Recombinant Candida albicans Iron-sulfur clusters transporter ATM1, mitochondrial (ATM1)

What is the structure and function of ATM1 in Candida albicans?

ATM1 in Candida albicans is a mitochondrial ATP-binding cassette (ABC) transporter that plays a crucial role in iron-sulfur cluster transport and iron homeostasis. Structurally, it belongs to the ABC transporter family characterized by transmembrane domains that create a passage across the mitochondrial membrane and nucleotide-binding domains that bind and hydrolyze ATP to power transport. The primary function of ATM1 appears to be the export of iron-sulfur cluster precursors or related compounds from the mitochondria to the cytosol, thereby connecting the mitochondrial iron-sulfur cluster (ISC) assembly pathway with the cytosolic iron-sulfur cluster assembly (CIA) pathway . Based on homology with yeast ATM1, the C. albicans transporter likely functions as a homodimer in the mitochondrial membrane, creating a channel for substrate transport .

How does C. albicans ATM1 compare to homologs in other organisms?

C. albicans ATM1 shares significant functional conservation with homologs in other organisms while maintaining species-specific adaptations. The yeast Saccharomyces cerevisiae ATM1 protein has been extensively characterized as a mitochondrial iron exporter, and deletion of the ATM1 gene results in mitochondrial iron overload, loss of respiratory function, and accumulation of mitochondrial DNA mutations . Studies have demonstrated that the human homolog (ABC7/ABCB7) can functionally complement yeast ATM1 deletion, indicating conservation of core functions across evolutionary distance . In apicomplexan parasites like Plasmodium falciparum and Toxoplasma gondii, ATM1 serves as a critical transporter in the mitochondrial membrane, bridging mitochondria-generated iron-sulfur clusters with cytosolic assembly pathways . This functional conservation suggests that C. albicans ATM1 likely plays similar essential roles in iron homeostasis while potentially harboring adaptations specific to C. albicans' dual lifestyle as both a commensal and pathogen.

What cellular processes depend on proper ATM1 function?

Proper ATM1 function is essential for numerous cellular processes in C. albicans. Based on research in related organisms, ATM1 is critical for:

• Mitochondrial iron homeostasis - preventing toxic iron accumulation in mitochondria

• Cytosolic iron-sulfur protein maturation - enabling the function of essential cytosolic and nuclear iron-sulfur proteins

• Maintenance of mitochondrial DNA integrity - protecting against oxidative damage caused by iron overload

• Respiratory function - supporting proper electron transport chain activity

• Oxidative stress resistance - preventing reactive oxygen species formation catalyzed by free iron

Disruption of these processes through ATM1 dysfunction can lead to widespread cellular defects affecting growth, metabolism, and stress responses that may impact both commensalism and pathogenicity in C. albicans.

What experimental systems are most effective for studying ATM1 function in C. albicans?

Several experimental systems have proven effective for investigating ATM1 function in C. albicans and related organisms:

• Gene deletion and conditional expression systems: Creating ATM1 knockout mutants (atm1Δ/Δ) or conditional expression strains allows assessment of phenotypic consequences of ATM1 loss or reduction. Due to the essential nature of ATM1 in many organisms, regulatable promoters may be necessary for controlled expression .

• Fluorescent protein tagging: GFP-tagging of ATM1 enables subcellular localization studies to confirm mitochondrial targeting, similar to approaches used for other mitochondrial proteins in C. albicans .

• Heterologous expression systems: Expression of C. albicans ATM1 in S. cerevisiae atm1Δ mutants can test functional complementation and conservation, as demonstrated for human ABC7 complementation of yeast ATM1 .

• Biochemical transport assays: Reconstitution of purified recombinant ATM1 in liposomes enables direct measurement of transport activities and substrate specificities, as shown for P. falciparum ATM1 .

• Iron and iron-sulfur cluster analytical techniques: Methods including atomic absorption spectroscopy, Mössbauer spectroscopy, and enzymatic assays of iron-sulfur proteins can measure the consequences of ATM1 dysfunction on cellular iron distribution and protein activities.

What methodologies allow for effective expression and purification of recombinant C. albicans ATM1?

Efficient expression and purification of C. albicans ATM1 requires specialized approaches for membrane proteins:

• Expression systems: Bacterial systems (E. coli) using specific strains optimized for membrane protein expression (C41/C43, Lemo21) may be suitable for ATM1 domains. Eukaryotic systems like P. pastoris, S. cerevisiae, or insect cells are often more effective for full-length membrane proteins with proper folding.

• Construct optimization: Designing constructs with removable tags (His, FLAG, MBP), solubilizing fusion partners, and potentially truncating N-terminal mitochondrial targeting sequences can improve expression yields.

