Recombinant Macaca fascicularis Calcium-binding mitochondrial carrier protein Aralar2 (SLC25A13)

What is Recombinant Macaca fascicularis Calcium-binding Mitochondrial Carrier Protein Aralar2 (SLC25A13)?

Recombinant Macaca fascicularis Calcium-binding Mitochondrial Carrier Protein Aralar2 (SLC25A13) is a laboratory-produced version of the naturally occurring Aralar2 protein found in crab-eating macaques (Cynomolgus monkeys). The protein functions as a mitochondrial electrogenic aspartate/glutamate antiporter that facilitates the efflux of aspartate and entry of glutamate and protons within the mitochondria as part of the malate-aspartate shuttle . Additionally, it mediates the uptake of L-cysteinesulfinate by mitochondria in exchange for L-glutamate and protons, and can exchange L-cysteinesulfinate with aspartate in their anionic form without proton translocation . The recombinant form is expressed with a full-length sequence of 674 amino acids and typically includes a tag determined during the production process .

How does the structure of Macaca fascicularis SLC25A13 compare to the human ortholog?

The Macaca fascicularis SLC25A13 protein shares high structural homology with its human counterpart, making it valuable for comparative studies and as a research model. Both contain four EF-hand Ca(2+) binding motifs in the N-terminal domain that regulate protein activity . The human SLC25A13 (UniProt: Q9UJS0) and Macaca fascicularis SLC25A13 (UniProt: Q8HXW2) demonstrate considerable sequence conservation, particularly in functional domains essential for calcium binding and substrate transport . The amino acid sequence of Macaca fascicularis SLC25A13 includes characteristic regions such as calcium-binding sites and transmembrane domains essential for its localization to the inner mitochondrial membrane and its function in metabolite exchange . This high conservation allows researchers to use the Macaca fascicularis model for studying human-relevant mechanisms with reasonable translational validity.

What are the functional domains of SLC25A13 and their physiological significance?

SLC25A13 contains several crucial functional domains that determine its physiological role:

• N-terminal Domain: Contains four EF-hand Ca(2+) binding motifs that respond to calcium fluctuations, enabling calcium-dependent regulation of transport activity .

• Transmembrane Domains: Allow proper insertion into the inner mitochondrial membrane where the protein functions as an antiporter .

• Substrate Binding Sites: Specialized regions that recognize and bind aspartate, glutamate, and L-cysteinesulfinate for transport across the mitochondrial membrane .

The physiological significance of these domains is multifaceted. The calcium-binding regions allow SLC25A13 to respond to cellular calcium signals, effectively linking metabolism to calcium homeostasis. The protein's antiporter activity is essential for the malate-aspartate shuttle, a critical pathway for transferring reducing equivalents across the mitochondrial membrane, thereby supporting cellular energy metabolism . Additionally, its role in amino acid transport has implications for nitrogen metabolism, particularly in the liver. Dysfunction of SLC25A13 leads to citrullinemia type II, highlighting its importance in the urea cycle and ammonia detoxification .

What experimental methods are recommended for studying recombinant SLC25A13 protein function?

Several methodological approaches are recommended for investigating recombinant SLC25A13 function:

• Mitochondrial Transport Assays: Researchers can use purified mitochondria or reconstituted proteoliposomes containing recombinant SLC25A13 to measure exchange rates of radiolabeled substrates such as 14C-aspartate, 14C-glutamate, or 14C-cysteinesulfinate. This approach allows for quantitative assessment of transport kinetics and substrate specificity.

• Calcium Sensitivity Analysis: Since SLC25A13 activity is modulated by calcium, researchers should assess transport activity under varied calcium concentrations to determine dose-response relationships . Calcium indicators and chelators can be employed to precisely control calcium levels during experiments.

• Site-Directed Mutagenesis: Systematic mutation of key residues, particularly within the calcium-binding domains and substrate recognition sites, provides insights into structure-function relationships. Comparing transport activities of wild-type and mutant proteins helps identify critical amino acids .

• Immunodetection Techniques: Western blot using specific antibodies (such as EPR9969(B)) can track protein expression, localization, and post-translational modifications . The antibody has been validated in human cell lines and is likely to cross-react with Macaca fascicularis SLC25A13 due to sequence homology.

