GO:0016226 iron-sulfur cluster assembly: Biosynthesis Pathway, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0016226 iron-sulfur cluster assembly is the biological process that incorporates iron and exogenous sulfur into metallo-sulfur clusters, which are ancient, versatile protein cofactors.
• The process is essential for the function of numerous proteins involved in respiration, DNA repair, ribosome biogenesis, and metabolism.
• In eukaryotes, assembly occurs in mitochondria and the cytosol, with the mitochondrial ISC machinery being the primary source of clusters.
• Defects in iron-sulfur cluster assembly are linked to human diseases including Friedreich ataxia, sideroblastic anemia, and various mitochondrial myopathies.
• Core assembly proteins include cysteine desulfurases (NFS1), scaffold proteins (ISCU), and chaperones (HSPA9) that facilitate cluster formation and transfer.
• Research methods such as CRISPR knockout, proteomics, and biochemical assays are crucial for dissecting the molecular details of this pathway.
Description
Iron-sulfur (Fe-S) clusters are among the most ancient and ubiquitous protein cofactors, essential for electron transfer, catalysis, and regulatory processes. The biological process of iron-sulfur cluster assembly (GO:0016226) encompasses the incorporation of iron and exogenous sulfur into these metallo-sulfur clusters, a highly conserved and tightly regulated pathway. This process is critical for the maturation of numerous Fe-S proteins, which are involved in fundamental cellular functions such as oxidative phosphorylation, DNA replication and repair, and ribosome biogenesis. In eukaryotes, Fe-S cluster assembly is compartmentalized, with the mitochondrial ISC (iron-sulfur cluster) machinery playing a central role in both mitochondrial and cytosolic Fe-S protein maturation. The cytosolic CIA (cytosolic iron-sulfur protein assembly) machinery relies on a sulfur-containing compound exported from mitochondria. The importance of this pathway is underscored by the severe consequences of its dysfunction, which are linked to a growing list of human diseases, including neurodegenerative disorders, anemias, and cancer. Understanding the molecular mechanisms of Fe-S cluster assembly is therefore not only a fundamental biological question but also a prerequisite for developing therapeutic strategies for related diseases.
iron-sulfur cluster assembly At A Glance
| GO ID | GO:0016226 |
|---|---|
| GO term | iron-sulfur cluster assembly |
| Ontology | biological_process |
| Synonym | iron-sulfur cluster biosynthesis, iron-sulphur cluster assembly |
| Major function | Incorporation of iron and sulfur into metallo-sulfur clusters for protein maturation |
| Subcellular location | Mitochondria and cytosol in eukaryotes; cytosol in bacteria |
| Key proteins | NFS1, ISCU, ISCS, HSPA9, GLRX5, FXN, CIAO1, etc. |
| Associated diseases | Friedreich ataxia, sideroblastic anemia, mitochondrial myopathies, cancer |
What Is GO:0016226?
According to the Gene Ontology, GO:0016226 iron-sulfur cluster assembly is defined as the incorporation of iron and exogenous sulfur into a metallo-sulfur cluster. This process involves the coordinated action of multiple proteins that provide sulfur, iron, and electrons, and facilitate the assembly and transfer of the cluster to target apoproteins.
Why Is iron-sulfur cluster assembly Important in Cell Biology?
Iron-sulfur cluster assembly is fundamental to life because Fe-S clusters are required for the activity of hundreds of proteins across all domains of life. These proteins participate in essential processes such as respiration, photosynthesis, nitrogen fixation, DNA repair, and gene regulation. In humans, defects in Fe-S cluster assembly lead to a range of diseases, including Friedreich ataxia, ISCU myopathy, and sideroblastic anemia, highlighting the clinical relevance of this pathway. Moreover, Fe-S clusters are sensitive to oxidative stress and iron availability, making their assembly a key node in cellular iron homeostasis and stress responses. Research into this process is therefore critical for understanding basic cell biology and for developing treatments for related disorders.
• Essential for the function of mitochondrial respiratory complexes I, II, and III, and thus for ATP production.
• Required for the activity of enzymes involved in DNA replication and repair, such as DNA primase and helicases.
• Critical for ribosome biogenesis and translation, as several ribosomal proteins and translation factors contain Fe-S clusters.
• Plays a key role in cellular iron homeostasis by regulating the iron-responsive element (IRE) binding activity of IRP1.
• Dysfunction is linked to neurodegenerative diseases like Friedreich ataxia due to frataxin deficiency.
• Mutations in ISCU cause myopathy with exercise intolerance and lactic acidosis.
• Implicated in cancer metabolism, as cancer cells often reprogram iron and sulfur metabolism to support proliferation.
• Target for antimicrobial drug development, as the pathway is essential in many pathogens.
• Involved in the biosynthesis of lipoic acid and biotin, which require Fe-S cluster enzymes.