• Membrane extraction: Careful selection of detergents (DDM, LMNG, GDN) is critical for maintaining ATM1 stability and function during extraction from membranes.

• Purification strategy: A typical approach involves:

  • Affinity chromatography (IMAC for His-tagged proteins)

  • Size exclusion chromatography to ensure homogeneity

  • Optional ion exchange chromatography for further purification

• Functional validation: ATP binding/hydrolysis assays and substrate interaction studies using techniques like microscale thermophoresis or surface plasmon resonance to confirm that purified protein retains functionality .

For structural studies, detergent screening and potential reconstitution into nanodiscs or amphipols may be necessary to maintain stability during crystallization attempts or cryo-EM analysis.

What is the relationship between ATM1 function and mitochondrial DNA stability?

Research in S. cerevisiae has established a critical link between ATM1 function and mitochondrial DNA (mtDNA) stability. Deletion of the ATM1 gene results in accumulation of mitochondrial DNA mutations . This relationship likely extends to C. albicans ATM1 through several mechanisms:

• Iron-catalyzed oxidative damage: ATM1 dysfunction leads to mitochondrial iron accumulation, which catalyzes the formation of highly reactive hydroxyl radicals through Fenton chemistry. These radicals directly damage mtDNA, causing mutations and deletions.

• Compromised DNA repair: Iron-sulfur clusters are essential cofactors for several DNA repair enzymes. ATM1 dysfunction impairs cytosolic iron-sulfur cluster assembly, potentially reducing the activity of repair enzymes that maintain mtDNA integrity.

• Respiratory chain dysfunction: ATM1 mutants typically show respiratory defects. Impaired electron transport chain function increases electron leakage and ROS production, further contributing to oxidative damage of mtDNA.

• Altered mitochondrial membrane potential: The electrochemical gradient across the inner mitochondrial membrane, which depends on proper respiratory chain function, may be disrupted in ATM1 mutants, potentially affecting mitochondrial quality control mechanisms that protect mtDNA.

How do iron levels in different host niches affect ATM1 function during C. albicans infection?

C. albicans encounters drastically different iron environments during infection, from the iron-limited bloodstream to potentially iron-replete niches like the gastrointestinal tract. These varying conditions likely modulate ATM1 function through several mechanisms:

• Transcriptional regulation: C. albicans has evolved a specialized iron-responsive transcriptional circuit involving Sef1, Sfu1, and Hap43 . In iron-limited environments (bloodstream), Sef1 activates iron uptake genes and Hap43, while Hap43 represses Sfu1 and iron utilization genes. In iron-replete environments (gut), Sfu1 represses Sef1 and iron uptake genes. ATM1 expression may be regulated within this circuit to optimize iron distribution based on environmental availability.

• Post-translational modifications: Iron levels may influence ATM1 activity through post-translational modifications or by affecting interaction with regulatory proteins, similar to how other ABC transporters are regulated.

• Substrate availability: The primary substrate for ATM1 is likely a glutathione-complexed iron-sulfur cluster precursor or related compound. Varying iron levels affect the formation rate of these substrates, potentially altering ATM1 transport kinetics.

• Metabolic adaptation: C. albicans shows remarkable metabolic flexibility during infection. Iron availability influences metabolic pathway usage, potentially altering the demand for iron-sulfur proteins and consequently affecting the importance of ATM1 function in different niches.

This niche-specific modulation of ATM1 function highlights the sophisticated iron homeostasis mechanisms that contribute to C. albicans' success as both a commensal and opportunistic pathogen .

What techniques can be used to monitor iron-sulfur cluster transport by ATM1 in vivo?

Monitoring iron-sulfur cluster transport by ATM1 in vivo requires sophisticated approaches that can detect changes in iron-sulfur cluster assembly and distribution within cellular compartments:

• Activity assays of iron-sulfur enzymes: Measuring the activities of cytosolic and nuclear iron-sulfur enzymes (e.g., aconitase, xanthine oxidase) can indirectly assess ATM1 transport function. Reduced activities in ATM1 mutants would suggest impaired transport of iron-sulfur cluster precursors.

• Iron-55/59 radiolabeling: Pulse-chase experiments with radioactive iron isotopes can track the movement of iron from mitochondria to cytosol, with reduced cytosolic incorporation in ATM1 mutants indicating transport defects.

• Fluorescent iron and sulfur probes: Chemical probes sensitive to iron or sulfide can be targeted to specific cellular compartments to monitor relative concentrations and transport dynamics.

• Protein-fragment complementation assays: Split fluorescent proteins fused to components of the cytosolic iron-sulfur cluster assembly machinery can report on successful assembly events dependent on ATM1 transport.

• Real-time monitoring of iron-sulfur cluster formation: Genetically encoded fluorescent sensors that change properties upon binding iron-sulfur clusters can provide spatiotemporal information about cluster assembly and distribution.

