Recombinant Mouse Transmembrane protein 120A (Tmem120a)

What is the basic structure of mouse Tmem120a protein?

Mouse Tmem120a shares structural homology with human TMEM120A, which forms a symmetrical homodimer. Each monomer consists of:

• An N-terminal soluble domain (NTD) on the cytosolic side containing two α-helices (intracellular helices 1 and 2, IH1 and IH2)

• A transmembrane domain (TMD) with six membrane-spanning helices (TM1-6) forming an asymmetric funnel

• A hinge-like motif (HM) between NTD and TMD containing two long loops and a short amphipathic α-helix

What are the known synonyms and alternative names for Tmem120a?

Tmem120a is known by several alternative names in scientific literature:

• NET29

• TMPIT

• RGD1311474 (in rat)

• Transmembrane protein induced by tumor necrosis factor alpha

• T120A_HUMAN (for the human ortholog)

These alternative designations may appear in different databases and publications, making it important for researchers to recognize these synonyms when conducting literature searches .

What are the proposed functions of Tmem120a?

Tmem120a has been associated with several potential functions:

• Adipocyte differentiation and metabolism regulation

• Previously proposed as a mechanosensitive ion channel (TACAN) involved in sensing mechanical pain

• Possible role as a membrane-embedded enzyme utilizing CoASH as a substrate or cofactor

• Potential function as a CoASH transporter

• Possible role as a CoASH-sensing receptor

• In C. elegans, TMEM120 homologs have been linked to incorporation of fatty acids into triacylglycerol

Recent research has questioned the ion channel function, as expression of TMEM120A was not sufficient to mediate poking- or stretch-induced currents in controlled experimental settings .

How does the CoASH binding affect Tmem120a structure and function?

Cryo-EM structural studies have revealed that TMEM120A exhibits distinct conformations depending on whether CoASH is bound or not. The CoASH molecule is hosted in a deep cavity within the transmembrane domain and forms specific interactions with nearby amino acid residues.

A central tryptophan residue plays a crucial role in CoASH binding; mutation of this residue dramatically reduces the binding affinity between TMEM120A and CoASH. If TMEM120A does function as an ion channel, the CoASH molecule may serve as a plug that blocks the intracellular entry, stabilizing the channel in a closed state. Channel activation might occur upon dissociation of the CoASH molecule, allowing the intracellular cavity to become accessible to ions .

• Regulation of a potential enzymatic function

• Modulation of a transport mechanism

• Signaling through conformational changes upon CoASH binding or release

What is the current evidence regarding Tmem120a's proposed mechanosensitive ion channel function?

The mechanosensitive ion channel function of Tmem120a (proposed name: TACAN) has become controversial based on recent findings:

Supporting evidence:

• Initial reports suggested heterologously expressed TMEM120A mediated a picoampere-level increase in stretch-induced currents

• TMEM120A appeared to be activated via pillar-based mechanical stimulation at the cell-substrate interface

• Reconstituted TMEM120A proteins reportedly mediated spontaneous channel activities with an estimated single-channel conductance of ~250 pS

Contradicting evidence:

• Under controlled experimental conditions, cells transfected with either mouse TMEM120A or human TMEM120A failed to show poking-induced currents

• Stretch-induced currents were not observable in TMEM120A-expressing cells under cell-attached or inside-out patch-clamp configurations

• The reported single-channel conductance differs dramatically between cellular measurements (11.5 pS) and reconstituted lipid membranes (~250 pS)

• When purified TMEM120A protein was reconstituted into giant unilamellar vesicles (GUVs), pressure-dependent channel activities were rarely observed during single-channel recording

Current consensus suggests that TMEM120A is not sufficient to form a channel capable of sensing poking or stretch of the cell membrane, though it might potentially respond to specific forms of mechanical stimuli such as perturbation at the cell-substrate interface .

How does Tmem120a relate to other membrane proteins structurally and functionally?

Tmem120a shares structural similarities with membrane-embedded enzymes called elongation of very long chain fatty acid (ELOVL) proteins. ELOVLs catalyze condensation reactions between acyl-CoA and malonyl-CoA to produce 3-keto acyl-CoA while releasing CoASH and CO₂ as byproducts.

