Recombinant Schizosaccharomyces pombe Iron-sulfur clusters transporter atm1, mitochondrial (atm1)

What is the primary function of Atm1 in Schizosaccharomyces pombe?

Atm1 in S. pombe functions as a mitochondrial ABC transporter essential for iron homeostasis and protection against oxidative stress. Its primary role involves exporting iron-sulfur cluster (ISC) precursors from mitochondria to the cytosol, facilitating the assembly of cytosolic and nuclear iron-sulfur cluster-containing proteins. This transport mechanism is crucial for maintaining proper cellular iron distribution and preventing oxidative damage. Atm1 is localized in the mitochondrial membrane, as confirmed by both fluorescence microscopy of GFP-tagged Atm1 and Western blot analysis of isolated mitochondrial fractions .

The importance of Atm1 extends beyond simple transport functions, as it plays integral roles in multiple cellular processes, including mitochondrial respiration, carbon source utilization, and maintenance of vacuolar functions. These diverse roles highlight the interconnected nature of iron metabolism with other cellular pathways in S. pombe and related fungal species .

How do Atm1 deletion mutations affect cellular phenotypes in fungi?

Deletion of Atm1 in fungal species produces several distinct phenotypic changes that highlight its essential functions:

• Growth defects: In Cryptococcus neoformans, atm1 deletion mutants exhibit significantly reduced growth rates in standard glucose media (YPD), with doubling times increasing from approximately 3.04 hours in wild-type cells to 6.81 hours in mutant cells .

• Carbon source utilization: Atm1 deletion renders cells unable to grow on non-fermentable carbon sources such as ethanol and acetate, indicating compromised mitochondrial respiratory function. This phenotype is conserved across several fungal species including S. cerevisiae, S. pombe, and C. neoformans .

• Enzyme activity reduction: The atm1 mutant shows diminished activity of cytosolic iron-sulfur cluster-containing proteins (such as Leu1) and heme-containing proteins (such as catalase), demonstrating the transporter's role in supplying essential components for these enzyme classes .

• Oxygen sensitivity: Atm1 deletion mutants show improved growth under reduced oxygen conditions, confirming the protein's involvement in mitochondrial respiratory functions that are primarily active in aerobic environments .

These phenotypic effects are largely conserved across different fungal species, suggesting evolutionary conservation of Atm1 function despite variations in iron regulatory networks.

How does Atm1 contribute to iron homeostasis in fungal species?

Atm1 plays a crucial role in fungal iron homeostasis by facilitating the export of iron-sulfur cluster precursors from mitochondria to the cytosol. This transport is essential for:

• Iron-sulfur cluster signaling: In fungi, iron-sulfur clusters serve as signaling molecules that regulate iron-responsive transcription factors. Atm1-mediated export of these precursors is necessary for the proper functioning of this signaling pathway .

• Glutaredoxin activity: Glutaredoxins (particularly those from the CGFS family) work alongside Atm1 to relay intracellular iron status to DNA-binding proteins that regulate iron metabolism genes. This partnership ensures appropriate cellular responses to changes in iron availability .

• Cytosolic iron-sulfur protein assembly: The exported precursors are utilized for the assembly of cytosolic and nuclear iron-sulfur proteins, which perform various essential cellular functions .

While the specific molecular mechanisms of iron regulation differ between fungal species (e.g., S. cerevisiae versus S. pombe), the involvement of Atm1 in iron-sulfur cluster transport represents a conserved theme. This conservation underscores the fundamental importance of iron-sulfur cluster signaling in fungal iron homeostasis across diverse species .

What are the key considerations when designing complementation studies for Atm1 across different fungal species?

When designing cross-species complementation studies involving Atm1, researchers should address several critical factors:

• Vector selection: Choose expression vectors appropriate for each host organism. For example, when testing C. neoformans ATM1 function in S. cerevisiae, episomal vectors like Yep352Gap-II have been successfully employed . Consider promoter strength, copy number, and compatibility with the host's replication machinery.