• Electron paramagnetic resonance (EPR) spectroscopy: Whole-cell EPR can detect and quantify different types of iron-sulfur clusters in vivo, potentially revealing changes in cluster distribution when ATM1 function is altered.

• Mössbauer spectroscopy: This technique can distinguish different iron species in cells, allowing assessment of how ATM1 dysfunction affects the distribution of various iron forms between mitochondria and cytosol.

These approaches can be combined with genetic manipulations of ATM1 expression to establish causative relationships between transporter function and observed phenotypes.

How does oxidative stress impact ATM1 function in C. albicans?

Oxidative stress likely affects C. albicans ATM1 function through multiple mechanisms, creating a complex relationship with implications for cellular homeostasis:

• Direct oxidative modification: ATM1 contains numerous cysteine residues that may be susceptible to oxidative modifications, potentially altering transport activity. These modifications might serve as a regulatory mechanism to adapt transporter function during oxidative stress.

• Substrate alterations: The putative substrate for ATM1, likely a glutathione-complexed iron-sulfur cluster precursor, can be directly affected by oxidative stress. Oxidized glutathione (GSSG) has been shown to modulate ATM1 homologs in other organisms, such as P. falciparum ATM1 .

• Increased transport demands: Oxidative stress damages iron-sulfur clusters, increasing demand for their de novo synthesis and potentially elevating the need for ATM1 transport activity to supply the cytosolic CIA machinery.

• Mitochondrial damage feedback: Oxidative stress damages mitochondria, potentially altering membrane potential and ATP availability, which could affect ATM1 function as an ATP-dependent transporter.

• Transcriptional regulation: Oxidative stress activates stress-responsive transcription factors that may regulate ATM1 expression. In C. albicans, the stress response regulator Srr1 has mitochondrial localization , suggesting specialized stress response pathways exist for mitochondrial proteins like ATM1.

Research in S. cerevisiae has shown that MDL1, another mitochondrial ABC transporter, is a high-copy suppressor of ATM1, providing evidence for a role in resistance to oxidative stress . This suggests complex interactions between mitochondrial transporters in responding to oxidative challenges, which likely extends to C. albicans.

What is the evolutionary significance of ATM1 conservation across diverse species?

The high conservation of ATM1 across evolutionarily diverse species from bacteria to humans highlights its fundamental importance in cellular metabolism and provides insights into mitochondrial evolution:

• Endosymbiotic origin: ATM1 represents a component of the mitochondrial protein import/export machinery that evolved from the original endosymbiotic event. Studies suggest that two-component signaling pathways found in mitochondria originated in prokaryotes and were inherited by eukaryotes through endosymbiotic lateral gene transfer from ancestral cyanobacteria . ATM1's conservation likely reflects its essential role in the evolutionary transition from endosymbiont to subcellular organelle.

• Specialized adaptation: While conserving core transport functions, ATM1 homologs show species-specific adaptations. In C. albicans, which must adapt to diverse host environments with varying iron availability, ATM1 likely has evolved specific regulatory mechanisms integrated with the unique iron homeostasis circuit involving Sef1, Sfu1, and Hap43 .

• Disease implications: The clinical importance of ATM1 is underscored by the fact that mutations in the human homolog ABC7/ABCB7 cause X-linked sideroblastic anemia and ataxia . This connection between iron-sulfur cluster transport and human disease highlights the fundamental cellular processes dependent on this ancient transport system.

• Bacterial connections: Interestingly, phylogenetic analysis of mitochondrial two-component signaling proteins in C. albicans indicates closer relationships to proteins found in marine bacteria than to other fungal proteins . This suggests unique evolutionary pressures and potential horizontal gene transfer events shaping mitochondrial transport systems in different fungal lineages.

The conservation of ATM1 across diverse species underscores how fundamental iron-sulfur cluster transport is to cellular life and provides a fascinating window into the evolution of eukaryotic cells and mitochondrial function.

How can comparative genomics inform our understanding of C. albicans ATM1 function?

Comparative genomics approaches provide valuable insights into C. albicans ATM1 function through several strategies:

• Sequence conservation analysis: Identifying highly conserved residues across ATM1 homologs can pinpoint functionally critical amino acids involved in substrate binding, ATP hydrolysis, or conformational changes during transport. Regions showing higher variability may represent species-specific adaptations.

• Domain architecture comparison: Analyzing differences in domain organization between C. albicans ATM1 and homologs in other species can reveal specialized features. For example, variations in the mitochondrial targeting sequence or regulatory domains might reflect adaptation to different cellular environments.

• Regulatory element analysis: Comparing promoter regions of ATM1 genes across fungal species can identify conserved transcription factor binding sites, potentially revealing how ATM1 expression is integrated into different regulatory networks. In C. albicans, this might show connections to the iron-responsive transcription factors Sef1, Sfu1, and Hap43 .

