Recombinant Emericella nidulans Iron-sulfur clusters transporter atm1, mitochondrial (atm1)

What is Emericella nidulans Iron-sulfur clusters transporter atm1?

Emericella nidulans Iron-sulfur clusters transporter atm1 (atm1) is a mitochondrial membrane protein belonging to the superfamily B of ABC transporters, often classified as a "half-transporter" that functions as a homodimer to facilitate active transport across the inner mitochondrial membrane . The protein contains an N-terminal transmembrane domain and a C-terminal nucleotide-binding domain (NBD) that work together through a classical switch mechanism to catalyze the transfer of cargo . Atm1 plays a crucial role in transporting iron-sulfur clusters or related compounds from the mitochondria to the cytosol, serving as a bridge between mitochondrial iron-sulfur cluster biogenesis and cytosolic iron-sulfur protein assembly . The full-length mature protein comprises amino acids 18-721, with a molecular weight consistent with other ABC transporters in this class . Structurally, it contains multiple transmembrane segments that anchor the protein in the mitochondrial membrane while positioning the catalytic domains for optimal substrate interaction and transport.

How does atm1 relate to other fungal ABC transporters?

Atm1 belongs to a phylogenetically distinct clade within the ABC transporter family in fungi, where it serves specialized functions beyond the typical drug extrusion or pheromone export roles often associated with ABC transporters . Unlike some fungal transporters like A. nidulans AtrD that participate in β-lactam secretion or A. fumigatus AbcB involved in siderophore peptide breakdown product excretion, atm1 specifically participates in iron-sulfur cluster transport pathways . Phylogenetic analyses of fungal ABC proteins reveal that atm1 homologs are highly conserved across diverse fungal species, indicating their fundamental importance in cellular metabolism . The conservation of these transporters extends beyond fungi to plants (ATM3) and mammals (ABCB7), suggesting an evolutionary ancient and essential role in eukaryotic cell functioning . In comparative genomic studies, Pezizomycotina species (including E. nidulans) can contain up to five members of related transporter subgroups, although the specific functional roles of many remain poorly characterized .

What is the potential substrate of atm1 transporters?

The precise substrate(s) of atm1 and its homologs remains somewhat elusive, presenting an active area of investigation . Current evidence suggests that Saccharomyces cerevisiae ATM1 (ScATM1) likely transports glutathione-coordinated [2Fe-2S] clusters, specifically [2Fe-2S]GS4 complexes, across the mitochondrial membrane . An alternative proposed cargo includes polysulfide adducts of oxidized glutathione that may play a role in cytosolic tRNA thiolation, as suggested by experiments with ScATM1 and Arabidopsis thaliana ATM3 . Biochemical characterization of homologous transporters in Apicomplexa parasites, like PfATM1 in Plasmodium falciparum, confirms their function as ABC transporters that are modulated by oxidized glutathione (GSSG) and [4Fe-4S] clusters . The activity of atm1 transporters appears to be closely tied to redox conditions, with oxidative stress significantly affecting their function and expression . Transport assays using reconstituted protein in liposomes have provided valuable insights, but definitive identification of the physiological substrate continues to be a challenge due to the transient nature of the transported species.

How is recombinant E. nidulans atm1 typically expressed and purified for research?

The recombinant full-length E. nidulans atm1 protein is typically expressed as a His-tagged fusion protein in E. coli expression systems, which provides a convenient method for affinity purification . The construct commonly used includes amino acids 18-721 of the mature protein fused to an N-terminal histidine tag, allowing for efficient one-step purification using immobilized metal affinity chromatography (IMAC) . Following initial purification, size exclusion chromatography is often employed to achieve higher purity and to separate monomeric from dimeric forms of the protein. The purified protein is typically obtained as a lyophilized powder that requires careful reconstitution in an appropriate buffer, usually a Tris/PBS-based buffer at pH 8.0 containing 6% trehalose as a stabilizing agent . For long-term storage, it is recommended to add glycerol to a final concentration of 5-50% (with 50% being standard) and to store aliquots at -20°C/-80°C to prevent repeated freeze-thaw cycles that can compromise protein integrity . Protein purity is commonly assessed by SDS-PAGE, with preparations typically yielding greater than 90% purity .