• Heterozygous vs. haploid systems: Since ATM1 deletion is lethal in haploid S. cerevisiae strains, heterozygous diploid mutants (ScATM1/Scatm1) must be used for complementation studies. In contrast, viable haploid atm1 mutants can be generated in C. neoformans and S. pombe .

• Expression verification: Implement multiple approaches to confirm proper expression:

  • RT-qPCR to verify transcript levels

  • Western blotting with species-specific antibodies

  • Subcellular localization studies using fluorescent tags

• Phenotypic rescue assessment: Evaluate complementation across multiple phenotypes:

• Evolutionary considerations: Account for potential functional divergence by analyzing sequence conservation in key functional domains. The core ABC transporter domains show higher conservation than regulatory regions, which may affect cross-species functionality .

How can researchers effectively study the interaction between Atm1 and glutaredoxins in iron-sulfur cluster transfer?

Investigating the functional relationship between Atm1 and glutaredoxins requires a multifaceted approach:

• Co-immunoprecipitation and proximity labeling studies:

  • Implement BioID or TurboID fusion proteins to identify proteins that interact directly with Atm1

  • Compare interactome data between wild-type and glutaredoxin mutant backgrounds

  • Validate interactions with reciprocal co-IP experiments

• Mutational analysis approach:

  • Generate point mutations in the putative interaction domains of Atm1

  • Focus on conserved residues in the ATP-binding cassette or transmembrane domains

  • Create a panel of mutants with varying degrees of functional impairment

• In vitro reconstitution of iron-sulfur cluster transfer:

  • Purify recombinant Atm1 and relevant glutaredoxins

  • Develop assays to measure iron-sulfur cluster transfer efficiency

  • Use spectroscopic methods (UV-visible, EPR) to monitor cluster transfer

• Real-time monitoring of iron-sulfur cluster dynamics:

  • Employ fluorescent iron sensors to track intracellular iron movement

  • Use split GFP systems to visualize protein-protein interactions in vivo

  • Implement time-resolved microscopy to capture transient interactions

• Genetic interaction mapping:

  • Construct double mutants between atm1 and various glutaredoxin genes

  • Perform synthetic genetic array analysis to identify genetic dependencies

  • Quantify epistatic relationships to determine pathway hierarchies

These approaches should be combined with careful controls, including non-interacting protein pairs and catalytically inactive mutants, to ensure specificity and biological relevance of the observed interactions .

What experimental approaches can distinguish direct versus indirect effects of Atm1 on mitochondrial function?

Discriminating between direct and indirect effects of Atm1 on mitochondrial function requires strategic experimental design:

• Temporal analysis of phenotypic changes:

  • Implement regulated expression systems (e.g., tetracycline-controlled or nitrogen-repressible promoters)

  • Monitor the sequential appearance of phenotypes after Atm1 depletion

  • Early-onset phenotypes are more likely direct consequences of Atm1 loss

• Targeted rescue experiments:

  • Supply downstream metabolites of Atm1-dependent pathways

  • Test whether specific mitochondrial defects can be reversed by:

    • Exogenous iron supplementation

    • Addition of antioxidants

    • Provision of cytosolic iron-sulfur cluster intermediates

• Compartment-specific measurements:

  • Isolate intact mitochondria from wild-type and atm1Δ strains

  • Assess respiratory chain complex activities, membrane potential, and reactive oxygen species production

  • Measure iron content and iron-sulfur cluster enzyme activities within purified mitochondria versus cytosolic fractions

• Genetic bypass experiments:

  • Overexpress alternative transporters or metabolic enzymes

  • Introduce mutations that activate compensatory pathways

  • Test whether these interventions can bypass specific Atm1-dependent phenotypes

• Structural and functional characterization:

  • Generate Atm1 variants with altered substrate specificity

  • Perform electron microscopy to assess mitochondrial ultrastructure

  • Conduct mitochondrial protein import assays to evaluate organelle integrity

A comprehensive experimental matrix comparing multiple mitochondrial parameters between wild-type, atm1Δ, and functionally characterized Atm1 point mutants can help distinguish primary defects from secondary adaptations to Atm1 loss .