• Phylogenetic profiling: Correlating the presence/absence or sequence features of ATM1 with other genes across species can identify functional partners. Proteins that show similar evolutionary patterns often participate in related processes.

• Natural variant analysis: Examining ATM1 sequence variations among clinical C. albicans isolates from different host niches or infection sites might reveal adaptations to specific host environments with different iron availabilities.

• Horizontal gene transfer assessment: The finding that some C. albicans mitochondrial proteins show closer relationships to marine bacterial proteins than to other fungal proteins suggests potential horizontal gene transfer events that could have introduced novel functions or regulatory mechanisms to C. albicans mitochondrial transporters.

Comparative genomics thus provides a powerful approach to understanding both the conserved core functions of ATM1 and the species-specific adaptations that might contribute to C. albicans' success as both a commensal and pathogen.

Could ATM1 serve as a potential antifungal target for C. albicans infections?

ATM1 represents a potentially attractive antifungal target for several compelling reasons:

• Essential function: Based on studies in related organisms, ATM1 likely provides essential functions in C. albicans, particularly during infection when iron acquisition and proper distribution are critical for survival in the host .

• Unique features compared to human homolog: While C. albicans ATM1 shares functional similarity with human ABCB7, structural and regulatory differences likely exist that could be exploited for selective targeting. The fact that fungi must adapt to more diverse iron environments than human cells might have driven unique adaptations in fungal ATM1 proteins.

• Integration with virulence mechanisms: C. albicans' ability to thrive in both iron-limited and iron-replete host niches depends on sophisticated iron homeostasis systems . Disrupting ATM1 function would likely impair this adaptation, potentially reducing virulence without completely blocking growth, which might reduce selective pressure for resistance development.

• Existing precedent for targeting iron homeostasis: Iron acquisition systems are already recognized as promising antifungal targets. Disrupting iron-sulfur cluster transport through ATM1 inhibition represents a novel angle for interfering with fungal iron utilization.

Target validation would require:

• Confirmation that ATM1 is essential for C. albicans virulence in animal models

• Development of assays suitable for high-throughput screening

• Structural characterization of C. albicans ATM1 to enable structure-based drug design

• Careful assessment of selectivity against human ABCB7 to avoid toxicity

The integration of ATM1 in iron homeostasis and mitochondrial function, both essential for C. albicans pathogenesis, makes it a promising though challenging antifungal target worthy of further investigation.

What experimental models are most suitable for investigating ATM1's role in C. albicans pathogenesis?

Several experimental models offer complementary advantages for investigating ATM1's role in C. albicans pathogenesis:

• Mammalian infection models:

  • Systemic candidiasis model: Intravenous infection of mice provides a well-established model for bloodstream infections, where iron limitation is a significant stress for C. albicans . This model would be particularly valuable for assessing how ATM1 contributes to survival in iron-limited environments.

  • Gastrointestinal colonization model: This model allows assessment of C. albicans persistence in the gut, an iron-replete environment where protection against iron toxicity is crucial . Comparing the fitness of ATM1 mutants in both bloodstream and gut models would reveal niche-specific requirements.

• Cell culture models:

  • Macrophage interaction assays: Macrophages restrict iron availability to pathogens. ATM1 mutants can be assessed for survival within macrophages to determine if iron-sulfur cluster transport is critical for resisting this host defense.

  • Epithelial adhesion and invasion assays: These assess the early stages of infection and can reveal if ATM1 dysfunction affects C. albicans' ability to adhere to and invade host tissues.

• Ex vivo models:

  • Reconstituted human epithelium: These 3D tissue models allow assessment of C. albicans invasion in a more physiologically relevant context than monolayer cultures.

  • Organoids: Intestinal organoids provide a complex model of the gut environment where iron levels can be manipulated to assess ATM1's role in adaptation to different iron conditions.

• Alternative host models:

  • Galleria mellonella (wax moth) larvae: This invertebrate model allows high-throughput virulence assessment and basic immune interaction studies.

  • Zebrafish embryos: The transparent nature of zebrafish embryos enables real-time visualization of infection progression and host-pathogen interactions.

• Conditional expression systems:

  • If ATM1 is essential, tetracycline-regulatable or other inducible systems allow virulence assessment with controlled ATM1 depletion during specific infection stages.

A combination of these models would provide comprehensive insights into how ATM1 contributes to C. albicans pathogenesis across different host niches and infection stages.

What are the major technical challenges in studying C. albicans ATM1?

Several significant technical challenges complicate research on C. albicans ATM1:

Addressing these challenges will require innovative approaches combining genetic, biochemical, and structural biology techniques tailored to the specific properties of C. albicans ATM1.