What reconstitution methods are most effective for functional studies of atm1?

For functional studies, recombinant atm1 must be properly reconstituted to maintain its native conformational state and transport activity. Initial reconstitution involves rehydration of the lyophilized protein in deionized sterile water to a concentration of 0.1-1.0 mg/mL . The reconstitution buffer should be carefully optimized to maintain protein stability, typically containing physiological salt concentrations, appropriate pH (7.5-8.0), and sometimes mild detergents to prevent aggregation. For transport assays, atm1 is often reconstituted into proteoliposomes using a mixture of phospholipids that mimic the mitochondrial membrane composition, typically through detergent-mediated reconstitution followed by detergent removal via dialysis or adsorption to bio-beads. ATP and magnesium are essential components for functional assays as they provide the energy required for the conformational changes associated with transport activity. To assess substrate transport, fluorescent or radioactively labeled potential substrates such as glutathione derivatives or iron-sulfur cluster precursors are commonly used. During reconstitution, it is critical to avoid repeated freeze-thaw cycles and to maintain the protein at 4°C when actively working with it to preserve its functional integrity .

What analytical techniques are suitable for studying atm1 structure and function?

Multiple complementary analytical techniques provide insights into atm1 structure and function. X-ray crystallography and cryo-electron microscopy have been employed with homologous transporters to determine three-dimensional structures in different conformational states, though these techniques often require extensive optimization of protein constructs and crystallization conditions. Circular dichroism spectroscopy offers valuable information about secondary structure content and thermal stability, while fluorescence spectroscopy can assess conformational changes upon substrate binding or nucleotide hydrolysis. For functional characterization, ATPase activity assays using colorimetric or bioluminescent detection of inorganic phosphate release provide quantitative measures of catalytic activity under various conditions. Transport assays using reconstituted proteoliposomes loaded with fluorescent probes allow real-time monitoring of transport activities. Site-directed mutagenesis coupled with functional assays helps identify critical residues involved in substrate recognition and transport. Mass spectrometry methods, particularly hydrogen-deuterium exchange mass spectrometry (HDX-MS), can provide insights into dynamic aspects of protein structure and ligand interactions without requiring crystallization. Molecular dynamics simulations complement experimental approaches by modeling the dynamic behavior of the transporter in a membrane environment.

How does atm1 depletion affect cellular iron-sulfur cluster metabolism?

Depletion or deletion of atm1 and its homologs results in profound effects on cellular iron-sulfur cluster metabolism across different species. In Saccharomyces cerevisiae, deletion of ScATM1 leads to increased sensitivity to oxidative stress and a remarkable 30-fold increase in mitochondrial iron accumulation, highlighting its critical role in maintaining iron homeostasis . Transcriptional analyses of yeast lacking ScATM1 reveal disruptions in multiple pathways, including upregulation of genes involved in cytosolic iron uptake (mediated by Aft1/2 transcription factors), decreased heme levels, and reduced activity of the heme-containing complex IV of the electron transport chain . In mammals, mutations in the homologous ABCB7 result in X-linked sideroblastic anemia with ataxia, characterized by loss of cytosolic iron-sulfur proteins due to cytosolic iron-sulfur cluster assembly (CIA) defects, mitochondrial iron accumulation, and impaired heme biosynthesis . Interestingly, in Toxoplasma gondii, depletion of TgATM1 does not specifically impair mitochondrial metabolism but instead affects the abundance of proteins involved in the transfer of iron-sulfur clusters to cytosolic proteins at the outer mitochondrial membrane, suggesting potential species-specific differences in metabolic integration .

What interactions exist between atm1 and the cytosolic iron-sulfur cluster assembly (CIA) machinery?