How should researchers interpret seemingly contradictory data between Atm1 function in S. pombe versus S. cerevisiae?

When faced with apparently contradictory data regarding Atm1 function across fungal species, researchers should systematically evaluate several factors:

• Evolutionary context: Despite sharing the core function of iron-sulfur cluster export, Atm1 operates within distinct iron regulatory networks in different fungi. For example, S. cerevisiae employs Aft1/Aft2 transcription factors while S. pombe uses different regulatory mechanisms. These divergent networks may place different functional demands on Atm1 .

• Methodological differences:

  • Assay sensitivity and specificity variations

  • Growth conditions that may not be perfectly matched

  • Different genetic backgrounds that can influence phenotypic manifestations

• Resolution through comparative analysis:

  • Identify conserved versus species-specific phenotypes

  • Determine whether differences are qualitative or merely quantitative

  • Create a hierarchical model of primary versus secondary functions

• Experimental validation approaches:

  • Perform heterologous expression studies under identical conditions

  • Conduct domain-swapping experiments between orthologs

  • Generate chimeric proteins to identify functionally divergent regions

• Integrative data analysis framework:

This systematic approach allows researchers to distinguish true functional differences from experimental artifacts and to develop unified models that accommodate species-specific variations while recognizing core conserved functions .

What bioinformatic approaches are most effective for analyzing conservation and divergence of Atm1 across fungal species?

To comprehensively analyze Atm1 conservation and divergence across fungal species, researchers should implement a multi-layered bioinformatic strategy:

• Sequence-based comparative analysis:

  • Multiple sequence alignment of Atm1 orthologs using MUSCLE or T-Coffee

  • Calculation of conservation scores for individual residues and domains

  • Identification of species-specific insertions, deletions, or rapidly evolving regions

  • Phylogenetic reconstruction to map evolutionary relationships and potential functional divergence

• Structural bioinformatics:

  • Homology modeling based on related ABC transporter structures

  • Mapping of conserved residues onto 3D models to identify functional surfaces

  • Molecular dynamics simulations to assess structural flexibility and substrate interactions

  • Prediction of potential posttranslational modification sites

• Genomic context analysis:

  • Assessment of synteny and gene neighborhood conservation

  • Identification of co-evolved gene clusters

  • Analysis of promoter regions to detect conserved regulatory elements

  • Comparison of intron-exon structures across species

• Integration with functional genomics data:

  • Incorporation of transcriptomic profiles across species

  • Analysis of protein-protein interaction networks

  • Correlation of genetic interaction profiles

  • Examination of phenotypic data from systematic deletion studies

• Prediction of functional impacts:

  • Computational prediction of substrate specificity

  • Identification of potential regulatory motifs

  • Assessment of codon usage bias as an indicator of expression levels

  • Analysis of selection pressure (dN/dS ratios) across different domains

This multi-faceted approach provides a comprehensive view of both structural and functional conservation, allowing researchers to generate testable hypotheses about species-specific adaptations in Atm1 function and regulation .

How can researchers accurately interpret changes in cytosolic Fe-S protein activities in atm1 mutants?

Accurate interpretation of changes in cytosolic iron-sulfur protein activities in atm1 mutants requires careful consideration of multiple factors:

• Direct versus indirect effects framework:

  • Primary effect: Reduced export of iron-sulfur cluster precursors from mitochondria

  • Secondary effects: Changes in iron homeostasis, oxidative stress, and adaptive responses

  • Tertiary effects: Altered gene expression, protein stability, and cellular metabolism

• Activity measurement considerations:

  • Enzyme-specific factors affecting measurements:

    • Protein expression levels (verify by Western blot)

    • Post-translational modifications affecting activity

    • Substrate availability and cofactor status

  • Assay-specific technical variables:

    • Reaction conditions (pH, temperature, ionic strength)

    • Sample preparation methods

    • Detection sensitivity and linear range

• Comprehensive analysis approach:

  • Measure multiple iron-sulfur enzymes (e.g., Leu1, aconitase, sulfite reductase)