• Serves as a model for studying metalloprotein assembly and protein-protein interactions.
What Happens During iron-sulfur cluster assembly?
Sulfur Mobilization by Cysteine Desulfurases
In simple terms: First, sulfur is taken from the amino acid cysteine and made available for cluster building.
The assembly process begins with the desulfurization of L-cysteine by cysteine desulfurases, such as NFS1 in eukaryotes and IscS in bacteria, to generate elemental sulfur and L-alanine. This sulfur is transiently bound to a conserved cysteine residue in the enzyme's active site as a persulfide intermediate. In eukaryotes, NFS1 is located in the mitochondria and is essential for both mitochondrial and cytosolic Fe-S cluster assembly. The sulfur is then transferred to scaffold proteins for cluster assembly.
Iron Delivery and Cluster Formation on Scaffold Proteins
In simple terms: Iron is brought in, and together with sulfur, a cluster is built on a scaffold protein.
Iron is delivered to the scaffold protein ISCU (IscU in bacteria) by chaperone proteins such as HSPA9 (mtHsp70) and the co-chaperone HSC20 (HscB). The scaffold protein provides a platform for the assembly of a [2Fe-2S] cluster, which is then transferred to target apoproteins. The assembly process requires electrons, which are provided by ferredoxin (Fdx) or ferredoxin reductase in some systems. In plants, the maturation of Complex I subunits involves a similar assembly process.
Cluster Transfer to Target Proteins
In simple terms: The finished cluster is handed off to the proteins that need it to function.
Once assembled, the Fe-S cluster is transferred from the scaffold to recipient apoproteins. This transfer is facilitated by chaperones and carrier proteins, such as glutaredoxin 5 (GLRX5) in eukaryotes, which can bind and deliver clusters. In the cytosol, the CIA machinery, including CIAO1 and MMS19, mediates the insertion of clusters into cytosolic and nuclear Fe-S proteins. The process is highly regulated to ensure that clusters are delivered to the correct targets under varying cellular conditions.
Mitochondrial Export and Cytosolic Assembly
In simple terms: A sulfur-containing molecule is exported from mitochondria to help build clusters in the cytosol.
The mitochondrial ISC machinery is not only responsible for mitochondrial Fe-S proteins but also for the maturation of cytosolic and nuclear Fe-S proteins. This is achieved through the export of a sulfur-containing compound, possibly a persulfide or a small molecule, via the ABC transporter Atm1 (ABCB7 in humans). In the cytosol, the CIA machinery utilizes this sulfur for the assembly of [4Fe-4S] clusters on target proteins. This compartmentalization allows for the coordination of iron and sulfur metabolism across the cell.
Regulation by Iron and Oxygen
In simple terms: The assembly process is turned up or down depending on how much iron and oxygen are around.
Fe-S cluster assembly is tightly regulated by iron availability and oxidative stress. In bacteria such as Escherichia coli, the IscR protein senses the status of Fe-S clusters and regulates the expression of the isc operon. In eukaryotes, iron deficiency activates the iron-responsive element (IRE)/IRP system, which increases the expression of proteins involved in iron uptake and storage, and can also affect Fe-S cluster assembly. Oxidative stress can damage Fe-S clusters, leading to the activation of stress responses and the upregulation of assembly factors.
Key Genes Involved in GO:0016226 iron-sulfur cluster assembly
The following genes and proteins are central to iron-sulfur cluster assembly, as evidenced by biochemical and genetic studies in model organisms and humans.
| Gene | Major Role | Research Relevance |
|---|---|---|
| NFS1 | Cysteine desulfurase; provides sulfur for cluster assembly | Knockout causes embryonic lethality; studied in mitochondrial function and cancer |
| ISCU | Scaffold protein for [2Fe-2S] cluster assembly | Mutations cause myopathy; target for structural and biochemical studies |
| HSPA9 | Mitochondrial chaperone; facilitates cluster transfer | Involved in iron-sulfur cluster assembly and neurodegeneration |
| GLRX5 | Glutaredoxin; cluster delivery and iron homeostasis | Deficiency causes sideroblastic anemia |
| FXN | Frataxin; iron chaperone and regulator of ISC | Deficiency causes Friedreich ataxia |
| CIAO1 | Cytosolic Fe-S protein assembly factor | Required for cytosolic Fe-S protein maturation |
| MMS19 | Cytosolic Fe-S protein assembly factor | Involved in DNA repair and transcription |
| ABCB7 | Mitochondrial ABC transporter; exports sulfur compound | Mutations cause X-linked sideroblastic anemia with ataxia |
| ISCS | Bacterial cysteine desulfurase | Model for sulfur mobilization |
| IscU | Bacterial scaffold protein | Model for cluster assembly |
| IscR | Fe-S cluster-containing regulator | Regulates isc operon in E. coli |
| SufU | Scaffold protein in SUF system | Alternative assembly system under stress |
| NfuA | Fe-S cluster carrier | Involved in cluster transfer |
| ErpA | A-type carrier protein | Essential for Fe-S protein maturation |
| BolA | Iron-sulfur cluster assembly regulator | Linked to oxidative stress response |
| ISC1 | Mitochondrial ISC component | Plant Complex I assembly |
How Is iron-sulfur cluster assembly Regulated?
