Recombinant Danio rerio Transmembrane protein 223 (tmem223)

What is tmem223 and what are its key structural features?

Tmem223 is a transmembrane protein that belongs to the TMEM70/TMEM186/TMEM223 protein family. In Danio rerio (zebrafish), the full-length protein consists of 248 amino acids . Structural analysis indicates that tmem223 has two putative transmembrane spans and lacks a defined N-terminal targeting sequence . The protein is an integral component of the inner mitochondrial membrane with both its N- and C-termini facing the mitochondrial matrix .

The protein structure is highly conserved across species, with zebrafish tmem223 being orthologous to human TMEM223 . This conservation suggests fundamental evolutionary importance in mitochondrial function. When studying the recombinant form, researchers should note that the commercially available protein typically includes an N-terminal His-tag to facilitate purification and detection in experimental settings .

The amino acid sequence of the full-length zebrafish tmem223 is: MAVQHLLFGVRRSCTFILACRRTAFQSSRAFTSIYTQFKVTDTKNIFRPLVFPVRVASAFTFTSAAVAKDVLLFEHDRTRFFRLLAIFCGGQFLFWAYLGHFAFTSLRDTRKYSEPQKVRTELGGFFSFDMNLGSNAWRYGFTSGCLIIGGGILALALLFSRRSVSRVILHKGGAKVSV YTQSPLGPQRSHHLTVPLSQVACYAHRQESHSFIPLKVKGYKFYFLLDKEGTVNNPKLFDI TVGAYRPL .

What are the predicted and confirmed functions of tmem223?

Tmem223 is primarily involved in mitochondrial functions. In zebrafish, it is predicted to enable mitochondrial ribosome binding activity and participate in mitochondrial cytochrome c oxidase assembly . Research on human TMEM223 has confirmed its role as a mitochondrial ribosome-interacting protein that is integral to the inner mitochondrial membrane .

Functional studies using TMEM223 knockout cell lines have demonstrated that TMEM223 is specifically involved in cytochrome c oxidase biogenesis . When TMEM223 is absent, there is a significant reduction in cytochrome c oxidase activity (approximately 62.5% of wild-type levels) . This finding establishes TMEM223 as an important factor in respiratory chain assembly, particularly for complex IV of the electron transport chain.

Interestingly, loss of TMEM223 also appears to affect the levels of other respiratory chain components. While complex IV (cytochrome c oxidase) shows reduced activity, complex III shows an increase in abundance (approximately 165% of wild-type) when TMEM223 is knocked out . This suggests that TMEM223 may have broader roles in maintaining the balance of respiratory chain complexes beyond its direct involvement in complex IV assembly.

How should recombinant Danio rerio tmem223 be stored and handled for optimal stability?

Recombinant tmem223 is typically supplied as a lyophilized powder and requires careful handling to maintain its stability and activity . For storage, it is recommended to keep the protein at -20°C to -80°C upon receipt, with aliquoting necessary for multiple use scenarios to avoid repeated freeze-thaw cycles, which can degrade protein quality .

For reconstitution, researchers should briefly centrifuge the vial prior to opening to bring the contents to the bottom. The protein should be reconstituted in deionized sterile water to a concentration of 0.1-1.0 mg/mL . Adding glycerol to a final concentration of 5-50% is recommended for long-term storage, with 50% being the standard concentration used by suppliers . Properly reconstituted and stored aliquots can be kept at 4°C for up to one week for ongoing experiments .

It's important to note that working aliquots should be maintained at 4°C to minimize protein degradation during experimental timeframes. The storage buffer typically consists of Tris/PBS-based buffer with 6% Trehalose at pH 8.0, which helps maintain protein stability .

What are the recommended applications for recombinant tmem223 in research?

• Mitochondrial ribosome interaction studies: Since tmem223 is confirmed to interact with the mitochondrial ribosome, the recombinant protein can be used in pull-down assays or co-immunoprecipitation experiments to identify specific ribosomal binding partners .

