Recombinant Saccharomyces cerevisiae Altered inheritance of mitochondria protein 37, mitochondrial (AIM37)

What is AIM37 and how was it identified?

AIM37 (also known as Mcs27 or Ynl100w) is a mitochondrial protein in Saccharomyces cerevisiae initially identified through a computational screen for genes with altered inheritance of mitochondria (AIM). It is one of 46 proteins resulting from this screen and was subsequently characterized as having significant roles in mitochondrial structure and function . AIM37 was later recognized as a component of a larger protein complex involved in determining cristae architecture .

What is the molecular structure of AIM37?

AIM37 contains regions predicted to form coiled-coil structures, which likely facilitate protein-protein interactions within the MitOS complex. Biochemical analysis suggests it is an integral membrane protein with specific topology in the mitochondrial membrane . The coiled-coil domains are particularly important as they contribute to the assembly and stability of the multi-protein complex that includes other components such as Fcj1, Aim5, and Aim13 . Unlike Aim5, which appears to be fungal-specific, AIM37 possesses conserved features suggesting evolutionary importance across species .

What cellular functions does AIM37 perform?

AIM37 plays a critical role in maintaining proper mitochondrial cristae architecture. Research demonstrates its involvement in:

• Formation and maintenance of cristae junctions (CJs) that connect the inner boundary membrane with the cristae membrane

• Contributing to the stability of the MitOS/MICOS complex that determines mitochondrial inner membrane organization

• Potentially functioning antagonistically to Mic60 (another protein in the complex), thus creating a balanced regulation of cristae junction formation

• Influencing respiratory growth, as demonstrated through phenotypic analysis of deletion strains

What are reliable methods for studying AIM37 protein interactions?

For investigating AIM37 protein interactions, the following methodological approaches have proven effective:

• Affinity Purification coupled with Mass Spectrometry: Chromosomal C-terminal FLAG-tagging of AIM37 allows for purification from digitonin-solubilized mitochondrial extracts followed by LC-MS/MS analysis. This approach successfully identified interactions between AIM37 and other complex components like Fcj1, Aim5, and Aim13 .

• Quantitative Western Analysis: Following purification of tagged proteins, quantitative western blotting provides verification of interactions and relative stoichiometry of complex components. This approach confirmed similar yields of adsorbed components in AIM37-associated purifications .

• Genetic Interaction Mapping: Genome-wide genetic interaction mapping, as demonstrated in the MITO-MAP, provides functional evidence of protein associations. The genetic interaction profile of AIM37 clustered with other MitOS components, strongly suggesting functional relationships .

How can researchers effectively visualize AIM37 localization and dynamics?

To study AIM37 localization and dynamics:

• Submitochondrial Localization Analysis: Protease protection assays using FLAG-tagged AIM37 can determine the protein's specific location within mitochondrial compartments .

• Transmission Electron Microscopy: Standard TEM provides visualization of mitochondrial ultrastructure in wild-type versus AIM37 deletion strains, allowing quantification of cristae junction numbers and morphology .

• Cryo-Electron Tomography: This advanced technique offers higher resolution analysis of cristae architecture in isolated mitochondria, revealing detailed effects of AIM37 deletion on cristae rim and junction structure .

• Fluorescence Microscopy of Tagged Proteins: Expression of AIM37 fused with fluorescent proteins enables live-cell visualization of its distribution and dynamics within mitochondria .

What are reliable approaches for genetic manipulation of AIM37 in yeast?

For genetic studies of AIM37, researchers can employ:

• Gene Deletion: Creating Δmic27 (AIM37 deletion) strains through homologous recombination, followed by phenotypic analysis under different growth conditions (fermentative vs. non-fermentative) and at varying temperatures (30°C vs. 37°C) .

• Chromosomal Tagging: Generation of C-terminal FLAG-tagged AIM37 expressed from its endogenous locus maintains native expression levels while enabling purification and detection .

• Double Deletion Analysis: Creating strains with combined deletions (e.g., Δmic60/Δmic27) allows assessment of genetic interactions and epistatic relationships .

• Drop Dilution Growth Assays: This method effectively quantifies growth phenotypes of AIM37 mutant strains under different conditions, revealing functional significance of the protein .

How does AIM37 contribute to cristae junction formation?