Despite these structural similarities, functional differences exist:

• TMEM120A has not exhibited ELOVL-like enzymatic activity when tested with malonyl-CoA and stearoyl-CoA as substrates

• TMEM120A may utilize different substrates or catalyze reactions distinct from ELOVLs

• TMEM120A appears to have evolved a specific CoASH-binding mechanism different from typical acyl-CoA utilizing enzymes

Additionally, TMEM120A has been reported to potentially interact with the mechanosensitive ion channel Piezo2. Co-expression of TMEM120A with Piezo2 reduced mechanically activated currents, suggesting TMEM120A may function as a negative modulator of Piezo2 channel activity, possibly by modifying the lipid content of the cell .

What expression systems are recommended for recombinant Tmem120a production?

Based on successful structural and functional studies, the following expression systems have proven effective for recombinant Tmem120a production:

Insect cell expression system:

• Spodoptera frugiperda Sf9 insect cells are effective for producing human and mouse TMEM120A

• Expression can be optimized using the following protocol:

  • Clone the codon-optimized full-length TMEM120A cDNA into pFastBac Dual vector

  • Add N-terminal tags (e.g., FLAG followed by twin-Strep-tags) for purification

  • Generate recombinant bacmid in DH10Bac cells identified by blue/white selection

  • Amplify the baculovirus through three generations

  • Culture Sf9 cells at 27°C for 24 hours followed by 20°C for 48 hours before harvesting

This approach has been validated for producing protein suitable for cryo-EM structural studies, isothermal titration calorimetry (ITC), and single-channel electrophysiology experiments .

Mammalian cell expression:

• HEK293T cells have been used to express TMEM120A for functional studies

• When investigating mechanosensitive properties, Piezo1 knockout HEK293T cells (P1-KO-HEK) are recommended to exclude any endogenous Piezo1-mediated mechanically activated currents

The choice of expression system should align with the specific experimental goals, with insect cells generally preferred for structural studies and mammalian cells for functional analyses in a more native-like environment .

What methods can be used to assess the CoASH binding properties of Tmem120a?

Several complementary approaches can be employed to investigate the CoASH binding properties of Tmem120a:

Isothermal Titration Calorimetry (ITC):

• Directly measures the thermodynamic parameters of CoASH binding

• Can determine binding affinity (Kd), stoichiometry, and thermodynamic parameters (ΔH, ΔS)

• Requires purified protein in detergent or reconstituted in nanodiscs

Mutagenesis studies:

• Strategic mutation of residues involved in CoASH binding (particularly the central tryptophan residue) can validate the binding site

• Mutants can be tested for CoASH binding using ITC or functional assays

• Comparison of wild-type and mutant binding properties can reveal the contribution of specific residues

Structural studies:

• Cryo-EM analysis of Tmem120a with and without CoASH can reveal conformational changes

• Comparison of apo and CoASH-bound structures provides insights into binding mechanisms

• Analysis of electron density maps can identify the precise CoASH binding pocket

These approaches should be used in combination to develop a comprehensive understanding of the CoASH binding properties and their functional implications .

What electrophysiological techniques are appropriate for evaluating potential ion channel activity of Tmem120a?

To thoroughly evaluate the potential ion channel activity of Tmem120a, researchers should consider multiple electrophysiological approaches:

Whole-cell patch-clamp recording:

• Useful for measuring poking-induced currents

• Use a piezo-driven blunted glass pipette to mechanically probe the cell membrane

• Record from cells expressing Tmem120a versus appropriate controls (vector-transfected cells)

• Include positive controls (e.g., cells expressing known mechanosensitive channels like Piezo1 or Piezo2)

Cell-attached patch-clamp recording:

• Appropriate for measuring stretch-induced currents

• Apply negative pressure from 0 to −120 mmHg to the membrane patch

• Maintains cellular integrity and preserves cytosolic factors

Inside-out patch-clamp recording:

• Alternative approach for measuring stretch-induced currents

• Allows direct access to the intracellular side of the membrane

• Useful for testing the effects of intracellular factors on channel activity

Reconstitution in artificial membranes:

• Purify Tmem120a and reconstitute into giant unilamellar vesicles (GUVs)

• Perform single-channel recordings to detect spontaneous or pressure-dependent channel activities

• Can help determine if Tmem120a alone is sufficient for channel formation

Pillar-based mechanical stimulation:

• Apply mechanical stimuli at the cell-substrate interface

• May detect channel activities that are not observable with conventional patch-clamp techniques

Using multiple approaches is critical for comprehensive evaluation, as different methodologies may reveal distinct aspects of channel function .