• Respiratory chain assembly research: Given its role in cytochrome c oxidase biogenesis, the recombinant protein is valuable for investigating the assembly mechanisms of complex IV components .

• Structural biology applications: Purified recombinant tmem223 can be used for crystallography or cryo-EM studies to determine its three-dimensional structure and inform its functional mechanisms within the inner mitochondrial membrane.

• Antibody production: The recombinant protein serves as an excellent antigen for generating antibodies to detect endogenous tmem223 in various experimental contexts, particularly for studying its subcellular localization and protein-protein interactions.

When designing experiments with recombinant tmem223, researchers should consider its transmembrane nature and the potential need for appropriate detergents or membrane mimetics to maintain its native conformation.

How does tmem223 deficiency affect mitochondrial translation and respiratory complex assembly?

In TMEM223 knockout cells, there is a significant decrease in the levels of newly synthesized COX1 (a mitochondrially-encoded complex IV subunit) compared to wild-type cells . This observation, coupled with the reduced cytochrome c oxidase activity, suggests that tmem223 plays a crucial role in the translation or early assembly steps of complex IV components.

Research methodologies to investigate this question should include:

• [35S]methionine labeling: This technique allows for tracking newly synthesized mitochondrial-encoded proteins and can reveal translation defects specific to certain subunits .

• Blue Native PAGE (BN-PAGE): This method can be used to analyze the integrity and abundance of respiratory chain complexes. In TMEM223-deficient cells, BN-PAGE analysis has shown reduced levels of fully assembled cytochrome c oxidase to approximately 60% of wild-type levels .

• Cytochrome c oxidase activity assays: Colorimetric assays can quantitatively assess the functional impact of tmem223 deficiency on complex IV activity .

• Protein stability assays: These can help determine whether tmem223 affects the stability of respiratory chain components or assembly factors. For example, thiamphenicol can be used to inhibit mitochondrial translation, allowing for the assessment of protein stability over time .

The increased abundance of complex III observed in TMEM223 knockout cells presents an interesting research direction, as it suggests potential compensatory mechanisms or regulatory connections between different respiratory chain complexes that warrant further investigation .

What are the differences in tmem223 function between species and their implications for model organism research?

While tmem223 is conserved across species, there may be functional differences that researchers should consider when using zebrafish as a model for human mitochondrial diseases. Comparative analysis of zebrafish tmem223 with its human ortholog shows similar predicted functions, including mitochondrial ribosome binding activity and involvement in cytochrome c oxidase assembly .

• Interaction partners: The specific proteins that interact with tmem223 may vary between zebrafish and humans, potentially affecting its functional outcomes.

• Developmental roles: The importance of tmem223 during development might differ between species, influencing the phenotypes observed in knockdown or knockout models.

• Compensation mechanisms: The ability to compensate for tmem223 deficiency through alternative pathways may vary between zebrafish and mammals.

To address these questions, researchers can employ cross-species complementation experiments, where human TMEM223 is expressed in zebrafish tmem223 knockouts to assess functional conservation. Additionally, comparative interactome studies can identify species-specific binding partners that might explain functional differences.

How do environmental chemicals affect tmem223 expression and function?

Several chemicals have been shown to modulate tmem223 expression, which may have implications for mitochondrial function under environmental stress conditions. Based on rat Tmem223 data, which has relevance to zebrafish due to evolutionary conservation, the following patterns have been observed:

These chemical interactions suggest that tmem223 expression is responsive to environmental stressors, potentially as part of cellular adaptation mechanisms to maintain mitochondrial function. When designing experiments involving recombinant tmem223, researchers should consider whether these chemicals might be present in their experimental systems, as they could confound results related to tmem223 function.

Methodologically, researchers can use quantitative PCR, western blotting, or reporter assays to monitor tmem223 expression changes in response to chemical exposures. Additionally, functional assays measuring cytochrome c oxidase activity or mitochondrial translation can help determine whether chemical-induced changes in tmem223 expression correlate with functional outcomes.