  • Compare with non-iron-sulfur enzymes as controls

  • Assess both activity and protein levels to distinguish between catalytic defects and protein abundance changes

• Rescue experiments for mechanistic insight:

  • Test whether activity can be restored in vitro by adding iron or reducing agents

  • Determine if activity correlates with iron content measured by ICP-MS

  • Evaluate whether activity can be rescued by overexpression of other components of iron-sulfur cluster assembly machinery

• Quantitative data interpretation framework:

By systematically assessing multiple parameters and controls, researchers can distinguish between specific defects in iron-sulfur cluster assembly versus broader metabolic adaptations or stress responses that might indirectly affect enzyme activities .

What are the optimal strategies for generating and validating recombinant S. pombe Atm1 for in vitro studies?

Generating high-quality recombinant S. pombe Atm1 for in vitro studies presents several challenges due to its membrane-bound nature. The following comprehensive strategy addresses these challenges:

• Expression system selection:

  • Bacterial systems (E. coli):

    • Advantages: High yield, cost-effective

    • Limitations: Membrane protein folding issues, lack of post-translational modifications

    • Optimization: Use specialized strains (C41/C43) and fusion tags (MBP, SUMO)

  • Yeast systems (S. cerevisiae, P. pastoris):

    • Advantages: Native-like membrane environment, proper folding

    • Recommended approach: Express in S. cerevisiae atm1Δ background for functional validation

  • Insect cell systems:

    • Advantages: High expression, mammalian-like glycosylation

    • Best for structural studies requiring large protein quantities

• Construct optimization:

  • Include His6/FLAG/STREP tandem affinity tags for purification

  • Consider GFP fusion to monitor expression and folding

  • Generate truncated constructs removing flexible termini for structural studies

  • Introduce thermostabilizing mutations based on computational predictions

• Purification strategy:

  • Membrane preparation: Differential centrifugation followed by sucrose gradient purification

  • Solubilization screening: Test panel of detergents (DDM, LMNG, GDN)

  • Chromatography sequence:

    • IMAC (immobilized metal affinity chromatography)

    • Size exclusion chromatography

    • Optional ion exchange for higher purity

• Functional validation methods:

  • ATPase activity assays:

    • Colorimetric phosphate release detection

    • Coupled enzyme assays for real-time monitoring

  • Substrate binding assays:

    • Microscale thermophoresis

    • Surface plasmon resonance

  • Transport reconstitution:

    • Proteoliposome preparation with controlled lipid composition

    • Fluorescent substrate analogs for transport measurement

• Quality control checkpoints:

  • Purity: SDS-PAGE with silver staining (>95% purity target)

  • Homogeneity: Dynamic light scattering, analytical ultracentrifugation

  • Structural integrity: Circular dichroism, limited proteolysis

  • Functional state: ATP binding capacity, substrate interaction

This methodical approach ensures production of functional recombinant Atm1 suitable for biochemical and structural characterization, while avoiding common pitfalls associated with membrane protein expression and purification.

What experimental controls are essential when studying the role of Atm1 in oxidative stress resistance?

When investigating Atm1's role in oxidative stress resistance, implementing the following essential controls ensures robust and interpretable results:

• Genetic controls:

  • Wild-type strain (positive control)

  • atm1Δ deletion mutant (experimental)

  • atm1Δ complemented with wild-type ATM1 (rescue control)

  • atm1Δ complemented with catalytically inactive ATM1 (specificity control)

  • Strains with deletions in known oxidative stress response genes (comparative controls)

• Oxidative stress induction controls:

  • Dose-response curves for each oxidative stressor

  • Time-course measurements to distinguish acute versus chronic effects

  • Multiple oxidative stress agents to differentiate specific sensitivities:

    • Hydrogen peroxide (general oxidative stress)

    • Menadione (superoxide generation)

    • tert-Butyl hydroperoxide (lipid peroxidation)

    • Heavy metals (specific iron-related stress)

• Physiological measurement controls:

  • Growth conditions standardization:

    • Defined media composition with controlled iron levels

    • Identical culture density at treatment initiation

    • Consistent growth phase (log phase recommended)

  • Multiple readout methods:

    • Survival plating (viable cell counting)

    • Growth curve analysis (real-time monitoring)

    • Metabolic activity assays (e.g., resazurin reduction)

• Molecular analysis controls:

  • Oxidative damage markers:

    • Protein carbonylation (control: heat-inactivated samples)

    • Lipid peroxidation (control: antioxidant-treated samples)

    • DNA oxidation (control: DNase-treated samples)

  • Antioxidant enzyme activities:

    • Include non-iron dependent enzymes as controls

    • Measure enzyme expression levels alongside activity

    • Include chemical inhibitors of each enzyme in parallel assays

• Rescue experiment controls:

  • Antioxidant supplementation (N-acetylcysteine, glutathione)

  • Iron chelation versus iron supplementation

  • Expression of alternative transporters

  • Genetic suppressors (e.g., deletion of iron regulators)

These systematically designed controls allow researchers to distinguish direct effects of Atm1 on oxidative stress resistance from indirect consequences of altered iron metabolism or mitochondrial dysfunction, ensuring accurate interpretation of experimental results .

What analytical techniques are most appropriate for studying iron-sulfur cluster transfer in S. pombe atm1 mutants?

Investigating iron-sulfur cluster transfer in S. pombe atm1 mutants requires specialized analytical techniques spanning multiple levels of biological organization:

• Spectroscopic methods for direct Fe-S cluster detection:

  • UV-visible spectroscopy:

    • Characteristic absorption peaks for [2Fe-2S] (320, 420, 460 nm) and [4Fe-4S] (390, 420 nm) clusters

    • Sample preparation: Anaerobic cell extracts or purified proteins

    • Limitations: Limited sensitivity, potential interference from other chromophores

  • Electron Paramagnetic Resonance (EPR):

    • Highly specific for paramagnetic Fe-S species

    • Can distinguish different cluster types and oxidation states

    • Sample requirements: Low-temperature measurements (liquid helium), concentrated samples

  • Mössbauer spectroscopy:

    • Provides detailed information on iron oxidation state and coordination

    • Requires 57Fe enrichment of cultures

    • Enables quantification of different iron species in whole cells

• Enzymatic activity measurements of Fe-S proteins:

  • Cytosolic markers:

    • Isopropylmalate isomerase (Leu1): Spectrophotometric assay measuring isopropylmalate → dimethylcitraconate conversion

    • Sulfite reductase: Measurement of sulfite-dependent NADPH oxidation

    • Aconitase: Coupled assay with isocitrate dehydrogenase

  • Mitochondrial markers:

    • Succinate dehydrogenase: Dichlorophenolindophenol reduction assay

    • Aconitase: Same assay as cytosolic, but with isolated mitochondria

  • Controls: Compare activities with total protein levels (Western blot) to distinguish assembly defects from expression changes

• Iron trafficking and distribution analysis:

  • Subcellular fractionation combined with ICP-MS:

    • Separate mitochondria, cytosol, and other compartments

    • Measure total iron content in each fraction

    • Compare with other metals (copper, zinc) as controls

  • Radioactive 55Fe labeling:

    • Pulse-chase experiments to track iron movement between compartments

    • Immunoprecipitation of specific Fe-S proteins to measure cluster incorporation

    • Time-resolved measurements to determine transfer kinetics

• Genetic reporter systems:

  • Fe-S-dependent transcription factor activity:

    • Reporter genes driven by iron-responsive promoters

    • Measurement of expression changes in response to atm1 deletion

  • Split reporter complementation:

    • Fusion of Fe-S scaffold proteins with split luciferase/GFP fragments

    • Signal generation upon successful cluster transfer

    • Live cell imaging to monitor cluster movement

• Proteomics approaches:

  • Differential protein expression analysis between wild-type and atm1Δ

  • Post-translational modification mapping (especially oxidative modifications)

  • Protein-protein interaction networks using proximity labeling in different compartments

Each of these techniques provides complementary information, and their combined application enables comprehensive characterization of how Atm1 deficiency affects iron-sulfur cluster biogenesis and distribution throughout the cell .