Atm1 serves as a critical link between mitochondrial iron-sulfur cluster biogenesis and the cytosolic iron-sulfur cluster assembly (CIA) pathway, though the exact mechanisms of this interaction remain incompletely understood . The transporter functions by exporting a sulfur-containing compound (potentially a glutathione-coordinated iron-sulfur cluster or related precursor) from the mitochondria to the cytosol, where it is utilized by the CIA machinery for assembly of cytosolic and nuclear iron-sulfur proteins . Proteomic analyses of TgATM1-depleted Toxoplasma gondii cells revealed that ATM1 expression levels inversely correlate with the abundance of proteins that participate in the transfer of iron-sulfur clusters to cytosolic proteins at the outer mitochondrial membrane, suggesting a compensatory relationship . Functional complementation studies using the well-characterized yeast ScATM1 in TgATM1-depleted cells have provided valuable insights into the conserved mechanisms of these interactions . The CIA machinery itself comprises multiple protein components including the cytosolic scaffold proteins, electron transfer components, and targeting factors that work coordinately to assemble iron-sulfur clusters on recipient apoproteins. Disruption of atm1 function impairs the activities of numerous cytosolic iron-sulfur proteins including enzymes involved in DNA metabolism, protein translation, and cellular redox control, highlighting the broad impact of this transporter on essential cellular processes.

How can functional complementation assays be designed to study atm1 orthologs?

Functional complementation assays provide powerful tools for studying the evolutionary conservation and specific functions of atm1 orthologs from different species. Such assays are typically designed using a deletion mutant of a well-characterized ortholog (often ScATM1 in Saccharomyces cerevisiae) as the background strain, which is then transformed with expression constructs containing atm1 orthologs from other species . For E. nidulans atm1, the experimental design would involve cloning the full-length coding sequence into an appropriate yeast expression vector, ensuring proper targeting to the mitochondria by retaining the native mitochondrial targeting sequence or replacing it with the equivalent sequence from the recipient species. Transformed yeast cells would then be assessed for rescue of the phenotypic defects associated with ScATM1 deletion, including growth defects (particularly under oxidative stress conditions), mitochondrial iron accumulation (measured by colorimetric assays or inductively coupled plasma mass spectrometry), and restoration of activities of cytosolic iron-sulfur proteins such as isopropylmalate isomerase or sulfite reductase. A quantitative measure of complementation efficiency can be obtained by comparing growth rates under various stress conditions, particularly in the presence of oxidative stress-inducing agents like hydrogen peroxide or menadione. More sophisticated analyses might include transcriptomic profiling to assess the extent of normalization of gene expression patterns or proteomic studies to evaluate the restoration of iron-sulfur protein abundance and activity.

What are the key structural domains of E. nidulans atm1 and their functions?

E. nidulans atm1 exhibits the characteristic domain architecture of half-transporters in the ABC superfamily B, with distinct functional domains that work cooperatively . The N-terminal region contains the transmembrane domain (TMD) comprising multiple α-helical segments that span the inner mitochondrial membrane, forming the substrate translocation pathway . The C-terminal region houses the nucleotide-binding domain (NBD) that binds and hydrolyzes ATP, providing the energy required for the conformational changes that drive substrate transport . The protein sequence (amino acids 18-721) includes conserved motifs characteristic of ABC transporters, including the Walker A and Walker B motifs in the NBD that are involved in ATP binding and hydrolysis, respectively . The amino acid sequence of E. nidulans atm1 reveals numerous charged and polar residues in predicted transmembrane segments that likely participate in substrate recognition and translocation . Structural models based on homologous transporters suggest that atm1 functions through a switch mechanism where ATP binding and hydrolysis induce conformational changes that alternately expose the substrate-binding site to opposite sides of the membrane, facilitating directional transport . As a half-transporter, functional atm1 requires dimerization to form a complete translocation pathway, with the two NBDs coming together to form two composite ATP-binding sites at the dimer interface .

How does the amino acid sequence of E. nidulans atm1 compare to homologs in other species?