How can researchers reconcile the contradictory findings regarding Tmem120a's mechanosensitive properties?

The contradictory findings regarding Tmem120a's mechanosensitive properties present a complex challenge. Researchers can approach this contradiction systematically through:

Critical experimental comparison:

• Examine methodological differences between studies reporting positive and negative results:

  • Cell types and expression systems used

  • Recording configurations and stimulation protocols

  • Presence of endogenous mechanosensitive channels

  • Environmental factors (temperature, ionic conditions)

• Consider the possibility of context-dependent function:

  • TMEM120A might require specific lipid environments or cofactors

  • Mechanosensitivity might depend on post-translational modifications

  • Interaction with accessory proteins might be necessary

• Evaluate signal-to-noise considerations:

  • The reported picoampere-level currents are near the detection limit

  • Distinguish between specific TMEM120A-mediated currents and background fluctuations

• Address the disparity in single-channel conductance:

  • Reconcile the dramatic difference between cellular measurements (11.5 pS) and reconstituted systems (~250 pS)

  • Consider whether the reconstituted activities might represent a different molecular entity

• Explore alternative stimulation modalities:

  • TMEM120A might respond specifically to pillar-based mechanical stimulation at the cell-substrate interface

  • Different forms of mechanical stimuli might activate the channel through distinct mechanisms

By systematically addressing these considerations, researchers can develop more nuanced hypotheses about TMEM120A's true functional role .

How should researchers interpret the structural data on Tmem120a in relation to its potential functions?

The structural data on Tmem120a provides valuable insights that can guide functional interpretation:

Structural features with functional implications:

• CoASH binding pocket:

  • The deep cavity hosting CoASH suggests a specific interaction rather than non-specific binding

  • The conserved tryptophan residue critical for CoASH binding indicates evolutionary selection

  • The distinct conformations observed with and without CoASH bound suggest functional relevance

• Homodimeric assembly:

  • The symmetrical homodimer structure with intertwined N-terminal domains suggests cooperative function

  • The interface between monomers could serve as a regulatory site

• Transmembrane domain architecture:

  • The asymmetric funnel with a wide intracellular opening and extracellular bottleneck resembles transporter structures

  • The pore profile calculated using HOLE program indicates a constriction that may regulate passage of molecules

• Structural similarity to ELOVL proteins:

  • Suggests potential enzymatic functions, possibly distinct from ELOVLs

  • May indicate evolution from a common ancestor with divergent functions

When interpreting these structural features, researchers should:

• Consider multiple potential functions rather than forcing data to fit a single hypothesis

• Design experiments to directly test structure-based functional predictions

• Compare structural features across species to identify conserved elements

• Develop structure-guided mutations to test specific functional hypotheses

• Integrate structural data with metabolic and physiological contexts

The structural data currently supports potential roles in CoASH transport, sensing, or metabolism more strongly than mechanosensitive channel function .

What are the implications of Tmem120a's potential role in lipid metabolism for experimental design?

Considering Tmem120a's potential involvement in lipid metabolism has significant implications for experimental design:

Key experimental considerations:

• Lipid environment:

  • Experiments should control for membrane lipid composition

  • Different lipid environments may dramatically affect protein function

  • Consider using defined lipid compositions in reconstitution experiments

• Metabolic state of cells:

  • The nutritional state and metabolic activity of experimental cells may influence results

  • Document and control feeding protocols, glucose concentrations, and serum levels

  • Consider conducting experiments under different metabolic conditions (fed vs. fasted)

• Adipocyte-specific studies:

  • Given Tmem120a's role in adipocyte differentiation, adipocyte models are particularly relevant

  • Consider using adipocyte differentiation protocols with mouse 3T3-L1 cells

  • Compare results between preadipocytes and mature adipocytes

• Enzymatic activity assays:

  • Design assays to test potential enzymatic activities related to CoA metabolism

  • Consider testing varied substrates beyond those used for ELOVLs

  • Monitor lipid profiles and CoA-related metabolites in Tmem120a knockout vs. wild-type cells

• Interaction with metabolic pathways:

  • Investigate Tmem120a's interaction with key lipid metabolism pathways

  • Consider its reported role in C. elegans for incorporating fatty acids into triacylglycerol

  • Measure triacylglycerol levels and lipid droplet size in response to Tmem120a manipulation

• Coordination with other proteins:

  • Investigate interaction with proteins identified through pull-down studies (GOT1, PPP1CC, proP24)

  • These interactions may provide clues to metabolic functions

By accounting for these metabolic considerations, researchers can design more informative experiments that may resolve current contradictions and reveal Tmem120a's true physiological functions .

What are the recommended protocols for purification of recombinant mouse Tmem120a?

Based on successful structural and functional studies, the following purification protocol is recommended for recombinant mouse Tmem120a:

• Cell lysis and membrane preparation:

  • Harvest cells expressing tagged Tmem120a (N-terminal FLAG and twin-Strep-tags recommended)

  • Resuspend in lysis buffer containing protease inhibitors

  • Disrupt cells using a homogenizer and remove debris by centrifugation

  • Isolate membranes by ultracentrifugation

• Solubilization:

  • Solubilize membranes using appropriate detergents (lauryl maltose neopentyl glycol has been successful)

  • Maintain sample at 4°C during solubilization

  • Remove insoluble material by ultracentrifugation

• Affinity purification:

  • Apply solubilized material to StrepTactin resin

  • Wash extensively to remove non-specific binding

  • Elute using desthiobiotin-containing buffer

• Size exclusion chromatography:

  • Further purify using size exclusion chromatography

  • Monitor protein quality by SDS-PAGE and Western blotting

  • Assess protein monodispersity by analytical size exclusion

• Reconstitution options:

  • For structural studies: reconstitute into lipid nanodiscs

  • For functional studies: reconstitute into liposomes or giant unilamellar vesicles (GUVs)

This protocol can be adapted based on specific experimental requirements and has been validated for producing protein suitable for cryo-EM structural studies and functional assays .

What approaches are effective for studying Tmem120a's role in adipocyte differentiation?

To investigate Tmem120a's role in adipocyte differentiation, researchers should consider the following methodological approaches:

• Cell culture models:

  • 3T3-L1 preadipocyte differentiation system (mouse)

  • Primary stromal vascular fraction (SVF) cells isolated from adipose tissue

  • Human SGBS (Simpson-Golabi-Behmel syndrome) preadipocytes

• Gene manipulation strategies:

  • CRISPR/Cas9-mediated knockout of Tmem120a

  • siRNA or shRNA-mediated knockdown for temporal studies

  • Overexpression systems with wild-type and mutant (particularly CoASH-binding deficient) variants

  • Inducible expression systems to control timing of expression

• Differentiation assessment:

  • Oil Red O staining to visualize and quantify lipid accumulation

  • Expression analysis of adipogenic markers (PPARγ, C/EBPα, FABP4, adiponectin)

  • Lipidomic profiling to assess changes in lipid composition

  • Metabolic assays (glucose uptake, lipolysis, de novo lipogenesis)

• Localization studies:

  • Immunofluorescence to track Tmem120a localization during differentiation

  • Co-localization with organelle markers (ER, Golgi, lipid droplets)

  • Live-cell imaging with fluorescently tagged Tmem120a

• Molecular interaction studies:

  • Proximity labeling approaches (BioID, APEX) to identify interaction partners

  • Co-immunoprecipitation to validate specific interactions

  • Yeast two-hybrid screening for potential binding partners

• In vivo approaches:

  • Adipose-specific Tmem120a knockout mice

  • Metabolic phenotyping (glucose tolerance, insulin sensitivity)

  • Histological analysis of adipose tissue development

These approaches should be used in combination to develop a comprehensive understanding of Tmem120a's role in adipocyte differentiation and function .

What structural biology techniques have proven most effective for studying Tmem120a?

