Cellular patch-clamp recording:

  • Cell-attached or excised patch configurations with controlled pressure application

  • Comparison with known mechanosensitive channels (e.g., TRAAK channels) using identical experimental setups

• Membrane reconstitution methods:

  • Giant unilamellar vesicles (GUVs) with controlled protein-to-lipid ratios

  • Planar lipid bilayers with precise control of membrane tension

  • Careful assessment of protein concentration effects to avoid membrane disruption artifacts

• Force application methods:

  • Pressure application to patches

  • Membrane stretching

  • Pillar-based mechanical stimulation

• In vivo validation:

  • Tissue-specific knockouts

  • Behavioral assessments

  • Primary sensory neuron recordings

Using multiple approaches is crucial given the controversial nature of TACAN's mechanosensitive properties.

How can researchers address the discrepancy between structural and functional data for TACAN?

To reconcile the conflicting data on TACAN's function, consider these research strategies:

• Structure-guided mutagenesis:

  • Target residues in the putative CoA binding site to assess impact on both enzymatic and channel activities

  • Mutate regions that distinguish TACAN from established ion channels

  • Examine the effect of mutations on the proposed mechanosensitivity

• Reconstitution systems with controlled variables:

  • Systematically vary protein-to-lipid ratios to establish the threshold at which membrane disruption occurs

  • Test TACAN function in various lipid compositions that might affect mechanosensitivity

  • Compare direct TACAN reconstitution with established ion channels in identical systems

• Multidisciplinary approaches:

  • Combine electrophysiology with fluorescence-based assays for conformational changes

  • Utilize computational molecular dynamics to model membrane interactions

  • Employ high-speed atomic force microscopy to directly visualize conformational changes

• Physiological context:

  • Examine TACAN in its native cellular environment with endogenous expression levels

  • Investigate potential interactions with other proteins that might modulate its function

What is the significance of TACAN's structural similarity to fatty acid elongases?

The structural similarity between TACAN and elongases for very long-chain fatty acids (ELOVL) despite low sequence homology raises important research questions:

• Enzymatic activity assessment:

  • Does TACAN possess elongase activity or similar enzymatic functions?

  • What are the potential substrates beyond CoA?

  • How does TACAN interact with fatty acid metabolism pathways?

• Evolutionary relationship:

  • Perform phylogenetic analysis to determine if TACAN evolved from ELOVLs

  • Investigate intermediate proteins that might share features of both families

  • Compare TACAN across species to identify conserved functional domains

• Dual functionality hypothesis:

  • Could TACAN function both as an enzyme and as a mechanosensitive element?

  • Might enzymatic activity be regulated by mechanical stimuli?

  • Could TACAN be part of a larger mechanosensitive complex?

• Structural comparisons:

  • Detailed comparison of active sites between TACAN and ELOVLs

  • Analysis of binding pockets and substrate specificity determinants

  • Investigation of conformational changes upon substrate binding

This similarity provides a compelling alternative hypothesis for TACAN's primary function that merits thorough investigation.

What are the key experimental parameters for cryo-EM studies of TACAN?

The following table summarizes the experimental parameters used in successful cryo-EM studies of TACAN:

Researchers aiming to reproduce or extend these structural studies should consider these parameters as a starting point for their experimental design .

What electrophysiological properties have been reported for TACAN?

The electrophysiological characterization of TACAN has yielded varied results across different experimental systems:

These varied results highlight the challenges in characterizing the electrophysiological properties of TACAN and underscore the importance of experimental context.

How does the tissue distribution of TACAN inform our understanding of its function?

The tissue distribution of TACAN provides important clues about its potential biological roles:

• Nociceptors: TACAN is expressed in a subset of nociceptors, consistent with a potential role in mechanical pain sensing

• Adipose tissue: TACAN is preferentially expressed in adipose tissue and plays an important role in adipocyte differentiation, suggesting metabolic functions

• Nuclear envelope: Initially identified as a nuclear envelope transmembrane protein (NET), raising questions about nuclear functions

Researchers should consider these diverse expression patterns when designing functional studies and interpreting results. The multi-tissue distribution might reflect different functions in different cellular contexts or a common biochemical function with tissue-specific consequences.

What are the most pressing unresolved questions about TACAN that researchers should address?

Several critical questions remain unresolved and should be prioritized in future research:

• Definitive functional identity:

  • What is TACAN's primary biological function: ion channel, enzyme, or both?

  • If it has enzymatic activity, what are its substrates and products?

  • If it functions as an ion channel, what are the gating mechanisms?

• Structural dynamics:

  • Does TACAN undergo conformational changes in response to mechanical stimuli?

  • What is the functional significance of its dimeric structure?

  • How does CoA binding influence protein function and dynamics?

• Physiological roles:

  • What is TACAN's precise role in nociception, if any?

  • How does it contribute to adipocyte differentiation and function?

  • Are there other physiological processes that involve TACAN?

• Disease relevance:

  • Is TACAN dysregulation associated with pain disorders?

  • Could it be involved in metabolic diseases given its expression in adipose tissue?

  • Does it represent a viable therapeutic target?

Addressing these questions will require multidisciplinary approaches combining structural biology, electrophysiology, biochemistry, and in vivo studies.

What novel methodological approaches might help resolve the controversy about TACAN's function?

Innovative approaches that could help resolve the functional controversy include:

• Advanced imaging techniques:

  • Single-molecule FRET to detect conformational changes in response to mechanical stimuli

  • Super-resolution microscopy to determine precise subcellular localization

  • Live-cell imaging to track dynamics in response to various stimuli

• Proteomics and interactomics:

  • Identify TACAN's interaction partners in different tissues

  • Characterize post-translational modifications that might regulate function

  • Investigate potential protein complexes that might include TACAN

• Metabolomics:

  • Assess changes in metabolite profiles upon TACAN manipulation

  • Focus on fatty acid metabolism given structural similarities to ELOVLs

  • Employ stable isotope labeling to track potential enzymatic activities

• Advanced genetic models:

  • Develop conditional and inducible knockout models with tissue-specific targeting

  • Create knock-in models with mutations affecting specific functions

  • Employ CRISPR-based approaches for precise genetic manipulation

• Computational approaches:

  • Molecular dynamics simulations to model mechanical force transmission

  • Virtual screening for potential ligands beyond CoA

  • Machine learning to identify patterns in experimental data across studies

These approaches, especially when used in combination, could provide comprehensive insights into TACAN's true biological function.