What methodologies are most effective for studying tmem223's interactions with the mitochondrial ribosome?

To investigate tmem223's interactions with the mitochondrial ribosome, several complementary approaches can be employed:

• Affinity purification coupled with mass spectrometry (AP-MS): This approach has successfully identified TMEM223 as a mitochondrial ribosome interactor . Researchers can use tagged recombinant tmem223 as bait to pull down associated ribosomal proteins and identify them by mass spectrometry.

• Proximity labeling techniques: Methods such as BioID or APEX2 can be used to identify proteins in close proximity to tmem223 in living cells, providing information about its dynamic interactions within the mitochondrial environment.

• Cryo-electron microscopy: This technique can visualize the structural integration of tmem223 with the mitochondrial ribosome, potentially revealing how it contributes to ribosome positioning or function.

• Ribosome profiling: This approach can determine whether tmem223 affects ribosome occupancy on specific mitochondrial transcripts, providing insights into its potential role in translational regulation.

• Cross-linking studies: Chemical cross-linking followed by mass spectrometry can identify the specific ribosomal proteins that interact directly with tmem223 and map the interaction interfaces.

The choice of methodology should be guided by the specific research question. For instance, if investigating whether tmem223 affects translation of specific mitochondrial transcripts, ribosome profiling would be most appropriate. If determining the structural basis of tmem223-ribosome interaction is the goal, cryo-EM would be more suitable.

What is the potential significance of tmem223 in mitochondrial disease models?

Given its role in cytochrome c oxidase assembly, tmem223 dysfunction could contribute to mitochondrial diseases characterized by complex IV deficiency. Researchers investigating disease models should consider several aspects:

• Patient mutation analysis: Screening for mutations in TMEM223 in patients with unexplained complex IV deficiencies could reveal novel disease associations.

• Phenotypic characterization: In zebrafish, knockdown or knockout of tmem223 could create a model for studying complex IV deficiency syndromes and testing potential therapeutic interventions.

• Tissue-specific effects: Investigating whether tmem223 deficiency affects tissues differentially based on their metabolic demands could explain the tissue-specific manifestations of mitochondrial diseases.

• Compensatory mechanisms: Studying how cells and organisms adapt to tmem223 dysfunction might reveal novel therapeutic targets for mitochondrial diseases.

The observation that TMEM223 knockout leads to increased complex III levels is particularly intriguing, as it suggests potential compensatory mechanisms that could be therapeutically relevant. Understanding these adaptive responses could inform strategies to enhance mitochondrial function in patients with cytochrome c oxidase deficiencies.

Zebrafish offer an excellent model system for such studies due to their genetic tractability, rapid development, and the ability to perform high-throughput drug screening. The availability of recombinant Danio rerio tmem223 facilitates the development and validation of tools for these investigations.

How can recombinant tmem223 be used to develop research tools for mitochondrial biology?

Recombinant tmem223 can serve as a platform for developing various research tools:

• Structure-based inhibitor design: With structural information about tmem223, researchers can design small molecules that modulate its interactions with the mitochondrial ribosome or other partners, creating tools to perturb its function in controlled ways.

• Fluorescent probes: Conjugating fluorescent dyes to recombinant tmem223 can create tools for tracking mitochondrial ribosome dynamics in live cells or in vitro.

• Antibody development: Recombinant tmem223 is an ideal immunogen for generating highly specific antibodies for detecting the endogenous protein in various experimental contexts.

• Interaction screening platforms: Immobilized recombinant tmem223 can be used to screen for novel interaction partners or to test compounds that modify these interactions.

• CRISPR gene editing controls: Purified recombinant tmem223 can serve as a control protein for assessing the specificity and efficiency of CRISPR gene editing approaches targeting the endogenous gene.

When designing such tools, researchers should consider the transmembrane nature of tmem223 and potential conformational dependencies of its interactions. The recombinant protein's His-tag may affect certain interactions or applications and may need to be removed for specific experiments.