Cryo-electron microscopy (cryo-EM) has emerged as the most effective structural biology technique for studying Tmem120a, with the following specific approaches yielding valuable insights:

• Cryo-EM of protein reconstituted in nanodiscs:

  • Provides a more native-like lipid environment

  • Has achieved resolution of 3.7 Å for human TMEM120A

  • Revealed detailed structural features including the CoASH binding site

  • Shows the protein in a lipid bilayer context

• Cryo-EM of detergent-solubilized protein:

  • Complementary approach that can reveal different conformational states

  • Useful for comparison with nanodisc structures to identify lipid-dependent features

• Single-particle analysis workflow:

  • Motion correction of movie stacks using MotionCor2

  • CTF estimation using CTFFIND4

  • Particle picking using Gautomatch

  • 2D classification, 3D classification, and 3D refinement using RELION

  • Post-processing with appropriate masking and sharpening

• Model building and refinement:

  • Initial model building in COOT guided by secondary structure prediction

  • Real-space refinement using phenix.real_space_refine

  • Validation using MolProbity

• Computational analysis of structures:

  • Pore profile calculation using HOLE program

  • Electrostatic potential surface calculations using ChimeraX or APBS

  • Structural comparisons with related proteins

These approaches have successfully revealed the homodimeric assembly, transmembrane domain architecture, and specific CoASH binding site of TMEM120A, providing critical insights into its potential functions .

What are the most promising avenues for resolving the functional role of Tmem120a?

Based on current knowledge and contradictions in the literature, the following research directions hold promise for clarifying Tmem120a's functional role:

• Comprehensive metabolomic profiling:

  • Compare metabolite profiles in wild-type versus Tmem120a knockout cells

  • Focus on CoA derivatives and lipid metabolites

  • Conduct studies under different nutritional conditions

• Investigation of enzymatic activities:

  • Design assays based on the CoASH binding capability

  • Test various CoA-related substrates beyond those tested for ELOVL activity

  • Conduct structure-guided enzyme activity screens

• Tissue-specific knockout models:

  • Generate adipose-specific and sensory neuron-specific knockout mice

  • Perform comprehensive metabolic and sensory phenotyping

  • Analyze tissue-specific effects on lipid metabolism

• High-resolution structural studies of different states:

  • Capture additional conformational states of the protein

  • Compare structures with various bound ligands

  • Conduct time-resolved structural studies if possible

• Investigation of protein-protein interactions:

  • Study the reported interaction with Piezo2 in detail

  • Explore interactions with metabolic enzymes

  • Investigate potential partnerships with lipid transport proteins

• Membrane microdomain localization:

  • Determine if Tmem120a localizes to specific membrane microdomains

  • Investigate how membrane composition affects its function

  • Study potential role in organizing membrane domains

• Single-molecule studies:

  • Apply single-molecule fluorescence techniques to study dynamics

  • Investigate conformational changes upon CoASH binding

  • Examine potential transport activities at the single-molecule level

By pursuing these complementary approaches, researchers can develop a more unified understanding of Tmem120a's true physiological functions .

How can structure-function relationships be better exploited to understand Tmem120a biology?

To better leverage structure-function relationships for understanding Tmem120a biology, researchers should consider:

• Strategic mutagenesis approaches:

  • Create mutations in the CoASH binding pocket to disrupt ligand binding

  • Modify the dimer interface to assess the importance of dimerization

  • Introduce cross-linking residues to trap specific conformational states

  • Perform alanine-scanning mutagenesis of conserved residues

• Chimeric protein design:

  • Create chimeras between Tmem120a and related proteins (e.g., TMEM120B)

  • Swap domains to identify functional modules

  • Engineer fusion proteins to probe subcellular localization requirements

• Molecular dynamics simulations:

  • Simulate protein behavior in various membrane environments

  • Study the dynamics of CoASH binding and release

  • Identify potential conformational changes relevant to function

• Structure-guided antibody development:

  • Design antibodies against specific structural epitopes

  • Create conformation-specific antibodies to detect different states

  • Use antibodies as tools to lock the protein in specific conformations

• Small molecule screening:

  • Design structure-based virtual screening for potential ligands

  • Develop assays to identify modulators of CoASH binding

  • Screen for compounds that alter Tmem120a's conformation

• In situ structural studies:

  • Apply emerging techniques like cryo-electron tomography

  • Study Tmem120a structure in its native cellular environment

  • Correlate structural features with cellular localization