• ELISA-Based Quantification: Sandwich ELISA techniques can provide sensitive quantification of SLC25A13 in various sample types, enabling researchers to measure protein levels in experimental conditions with sensitivity down to 0.12 ng/mL .

How should recombinant SLC25A13 be stored and handled to maintain optimal activity?

Proper storage and handling of recombinant SLC25A13 is crucial for maintaining its structural integrity and functional activity:

• Storage Conditions: Store the protein at -20°C for routine use, or at -80°C for extended storage periods . The protein is typically supplied in a Tris-based buffer with 50% glycerol to ensure stability during freeze-thaw cycles.

• Aliquoting: To avoid repeated freeze-thaw cycles that can denature the protein, prepare small working aliquots upon receipt of the recombinant protein. Working aliquots can be stored at 4°C for up to one week for ongoing experiments .

• Temperature Management: When working with the protein, keep it on ice or at 4°C to minimize degradation. Avoid exposing the protein to temperatures above 25°C for extended periods.

• Buffer Considerations: If buffer exchange is necessary for specific applications, use gentle methods such as dialysis or size exclusion chromatography rather than harsh techniques that might denature the protein. Maintain a pH range of 7.0-8.0 for optimal stability.

• Protease Inhibitors: Consider adding protease inhibitor cocktails when working with the protein in complex biological samples to prevent enzymatic degradation.

Following these handling procedures will help ensure experimental reproducibility and maximize the utility of recombinant SLC25A13 in research applications .

What are the optimal conditions for using recombinant SLC25A13 in transport assays?

Optimal conditions for transport assays with recombinant SLC25A13 should mimic the physiological environment of the mitochondrial inner membrane:

For accurate transport measurements, researchers should establish baseline conditions with appropriate controls including:

• Non-functional protein variants (e.g., calcium binding site mutants)

• Specific transport inhibitors when available

• Temperature-dependent inactivation controls

• Substrate-free conditions to assess background transport

Time-course measurements ranging from seconds to minutes are recommended to capture both initial rates and steady-state kinetics of transport . For optimal results, reconstitute the recombinant protein in proteoliposomes with a defined lipid composition that includes cardiolipin, a key mitochondrial phospholipid that often influences carrier protein function.

How can researchers validate the functional activity of recombinant SLC25A13?

Validating functional activity of recombinant SLC25A13 requires multiple complementary approaches:

• Substrate Transport Assays: The gold standard for functional validation involves measuring transport of radiolabeled substrates (aspartate, glutamate) across membranes containing the reconstituted protein. Successful transport that is inhibited by known transport blockers confirms functionality.

• Calcium-Binding Assays: Since SLC25A13 contains four EF-hand calcium-binding motifs, researchers can employ techniques like isothermal titration calorimetry (ITC) or calcium-dependent fluorescence changes to verify proper calcium binding . Active protein should demonstrate calcium binding with physiologically relevant affinities.

• Structural Integrity Assessment: Circular dichroism (CD) spectroscopy can verify proper protein folding by analyzing secondary structure elements. Properly folded SLC25A13 should exhibit spectral characteristics consistent with its predicted alpha-helical transmembrane domains.

• Immunological Reactivity: Using validated antibodies like EPR9969(B) in Western blotting can confirm structural integrity by detecting the expected ~70-74 kDa band . Comparison with positive controls (e.g., native mitochondrial extracts) and negative controls (knockout samples) strengthens validation.

• Functional Complementation: Expression of recombinant SLC25A13 in cellular models deficient in endogenous protein (either through genetic knockout or siRNA) should rescue phenotypes associated with SLC25A13 deficiency if the recombinant protein is functional.

These validation steps should be performed systematically before proceeding with more complex experimental applications to ensure scientific rigor and reproducibility.

What are common technical challenges when working with recombinant SLC25A13 and how can they be addressed?

Researchers frequently encounter several technical challenges when working with recombinant SLC25A13:

• Protein Aggregation: As a membrane protein, SLC25A13 has hydrophobic regions that can promote aggregation.

  • Solution: Include mild detergents (0.1% DDM or 1% CHAPS) in buffers; avoid freeze-thaw cycles; maintain glycerol concentration at 10-20% during experiments .

• Low Functional Activity: Recombinant expression can sometimes yield structurally intact but functionally impaired protein.