The amino acid sequence of E. nidulans atm1 shares significant homology with ABC transporters in diverse species, reflecting the evolutionary conservation of this essential protein family . The full amino acid sequence of E. nidulans atm1 (residues 18-721) reveals characteristic features of mitochondrial ABC transporters, with an N-terminal region that likely serves as a mitochondrial targeting sequence followed by alternating hydrophobic transmembrane segments and hydrophilic loops . Sequence comparison with homologs in other organisms reveals highest conservation in the nucleotide-binding domain, particularly in motifs directly involved in ATP binding and hydrolysis, such as the Walker A motif (GXXGXGKS/T), Walker B motif (hhhhD, where h is a hydrophobic residue), and the ABC signature motif (LSGGQ) . The transmembrane domains show more sequence diversity, though certain residues implicated in substrate binding and translocation remain conserved. Notably, the E. nidulans sequence shows significant similarity to Saccharomyces cerevisiae ATM1, enabling functional complementation in heterologous expression systems . Comparison with mammalian ABCB7 reveals conservation of residues associated with human disease mutations in X-linked sideroblastic anemia with ataxia, providing insights into structure-function relationships. The protein also shares sequence features with atm1 homologs in Apicomplexa parasites (Toxoplasma gondii and Plasmodium falciparum), reflecting the broad conservation of this transporter across diverse eukaryotic lineages .

What experimental models are suitable for studying atm1 function in vivo?

Several experimental models offer complementary advantages for investigating atm1 function in vivo. Saccharomyces cerevisiae represents a powerful model system due to its genetic tractability, with ScATM1 deletion strains providing a well-characterized background for functional studies and complementation assays with E. nidulans atm1 . Filamentous fungi, including E. nidulans itself, offer the advantage of studying the protein in its native context, though genetic manipulation may be more challenging than in yeast. For biochemical characterization, heterologous expression in E. coli remains the method of choice, allowing production of sufficient quantities of recombinant protein for structural and functional analyses . Toxoplasma gondii and Plasmodium falciparum provide insights into atm1 function in divergent eukaryotes, with conditional knockdown systems enabling temporal control of protein expression to study essential genes . Mammalian cell culture models expressing E. nidulans atm1 can be used to investigate functional conservation across greater evolutionary distances and to assess potential differences in substrate specificity or transport mechanisms. In all models, fluorescent protein tagging provides valuable information on subcellular localization and dynamics, while epitope tagging facilitates proteomic analyses of interaction partners. Emerging technologies such as CRISPR-Cas9 genome editing enable precise manipulation of endogenous genes to create specific mutations or regulatory changes for detailed mechanistic studies.

What are the optimal conditions for measuring atm1 transport activity in vitro?

Establishing optimal conditions for measuring atm1 transport activity in vitro requires careful consideration of multiple parameters. The protein should be reconstituted into liposomes composed of a lipid mixture that mimics the mitochondrial inner membrane, typically including phosphatidylcholine, phosphatidylethanolamine, and cardiolipin in appropriate ratios. The buffer composition is critical, generally containing physiological concentrations of potassium or sodium chloride (100-150 mM), a buffering agent such as HEPES or Tris (20-50 mM, pH 7.4-8.0), and magnesium chloride (1-5 mM) as a cofactor for ATP hydrolysis . ATP should be included at a concentration of 1-5 mM to drive transport activity, with an ATP regenerating system (creatine phosphate and creatine kinase) often added to maintain ATP levels during extended assays. Temperature control is essential, with most assays performed at 25-30°C to balance protein stability with activity. Potential substrates for transport assays include glutathione derivatives, particularly oxidized glutathione (GSSG) which has been shown to modulate the activity of atm1 homologs . Substrate transport can be monitored using radioisotope-labeled compounds, fluorescent probes, or coupled enzymatic assays that detect substrate appearance inside the liposomes or depletion from the external medium. Control experiments should include liposomes without protein, assays in the absence of ATP, and inclusion of transport inhibitors such as vanadate to confirm the specificity of the measured activity.

How can researchers address challenges in expressing and purifying functional atm1?

