Recombinant Mouse Protein FAM173A (Fam173a)

What is FAM173A and what is its function in mitochondria?

FAM173A (also known as ANTKMT) is a mitochondrial lysine-specific methyltransferase that primarily targets adenine nucleotide translocases (ANT), specifically ANT2 and ANT3, trimethylating them at Lys-52 . The protein localizes to mitochondria through a noncanonical targeting sequence and contains a methyltransferase domain characteristic of the seven-β-strand (7BS) methyltransferase family . Functionally, FAM173A regulates mitochondrial respiration, with FAM173A-deficient cells displaying significantly increased oxygen consumption rates compared to FAM173A-proficient cells .

How does mouse Fam173a differ from human FAM173A?

Mouse Fam173a shares high sequence homology with human FAM173A, particularly in the catalytic methyltransferase domain. Both proteins function similarly as mitochondrial lysine methyltransferases targeting ANT proteins. The mouse protein is encoded by the Fam173a gene located on chromosome 16 and comprises 229 amino acids, while the human counterpart consists of 235 amino acids . Functionally conserved regions include the methyltransferase domain with the characteristic 7BS topology and the mitochondrial targeting sequence .

What experimental models are appropriate for studying Fam173a function?

Several experimental models have proven effective for studying Fam173a function:

For optimal results, researchers should select models based on their specific research questions, with cell lines offering controlled conditions for mechanistic studies, while in vivo models provide physiological relevance .

How should researchers design experiments to investigate Fam173a methyltransferase activity?

To investigate Fam173a methyltransferase activity, researchers should implement a multi-faceted approach:

• In vitro methyltransferase assays: Incubate purified recombinant Fam173a with [³H]-S-adenosyl-L-methionine (SAM) as methyl donor and potential substrate proteins. Analyze methylation by fluorography or mass spectrometry .

• Site-directed mutagenesis: Generate enzymatically inactive mutants (e.g., E105A mutation in the catalytic domain) to serve as negative controls .

• Substrate identification: Employ immunoblotting with methyllysine-specific antibodies to detect methylated proteins, followed by mass spectrometry for site identification .

• Complementation experiments: In Fam173a knockout cells, express wild-type or enzymatically dead Fam173a to confirm methyltransferase-dependent effects .

• Functional readouts: Measure mitochondrial respiration using Seahorse technology to assess oxygen consumption rates under various conditions (basal, ADP-stimulated, oligomycin-inhibited, and FCCP-uncoupled) .

The inclusion of appropriate controls (enzyme-dead mutants, substrate-free conditions) is essential for result validation .

What are the challenges in purifying active recombinant Fam173a and how can they be overcome?

Purifying active recombinant Fam173a presents several challenges due to its mitochondrial localization and membrane association:

For optimal activity, consider dual-tag systems (His and additional affinity tag) for two-step purification to enhance purity while maintaining native conformation .

How does Fam173a-mediated methylation of ANT affect mitochondrial function and what methods best measure these effects?

Fam173a-mediated methylation of ANT at Lys-52 significantly impacts mitochondrial function, particularly respiration. Research indicates that:

• Respiration effects: FAM173A-deficient cells show approximately 50% increase in State II and State III respiration compared to FAM173A-proficient cells, suggesting methylation normally restricts ADP/ATP exchange .

• Measurement methods:

  • Oxygen consumption rate (OCR): Use Seahorse XF Analyzer to measure different respiratory states (basal, ADP-stimulated, oligomycin-inhibited, FCCP-uncoupled) .

  • Mitochondrial membrane potential: Monitor with potentiometric dyes (TMRM, MitoTrackerRedCMXROS) .

  • ROS production: Assess using fluorescent indicators like dihydroethidium (DHE) or MitoSox .

• Experimental design: Compare FAM173A-proficient conditions (wild-type or complemented knockout) with FAM173A-deficient conditions (knockout or enzyme-dead mutant). Include controls for mitochondrial content by measuring levels of mitochondrial proteins (COXIV, ATPSc, ATP5A) .

Results suggest methylation of ANT reduces translocase activity, potentially limiting ADP/ATP exchange and thereby decreasing ATP synthesis and respiration in FAM173A-proficient mitochondria .

How does Fam173a compare to its paralogue Fam173b in structure and function?

Fam173a and Fam173b share significant structural similarities but target different substrates and have distinct functional implications:

What experimental approaches can distinguish the specific effects of Fam173a from other mitochondrial methyltransferases?