  • Solution: Verify proper folding using spectroscopic methods; optimize expression systems (consider mammalian or insect cell expression for complex proteins); ensure co-expression of potential chaperones.

• Calcium-Dependent Artifacts: Since SLC25A13 is calcium-regulated, undefined calcium levels can introduce variability.

  • Solution: Standardize calcium concentrations using calibrated buffers; include calcium chelators (EGTA) to establish baseline activity; create a calcium-response curve for your specific protein preparation .

• Detection Specificity Issues: Cross-reactivity in immunodetection methods can complicate analysis.

  • Solution: Validate antibodies using knockout controls; consider epitope-tagged recombinant protein for specific detection; employ multiple detection methods .

• Reconstitution Inefficiency: Membrane protein incorporation into artificial membranes is often inefficient.

  • Solution: Optimize lipid composition to include cardiolipin; try different reconstitution methods (direct incorporation, detergent dialysis, or liposome fusion); quantify protein incorporation using protein/lipid ratio measurements.

Addressing these challenges requires systematic optimization and appropriate controls to ensure reliable and reproducible research outcomes.

How can researchers differentiate between artificial effects of the recombinant system and native protein properties?

Distinguishing between artifacts of recombinant expression and native properties requires careful experimental design:

• Comparative Analysis with Native Protein: When possible, isolate native SLC25A13 from mitochondria and compare its properties with the recombinant version. Key parameters to compare include:

  • Substrate affinity and transport kinetics

  • Calcium sensitivity profiles

  • Protein-protein interactions

  • Post-translational modification patterns

• Expression System Considerations: Different expression systems can introduce system-specific artifacts:

  • Bacterial systems lack post-translational modifications

  • Insect cells may have different membrane composition

  • Mammalian cells closest to native environment but may have endogenous protein

  Always consider how the expression system might influence protein properties and validate findings across multiple systems when possible.

• Tag Interference Assessment: Expression tags can alter protein function.

  • Compare tagged and untagged versions of the protein

  • Test multiple tag positions (N-terminal, C-terminal)

  • Use cleavable tags and compare properties before and after tag removal

• Functional Complementation Studies: Express the recombinant protein in cells lacking endogenous SLC25A13 and assess whether it rescues known phenotypes. Successful complementation suggests the recombinant protein maintains essential native functions .

• Cross-Species Validation: Compare properties of recombinant SLC25A13 from different species (human, Macaca fascicularis, etc.). Conserved properties across species are more likely to reflect intrinsic protein characteristics rather than artifacts .

By systematically addressing these considerations, researchers can more confidently distinguish between artificial effects and genuine biological properties.

How can recombinant SLC25A13 be used to study citrullinemia type II and other associated disorders?

Recombinant SLC25A13 provides valuable tools for investigating citrullinemia type II and related disorders:

• Mutation Analysis Platform: Recombinant expression systems allow researchers to introduce disease-associated mutations into SLC25A13 and assess their functional consequences. This approach enables systematic characterization of how specific genetic variants affect:

  • Protein expression and stability

  • Subcellular localization

  • Transport activity

  • Calcium sensitivity

  • Protein-protein interactions

• Structure-Function Relationships: By comparing wild-type and mutant recombinant proteins, researchers can map critical functional domains and identify mechanistic links between protein structure and disease manifestations. This information is crucial for understanding pathophysiology and developing targeted therapies .

• Drug Screening Platform: Stabilized recombinant SLC25A13 embedded in artificial membrane systems can serve as a platform for high-throughput screening of compounds that might:

  • Enhance residual activity of disease-associated variants

  • Stabilize partially misfolded mutants

  • Act as chemical chaperones to improve protein folding

• Biomarker Development: Antibodies developed against recombinant SLC25A13 can be used to establish sensitive ELISA assays for detecting the protein in patient samples, potentially helping with diagnosis or monitoring disease progression . Quantitative assays with detection ranges of 0.312-20 ng/mL provide sufficient sensitivity for clinical applications.

• Metabolic Flux Analysis: Reconstituted systems with recombinant SLC25A13 enable controlled studies of how protein dysfunction affects metabolic flux through the malate-aspartate shuttle and connected pathways, offering insights into the metabolic consequences of SLC25A13 deficiency that underlie disease manifestations .