Expression and purification of functional membrane proteins like atm1 present numerous challenges that require systematic optimization. For heterologous expression in E. coli, selection of an appropriate strain is crucial, with options like C41(DE3) or C43(DE3) often proving superior for membrane protein expression . Expression vectors should include a strong but regulatable promoter (such as T7) to control expression levels, preventing toxic accumulation of the protein. The inclusion of fusion tags like His6 at either the N- or C-terminus facilitates purification while fusion partners such as maltose-binding protein (MBP) or thioredoxin can enhance solubility . Induction conditions require careful optimization, with lower temperatures (16-20°C) and reduced inducer concentrations often yielding better results for membrane proteins. For extraction from membranes, selection of an appropriate detergent is critical—mild detergents like n-dodecyl-β-D-maltoside (DDM) or lauryl maltose neopentyl glycol (LMNG) generally preserve protein structure and function better than harsh detergents like Triton X-100. Purification should proceed rapidly at 4°C to minimize protein degradation, with chromatography buffers containing stabilizing agents such as glycerol (10-20%) and sometimes lipids to maintain the native environment . For particularly challenging constructs, alternative expression systems including yeast (Pichia pastoris), insect cells (using baculovirus), or cell-free systems might prove more successful. Finally, stability screening using techniques like differential scanning fluorimetry can identify optimal buffer conditions for maintaining protein integrity throughout purification and subsequent functional studies.

How does understanding atm1 contribute to research on human diseases?

Research on E. nidulans atm1 and its homologs provides critical insights into human diseases associated with defects in iron-sulfur cluster biogenesis and transport. The human homolog ABCB7 is directly implicated in X-linked sideroblastic anemia with ataxia (XLSA/A), a rare genetic disorder characterized by impaired heme synthesis, mitochondrial iron overload, and progressive ataxia . Comparative studies between fungal atm1 and human ABCB7 can reveal conserved structural features and functional mechanisms that help interpret human disease mutations. The efficient expression and purification systems developed for E. nidulans atm1 provide templates for producing human ABCB7 for structural studies and drug screening . Beyond XLSA/A, dysfunction in iron-sulfur cluster biogenesis has been implicated in multiple human disorders including Friedreich's ataxia, various myopathies, and certain neurodegenerative conditions. Fungal models expressing E. nidulans atm1 variants designed to mimic human disease mutations can serve as valuable tools for investigating pathological mechanisms and screening potential therapeutic compounds. The fundamental role of atm1 in cellular iron homeostasis also connects to more common disorders of iron metabolism, including hereditary hemochromatosis and various anemias. Furthermore, understanding the role of atm1 in oxidative stress responses provides insights into pathological conditions associated with redox imbalance, including aspects of aging, cancer, and inflammatory diseases.

What biotechnological applications might utilize recombinant atm1?

Recombinant atm1 offers several promising biotechnological applications beyond its value in basic research. The protein could serve as a target for developing selective antifungal agents, particularly against pathogenic Aspergillus species, by exploiting structural or functional differences between fungal and human homologs. Engineered variants of atm1 with enhanced transport capacity could potentially be used in bioremediation applications for removing heavy metals or toxins from contaminated environments, leveraging the protein's ability to transport diverse substrates across membranes. The well-characterized expression and purification system for recombinant atm1 provides a template for producing other challenging membrane proteins of biotechnological interest . In protein engineering applications, the nucleotide-binding domain of atm1 could be fused with alternative transmembrane domains to create chimeric transporters with novel substrate specificities for synthetic biology applications. As a component of iron-sulfur cluster biosynthesis pathways, atm1 could be incorporated into metabolic engineering efforts aimed at enhancing production of iron-sulfur enzymes for industrial biocatalysis. In drug discovery platforms, purified recombinant atm1 could serve as a target for high-throughput screening of compound libraries to identify inhibitors with potential therapeutic applications in fungal infections or as tools for dissecting transporter mechanisms. Finally, the protein could be utilized in nanotechnology applications, such as incorporation into artificial membrane systems for developing sensors or controlled release systems.