AIM37 plays a critical role in cristae junction (CJ) formation through complex regulatory mechanisms:

• Antagonistic Relationship with Mic60: Experimental evidence demonstrates that AIM37 functions in opposition to Mic60 in regulating CJ formation. The deletion of AIM37 in cells lacking Mic60 partially restores CJ formation, suggesting a balanced regulatory system between these components .

• Structural Contribution: As part of the MitOS/MICOS complex, AIM37 contributes to the physical structure of CJs. In AIM37-deficient cells, CJs appear broader and less defined, showing more flattened morphology compared to the narrow, sharp-edged CJs in wild-type cells .

• Complex Component Interactions: AIM37 functions through interactions with other MitOS components including Fcj1/Mic60, Aim5/Mic12, and Aim13/Mic19, collectively determining the architecture of the inner mitochondrial membrane .

The following data shows the effect of AIM37 deletion on CJ number:

What is the relationship between AIM37 and respiratory function?

AIM37 influences respiratory function through its impact on mitochondrial architecture:

• Growth on Non-fermentable Carbon Sources: Unlike strains lacking Mic60, which show growth defects on non-fermentable carbon sources (YPEG), AIM37 deletion strains (Δmic27) do not exhibit growth defects under respiratory conditions. Interestingly, they may even show slight growth improvement at elevated temperatures (37°C) .

• Epistatic Relationship: The additional deletion of AIM37 in Mic60-deficient cells (Δmic60/Δmic27) rescues the growth phenotype on YPEG caused by the loss of Mic60 alone. This provides clear evidence of a positive genetic interaction between Mic60 and AIM37, indicating that AIM37 loss is epistatic to Mic60 or that they function antagonistically .

• Contrasting Functions: The relationship between AIM37 and respiratory function contrasts with other complex components like Mic26, which appears to act synergistically with Mic60 .

How does AIM37 integrate into the MitOS/MICOS complex?

AIM37 (Mcs27) is a core component of the mitochondrial organizing structure (MitOS)/MICOS complex:

• Physical Interactions: Affinity purification and mass spectrometry analyses demonstrate that AIM37 physically interacts with Fcj1, Aim5, Aim13, as well as Ycl057c (Mos1) and Ygr235c (Mos2) . Purifications of AIM37-FLAG contain significant peptides/coverage of these interacting partners .

• Complex Assembly: The coiled-coil domains in AIM37 likely facilitate its integration into the MitOS complex. These structural features are shared with other complex components including Fcj1, Aim13, and Mos2, suggesting a common interaction mechanism .

• Membrane Integration: AIM37 is predicted to be an integral membrane protein, which positions it strategically within the mitochondrial membrane to participate in complex formation and function .

• Conserved Features: Unlike some components such as Aim5 (which appears fungal-specific), AIM37 possesses conserved features suggesting evolutionary importance of its role in the complex across species .

What experimental evidence supports AIM37's role in the MitOS complex?

Multiple experimental approaches provide compelling evidence for AIM37's integration in the MitOS complex:

How do AIM37 deletions affect mitochondrial function and morphology?

Deletion of AIM37 (Δmic27) produces specific effects on mitochondrial function and structure:

• Cristae Morphology Alterations: Electron microscopy reveals that AIM37 deletion alters cristae morphology, though less severely than deletion of Mic60. Cryo-electron tomography shows that in AIM37-deficient cells, cristae junctions appear broader and less defined compared to the narrow, sharp-edged junctions in wild-type cells .

• Respiratory Growth: Interestingly, unlike Mic60 deletion strains, AIM37 deletion does not impair growth on non-fermentable carbon sources (YPEG), suggesting that the protein's absence does not critically compromise respiratory function .

• Temperature Response: AIM37 deletion strains may even show slightly improved growth at elevated temperatures (37°C) on respiratory media, indicating complex regulatory effects on mitochondrial function .

• Antagonistic Effects: The additional deletion of AIM37 in Mic60-deficient cells partially restores cristae junction formation and rescues respiratory growth defects, demonstrating an antagonistic relationship between these proteins .

What genetic interactions has AIM37 been shown to participate in?

AIM37 exhibits several significant genetic interactions:

• Positive Genetic Interaction with Mic60: The double deletion Δmic60/Δmic27 shows improved growth on non-fermentable carbon sources compared to Δmic60 alone, indicating that AIM37 loss is epistatic to Mic60 or that they function antagonistically .

