Recombinant Aspergillus terreus Altered inheritance of mitochondria protein 31, mitochondrial (aim31)

What is aim31 protein and what is its primary function in Aspergillus terreus?

Aim31 (also known as Rcf1 - Respiratory supercomplex factor 1) is a mitochondrial protein belonging to the Hypoxia-Induced Gene 1 (Hig1) protein family. In A. terreus, aim31 is encoded by the ATEG_05390 gene . The primary function of aim31 involves association with the cytochrome bc1-COX supercomplex in the mitochondria, where it plays a critical role in respiratory chain function . Research indicates that aim31/Rcf1 can be found in physical association with both the cytochrome bc1 and COX enzyme domains of the respiratory supercomplex, suggesting it acts as a bridge supporting the assembly of the supercomplex state .

How does aim31 relate to mitochondrial inheritance patterns in fungi?

Aim31 was originally discovered in a genetic screen designed to identify genes encoding proteins whose absence caused an altered inheritance of mitochondrial DNA (AIM) . In fungi, mitochondrial inheritance often follows a uniparental pattern, but the mechanisms controlling this process vary across species. In Saccharomyces cerevisiae, research suggests that Aim31/Rcf1, along with Aim38/Rcf2, influences mitochondrial DNA inheritance through their functions in respiratory complex assembly . The specific mechanisms by which aim31 affects mitochondrial inheritance in A. terreus remain an active area of research, but it likely involves the protein's role in maintaining mitochondrial integrity and function.

What methods are used to express recombinant aim31 protein for research purposes?

Several expression systems have been developed for producing recombinant aim31:

For A. terreus aim31 specifically, expression in E. coli with an N-terminal His-tag has been documented . The recombinant protein is typically purified through affinity chromatography, with storage recommendations including 50% glycerol in Tris-based buffer at -20°C or -80°C for extended storage .

How does aim31/Rcf1 contribute to respiratory supercomplex assembly and function?

Aim31/Rcf1 plays a crucial role in the assembly and stability of respiratory supercomplexes. Research shows that:

• Aim31/Rcf1 physically associates with both cytochrome bc1 and COX enzyme domains of the respiratory supercomplex .

• The protein displays a closer physical relationship with the Cox3 protein within the COX complex .

• Deletion of both aim31/Rcf1 and aim38/Rcf2 (but not individually) significantly impacts COX enzyme activity and assembly of peripheral COX subunits Cox12 and Cox13 .

• Aim31/Rcf1 may act as a "bridge" to support the assembly of the supercomplex state .

Methodologically, these interactions have been studied using affinity purification with histidine-tagged cytochrome c1 and Aac2 derivatives under mild digitonin solubilization conditions to maintain supercomplex organization .

What phenotypic changes are observed in aim31 deletion mutants in various fungi?

Phenotypic changes in aim31/Rcf1 deletion mutants vary somewhat across fungal species but generally show respiratory deficiencies:

In Saccharomyces cerevisiae:

• Compromised colony growth and impaired utilization of non-fermentable carbon sources

• Altered cytochrome bc1-COX supercomplex assembly

In Aspergillus fumigatus:

• Abnormal mitochondrial membrane potential

• Reduced reactive oxygen species (ROS) production

• Increased resistance to various antifungal agents, including azoles, terbinafine, and simvastatin

• Upregulation of multiple efflux pumps

These phenotypes are typically assessed through growth assays on different carbon sources, measurement of respiratory enzyme activities, analysis of supercomplex assembly by blue native gel electrophoresis (BN-PAGE), and antifungal susceptibility testing .

How does aim31 protein sequence compare across different Aspergillus species?

Alignment analysis of aim31 sequences from different Aspergillus species reveals both conservation and species-specific variations:

The protein maintains conserved functional domains across species, particularly in regions involved in respiratory supercomplex interactions. Multiple sequence alignment shows that the N-terminal region containing the mitochondrial targeting sequence exhibits greater variability than the core functional domain .

What experimental approaches are most effective for studying aim31 protein-protein interactions?

Several complementary approaches have proven effective for studying aim31/Rcf1 protein-protein interactions:

• Affinity Purification with Tagged Proteins:

  • Expression of His-tagged aim31 followed by pull-down assays to identify interacting partners

  • Use of digitonin for mild solubilization to preserve native protein complexes

• Blue Native PAGE (BN-PAGE):

  • Effective for visualizing intact respiratory supercomplexes

  • Can be combined with second-dimension SDS-PAGE for detailed component analysis

• Colocalization Studies:

  • Expression of fluorescently tagged aim31 (e.g., aim31-GFP) to visualize localization

  • Combination with mitochondrial markers to confirm subcellular localization

• Crosslinking Mass Spectrometry (XL-MS):

  • For capturing transient or weak interactions between aim31 and partner proteins

  • Particularly useful for identifying contact points within large complexes

• Yeast Two-Hybrid and Split-GFP Systems:

  • For validation of specific protein-protein interactions

  • Useful for mapping interaction domains

These methods have revealed that aim31/Rcf1 interacts with various components of the respiratory chain, particularly showing a close physical relationship with the Cox3 protein of the COX complex .

How does aim31 function differ under normal versus stress conditions in fungal cells?

Normal Conditions:

• Facilitates cytochrome bc1-COX supercomplex assembly

• Supports optimal respiratory chain function

• Contributes to normal mitochondrial inheritance patterns

Stress Conditions:

• May play a role in oxidative stress responses

• Could be involved in cellular adaptation to antifungal drugs

• Potentially contributes to alternative respiratory pathways

In A. fumigatus, deletion of mitochondrial proteins affects the response to antifungal drugs, suggesting that proteins like aim31 may function differently under drug stress . Similarly, in A. terreus, which exhibits intrinsic resistance to amphotericin B, mitochondrial functions and oxidative stress responses appear central to this resistance .

Research in related fungi suggests that under stress conditions, alternative respiratory pathways may be activated, potentially involving altered functions of proteins like aim31. For example, S. reilianum employs an alternative oxidase (AOX) under conditions of classical respiratory chain inhibition, which is upregulated in the teliospore stage of the fungus .

What are the implications of aim31 for understanding fungal pathogenicity and developing novel antifungal approaches?

Understanding aim31's role in mitochondrial function has significant implications for fungal pathogenicity and antifungal strategies:

• Pathogenicity Mechanisms:

  • A. terreus is an emerging pathogen affecting immunocompromised patients, causing infections that are often difficult to treat

  • Mitochondrial functions regulated by proteins like aim31 influence fungal stress responses and adaptation to host environments

  • A. terreus employs unique survival strategies, such as remaining viable within macrophages without germination, unlike A. fumigatus which escapes by germination

• Antifungal Resistance:

  • A. terreus shows intrinsic resistance to amphotericin B, a polyene antifungal

  • Mitochondrial proteins may influence drug resistance by altering:

    • Cellular redox status

    • Drug efflux pump expression

    • Membrane composition and fluidity

• Novel Therapeutic Approaches:

  • Targeting mitochondrial proteins like aim31 could represent a new strategy for overcoming antifungal resistance

  • Combination therapies that include pro-oxidants with amphotericin B show promise for treating resistant A. terreus isolates

  • Diagnostic approaches using monoclonal antibodies to A. terreus-specific proteins could improve early detection