What are the most promising future research directions for S. pombe Atm1 studies?

The field of S. pombe Atm1 research offers several promising avenues for future investigation that could significantly advance our understanding of iron-sulfur cluster metabolism and its connections to cellular physiology:

• Structural biology approaches:

  • High-resolution structure determination of S. pombe Atm1 using cryo-electron microscopy

  • Comparison with orthologs from pathogenic fungi to identify unique structural features

  • Structure-guided design of specific inhibitors as potential antifungal agents

  • Visualization of substrate binding and conformational changes during transport cycle

• Systems biology integration:

  • Multi-omics analysis comparing wild-type and atm1 mutants across different growth conditions

  • Mathematical modeling of iron distribution networks incorporating Atm1-mediated transport

  • Genome-wide genetic interaction mapping to identify synthetic lethal partners

  • Metabolic flux analysis to quantify the impact of Atm1 deficiency on cellular metabolism

• Translational research opportunities:

  • Exploration of Atm1's role in fungal pathogenesis and virulence

  • Development of small molecule modulators of Atm1 activity as research tools

  • Investigation of human orthologs (ABCB7) in connection with sideroblastic anemia

  • Comparative studies with bacterial iron-sulfur transporters for evolutionary insights

• Advanced methodological developments:

  • Single-molecule tracking of iron-sulfur cluster transfer in living cells

  • Development of specific iron-sulfur cluster biosensors

  • Application of optogenetic approaches to modulate Atm1 activity with spatial and temporal precision

  • CRISPR-based screening for novel components of the Atm1-dependent pathway

• Integration with cellular stress response networks:

  • Investigation of Atm1's role in connecting iron metabolism with other stress response pathways

  • Analysis of post-translational modifications regulating Atm1 activity

  • Exploration of potential non-canonical functions beyond iron-sulfur cluster export

  • Examination of the relationship between Atm1 and cellular aging/lifespan regulation

These research directions leverage emerging technologies while addressing fundamental questions about the role of Atm1 in cellular iron homeostasis. The evolutionary conservation of iron-sulfur cluster signaling across fungal species suggests that insights gained from S. pombe can inform our understanding of iron metabolism in both model organisms and pathogenic fungi, potentially leading to novel therapeutic strategies .

How might comparative studies of Atm1 across fungal species inform potential antifungal drug development?

Comparative analysis of Atm1 across fungal species offers valuable insights for antifungal drug development through several key approaches:

• Identification of pathogen-specific vulnerabilities:

  • Detailed sequence and structural comparisons between pathogenic (Candida, Aspergillus, Cryptococcus) and non-pathogenic (S. cerevisiae, S. pombe) Atm1 orthologs

  • Mapping of unique domains or motifs present only in pathogenic species

  • Functional analysis of these regions to determine their importance for virulence

• Targeting iron metabolism as a virulence strategy:

  • Atm1's essential role in iron homeostasis makes it an attractive target, as iron acquisition is a critical virulence determinant for fungal pathogens

  • Comprehensive phenotypic analysis of atm1 mutants in infection models

  • Identification of iron-dependent processes specific to host-pathogen interaction

• Structure-based drug design approach:

  • Comparative homology modeling of Atm1 transporters from multiple fungal species

  • Virtual screening campaigns targeting pathogen-specific binding pockets

  • Design of selective inhibitors that exploit structural differences while sparing human orthologs

• Target validation framework:

  • Generation of conditional atm1 mutants in pathogenic fungi to confirm essentiality

  • Testing potential inhibitors in both cellular and infection models

  • Development of resistance mechanisms to understand potential clinical limitations

• Combination therapy strategies:

  • Identification of synthetic lethal interactions with Atm1 across species

  • Design of drug combinations targeting both Atm1 and synergistic pathways

  • Leveraging iron chelation approaches alongside Atm1 inhibition