• Contrasting Interaction Patterns: Unlike AIM37, Mic26 appears to have a synergistic relationship with Mic60, as the Δmic60/Δmic26 strain shows more severe phenotypes. This suggests opposing functions between AIM37 and Mic26 .

• MITO-MAP Cluster: AIM37's genetic interaction profile in the mitochondrial-focused genetic interaction map (MITO-MAP) clusters with profiles of other MitOS components including Fcj1, Aim5, and Aim13, providing strong evidence for functional relationships .

• Complex Component Interactions: The similar genetic interaction profiles of MitOS components suggest they function in a common pathway or process related to mitochondrial organization and cristae formation .

How can researchers reconcile the antagonistic relationship between AIM37 and Mic60?

The antagonistic relationship between AIM37 and Mic60 represents an intriguing regulatory mechanism that warrants deeper investigation:

• Balanced Regulation Hypothesis: One explanation is that AIM37 and Mic60 exert opposing forces on cristae junction formation, creating a balanced regulatory system. When both are absent, the opposing forces are removed, allowing for partial restoration of function .

• Methodological Approaches: To further investigate this relationship, researchers could:

  • Perform structure-function analyses to identify specific domains mediating the antagonistic effects

  • Utilize super-resolution microscopy to visualize the spatial relationship between these proteins

  • Employ in vitro reconstitution studies to test direct effects on membrane curvature

  • Develop inducible expression systems to study the temporal aspects of their antagonism

• Molecular Mechanism Investigation: Future research should focus on determining whether this antagonism occurs through:

  • Direct physical inhibition

  • Competition for binding to other complex components

  • Opposing effects on membrane curvature

  • Regulation of different steps in cristae junction formation

What are emerging techniques that could advance our understanding of AIM37?

Several cutting-edge techniques show promise for deeper insights into AIM37 function:

• Cryo-Electron Tomography with Subtomogram Averaging: This technique could provide higher-resolution visualization of AIM37's position within the MitOS complex and at cristae junctions .

• Proximity Labeling Proteomics: Techniques like BioID or APEX2 fused to AIM37 could identify transient or weak interactions not captured by traditional pull-down approaches.

• Single-Cell Screening Platforms: Advanced single-cell analysis could reveal cell-to-cell variability in AIM37 function and localization under different stress conditions .

• CRISPR-Based Genetic Screens: Genome-wide CRISPR screens in yeast could identify additional genetic interactions and regulatory pathways affecting AIM37 function.

• In Vitro Membrane Deformation Assays: Reconstitution of purified AIM37 with artificial membranes could directly test its role in membrane curvature and cristae junction formation.

How might findings about AIM37 in yeast translate to understanding human mitochondrial disorders?

Understanding AIM37's function could inform human mitochondrial disease research:

• Conserved MICOS Components: While Aim5 appears fungal-specific, AIM37 possesses conserved features suggesting its function may be preserved in human mitochondria . Identifying human orthologs or functional equivalents could provide insights into disease mechanisms.

• Cristae Remodeling in Disease: Abnormal cristae structure is implicated in various human diseases including neurodegenerative disorders. The antagonistic relationship between AIM37 and Mic60 may suggest similar regulatory mechanisms in human cells that could be targeted therapeutically .

• Experimental Approaches: To translate these findings:

  • Compare the effects of manipulating human MICOS components to the phenotypes observed in yeast

  • Introduce human MICOS components into yeast deletion strains to test functional conservation

  • Examine whether disease-associated variants of human MICOS components display altered interaction patterns similar to those observed in yeast models

What methodological approaches are most effective for studying AIM37's role in protein secretion pathways?

While AIM37 primarily functions in mitochondrial organization, understanding potential connections to protein secretion requires specific approaches:

• Integrative Analysis: Researchers investigating potential links between mitochondrial function and protein secretion should:

  • Assess secretion of reporter proteins (like α-amylase) in AIM37 deletion strains

  • Monitor ER stress markers to identify potential retrograde signaling mechanisms

  • Examine whether altered mitochondrial architecture affects ER-mitochondria contact sites that might influence secretory processes

• Stress Response Integration: Given that protein secretion stress can trigger reactive oxygen species production, investigating how AIM37 deletion affects this response could reveal unexpected connections between mitochondrial organization and secretory pathway function .

• Systematic Analysis: Approaches like those used to study SEC16's effects on protein secretion could be applied to investigate potential roles of AIM37, examining effects on secretion efficiency, ER volume, and stress responses .