To distinguish specific Fam173a effects from other mitochondrial methyltransferases, researchers should employ several complementary approaches:

• Knockout specificity validation:

  • Generate single knockouts of Fam173a and other mitochondrial methyltransferases

  • Create double/multiple knockouts to identify compensatory or synergistic effects

  • Use rescue experiments with wild-type versus catalytically inactive mutants

• Substrate specificity assays:

  • Perform methyltransferase assays with purified substrates

  • Use point mutations in substrate lysine residues (e.g., K52R in ANT2/ANT3)

  • Conduct competitive assays with multiple methyltransferases and substrates

• PTM-specific detection methods:

  • Employ antibodies recognizing specific methylation patterns

  • Use mass spectrometry to identify methylation sites with residue-level resolution

  • Quantify methylation stoichiometry at specific sites

• Functional readouts:

  • Measure parameters altered by Fam173a deficiency (oxygen consumption, membrane potential)

  • Compare with effects of other methyltransferase knockouts

  • Assess cumulative effects in multiple knockout models

These approaches enable researchers to isolate Fam173a-specific effects while accounting for potential functional overlap with other mitochondrial methyltransferases .

What are the optimal conditions for expressing and purifying recombinant mouse Fam173a protein for in vitro studies?

For optimal expression and purification of recombinant mouse Fam173a:

Purification protocol:

• Cell lysis: Use gentle detergents (0.5% NP-40 or 1% Triton X-100) to solubilize membrane-associated protein

• IMAC purification: Ni-NTA resin with imidazole gradient (20-250mM)

• Size exclusion chromatography: Remove aggregates and ensure homogeneity

• Buffer optimization: Include stabilizers (5-10% glycerol, 1mM DTT) and cofactor (SAM)

• Activity verification: Methyltransferase assay with artificial (poly-lysine) or natural (ANT) substrates

For constructs lacking the transmembrane domain (Fam173aΔ42), higher yields and solubility can be achieved while maintaining catalytic activity .

How can researchers effectively assess the methylation status of ANT proteins in cells or tissues?

To effectively assess ANT protein methylation status:

• Immunoblotting approach:

  • Use methyllysine-specific antibodies to detect methylated proteins in mitochondrial extracts

  • Compare results between wild-type and Fam173a-knockout samples

  • Quantify band intensity to estimate methylation levels

• Mass spectrometry analysis:

  • Isolate mitochondrial membrane proteins by SDS-PAGE

  • Digest gel bands corresponding to ~32 kDa with chymotrypsin

  • Identify methylated peptides by LC-MS/MS

  • Calculate methylation stoichiometry by comparing modified vs. unmodified peptide intensities

• Experimental verification protocol:
  a. Prepare mitoplast extracts enriched in mitochondrial membranes
  b. Resolve proteins by SDS-PAGE
  c. For immunoblotting: Transfer proteins to membrane and probe with methyllysine antibodies
  d. For MS analysis: Excise bands, digest, and analyze by LC-MS/MS
  e. Confirm site-specificity through MS/MS fragmentation analysis

This combinatorial approach provides both qualitative (site identification) and quantitative (modification stoichiometry) information about ANT methylation status .

What are the potential therapeutic implications of modulating Fam173a activity in mitochondrial disorders?

The role of Fam173a in regulating mitochondrial respiration suggests several therapeutic implications:

• Mitochondrial disorders: Given that FAM173A deficiency increases mitochondrial respiration, inhibiting FAM173A could potentially benefit conditions characterized by respiratory chain deficiencies or reduced ATP production .

• Metabolic disorders: Modulating ANT methylation could influence ADP/ATP exchange rates, potentially affecting cellular energy homeostasis in metabolic conditions like diabetes or obesity .

• Neurodegenerative diseases: Since neurodegeneration often involves mitochondrial dysfunction, targeting FAM173A might mitigate energy deficits in conditions like Alzheimer's or Parkinson's disease .

• Therapeutic approaches:

  • Small molecule inhibitors targeting FAM173A methyltransferase activity

  • Gene therapy approaches to modulate FAM173A expression

  • Substrate peptide mimetics to compete with natural substrates

These associations highlight the potential of FAM173A as a target for therapeutic strategies aiming to mitigate the effects of these conditions by restoring proper mitochondrial function .

How might the evolutionary conservation of Fam173a across vertebrates inform its essential biological functions?

The evolutionary conservation of Fam173a exclusively in vertebrates provides important insights into its biological functions:

• Vertebrate-specific adaptation: Unlike its paralogue FAM173B (ATPSc-KMT) which is present across all metazoans, FAM173A emerged only in vertebrates, suggesting it evolved to address vertebrate-specific energy metabolism needs .

• Conservation analysis:

  • The preMT sequence responsible for mitochondrial localization is highly conserved across vertebrate species

  • The catalytic methyltransferase domain shows strong conservation, particularly around the SAM-binding motifs

  • The target lysine (K52) in ANT proteins is invariant across vertebrates

• Functional implications:

  • Conservation suggests an important role in fine-tuning energy metabolism in vertebrates

  • The exclusive trimethylation of Lys-52 in ANT across species indicates a constitutive rather than regulatory modification

  • The emergence in vertebrates may correlate with the increased energy demands of vertebrate physiology and homeothermy

This evolutionary pattern suggests Fam173a plays an essential role in vertebrate-specific energy metabolism regulation, potentially related to the higher and more complex energy demands of vertebrate tissues .