These approaches collectively provide powerful tools for translational research aimed at understanding and treating SLC25A13-associated disorders.

What are the current challenges in studying calcium-dependent regulation of SLC25A13 and potential methodological solutions?

Studying calcium-dependent regulation of SLC25A13 presents several specific challenges with corresponding methodological solutions:

• Challenge: Precisely controlling calcium concentrations in experimental systems.
  Solution: Employ calcium buffer systems (EGTA/CaEGTA) calibrated using calcium-sensitive dyes or electrodes. Create standard curves for calcium concentration validation. For cellular studies, use genetically encoded calcium indicators (GECIs) for real-time monitoring of calcium fluctuations alongside SLC25A13 activity .

• Challenge: Determining physiologically relevant calcium concentrations at the mitochondrial surface.
  Solution: Use targeted calcium sensors localized to the mitochondrial outer membrane to measure local calcium microdomains. Combine with super-resolution microscopy to correlate calcium fluctuations with SLC25A13 localization and activity.

• Challenge: Linking calcium binding to conformational changes and transport activity.
  Solution: Implement FRET-based reporters by introducing fluorescent protein pairs at strategic positions in recombinant SLC25A13. Calcium-induced conformational changes will alter FRET efficiency, providing real-time readouts of protein structural dynamics. Complement with hydrogen-deuterium exchange mass spectrometry (HDX-MS) to map specific regions undergoing calcium-dependent structural changes.

• Challenge: Distinguishing between direct calcium effects on SLC25A13 and indirect effects through interacting partners.
  Solution: Use purified recombinant protein in reconstituted systems to isolate direct effects. Compare with results in cellular systems to identify divergences that suggest involvement of additional factors. Apply proximity labeling techniques (BioID, APEX) to identify calcium-dependent protein interactions.

• Challenge: Correlating in vitro calcium sensitivity with physiological regulation.
  Solution: Develop cell models expressing calcium-insensitive SLC25A13 variants (mutation of EF-hand domains) and assess metabolic consequences using isotope tracing and metabolomics. Compare cellular phenotypes under varied calcium signaling conditions to bridge in vitro observations with cellular physiology .

These methodological approaches provide a framework for addressing the complex relationship between calcium signaling and SLC25A13 regulation.

How can cross-species comparison of SLC25A13 (human vs. Macaca fascicularis) inform evolutionary conservation of carrier function?

Cross-species comparison of SLC25A13 between human and Macaca fascicularis provides valuable insights into evolutionary conservation of mitochondrial carrier protein structure and function:

• Sequence Conservation Analysis: Systematic alignment of human and Macaca fascicularis SLC25A13 sequences reveals evolutionary pressure on specific domains. Highly conserved regions likely represent functionally critical motifs essential for core transport functions, while variable regions may indicate species-specific adaptations . Researchers should focus on:

  • Conservation patterns in calcium-binding domains

  • Substrate binding sites

  • Transmembrane regions

  • Regulatory phosphorylation sites

• Comparative Functional Studies: Side-by-side analysis of recombinant human and Macaca fascicularis SLC25A13 transport properties can reveal evolutionary conservation or divergence in:

  • Substrate specificity and selectivity

  • Transport kinetics (Km and Vmax values)

  • Calcium sensitivity thresholds

  • Response to regulatory factors

• Disease-Associated Variant Analysis: Testing equivalent disease-associated mutations in both species provides insight into pathophysiological mechanisms. If a mutation causes similar functional defects in both species, the affected residue likely plays a fundamental role in carrier function conserved through primate evolution .

• Interactome Conservation: Using techniques like co-immunoprecipitation followed by mass spectrometry, researchers can identify and compare protein interaction partners of human and Macaca fascicularis SLC25A13. Conserved interactions likely represent fundamental regulatory mechanisms, while species-specific interactions may indicate evolutionary adaptation.

• Physiological Context Comparison: Differences in metabolic demands between species may drive subtle functional adaptations in SLC25A13. Researchers should consider species-specific metabolic profiles when interpreting functional differences between human and Macaca fascicularis SLC25A13 .

This comparative approach not only enhances understanding of SLC25A13 evolution but also helps distinguish fundamental functional properties from species-specific adaptations, ultimately strengthening translational research applications.