The iron-sulfur cluster assembly pathway is regulated at multiple levels in response to iron availability, oxidative stress, and cellular demands. In bacteria, the IscR protein, which contains a [2Fe-2S] cluster, senses the cluster status and represses or activates the isc operon accordingly. In eukaryotes, iron deficiency leads to the activation of iron regulatory proteins (IRP1 and IRP2), which stabilize mRNAs encoding iron uptake and storage proteins, and can also modulate the expression of Fe-S cluster assembly factors. Additionally, the assembly process is influenced by the availability of cysteine, the substrate for sulfur mobilization, and by the redox state of the cell. In plants, the assembly of mitochondrial Complex I is coordinated with the availability of iron and the expression of assembly factors.
iron-sulfur cluster assembly and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| FXN | Friedreich ataxia | Knockout or knockdown in neuronal cell lines; patient-derived iPSCs |
| ISCU | Myopathy with exercise intolerance | Knockout in muscle cell lines; knock-in of patient mutations |
| GLRX5 | Sideroblastic anemia | Knockout in erythroid cell lines; zebrafish models |
| ABCB7 | X-linked sideroblastic anemia with ataxia | Knockout in HeLa or HEK293 cells; yeast models |
| NFS1 | Cancer proliferation | Conditional knockout in cancer cell lines; xenograft models |
Friedreich Ataxia and Neurodegeneration
Friedreich ataxia is caused by reduced expression of frataxin (FXN), a mitochondrial protein involved in iron-sulfur cluster assembly. Frataxin deficiency leads to impaired Fe-S cluster biogenesis, mitochondrial iron accumulation, and oxidative stress, resulting in progressive neurodegeneration and cardiomyopathy. Studies in model organisms have shown that restoring Fe-S cluster assembly can partially rescue the phenotype, highlighting the central role of this pathway in the disease.
Sideroblastic Anemia and Iron Metabolism Disorders
Mutations in genes encoding Fe-S cluster assembly proteins, such as GLRX5 and ABCB7, cause sideroblastic anemia, a group of disorders characterized by defective heme synthesis and iron overload in mitochondria. GLRX5 deficiency impairs the delivery of Fe-S clusters to target proteins, leading to iron accumulation and ineffective erythropoiesis. ABCB7 mutations disrupt the export of a sulfur-containing compound from mitochondria, affecting both mitochondrial and cytosolic Fe-S protein maturation.
Cancer and Metabolic Reprogramming
Cancer cells often exhibit altered iron and sulfur metabolism to support rapid proliferation. Fe-S cluster assembly is upregulated in some cancers to meet the demand for Fe-S proteins involved in DNA replication and metabolism. Targeting the Fe-S cluster assembly pathway, such as by inhibiting NFS1, has been proposed as a therapeutic strategy for cancers with high iron demand. However, the role of Fe-S clusters in cancer is complex, as they can also promote oxidative stress-induced cell death.
Mitochondrial Myopathies and Metabolic Disorders
Defects in Fe-S cluster assembly can lead to mitochondrial myopathies, such as those caused by mutations in ISCU. ISCU myopathy is characterized by exercise intolerance, muscle weakness, and lactic acidosis, due to impaired mitochondrial respiration. Other mitochondrial disorders, including Complex I deficiency, can result from mutations in assembly factors like ISC1 in plants, which have provided insights into the human disease mechanisms.
From iron-sulfur cluster assembly-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| What is the effect of complete loss of ISCU on cell viability? | CRISPR knockout in HEK293 or HeLa cells |
| How does a specific patient mutation in FXN affect Fe-S cluster assembly? | CRISPR point mutation knock-in in patient-derived fibroblasts |
| Can wild-type NFS1 rescue the phenotype of NFS1 knockout cells? | CRISPR knock-in of tagged NFS1 for rescue experiments |
| What is the interactome of ISCU during cluster assembly? | Knock-in of FLAG-tagged ISCU followed by immunoprecipitation and mass spectrometry |
| Does overexpression of GLRX5 protect against oxidative stress? | Overexpression of GLRX5 in neuronal cell lines |
| Which genes are essential for Fe-S cluster assembly in cancer cells? | CRISPR library screening targeting all Fe-S assembly genes |
How to Study the iron-sulfur cluster assembly Process
| Method | What It Measures | Typical Application |
|---|---|---|
| UV-visible spectroscopy | Formation of Fe-S clusters | In vitro reconstitution assays |
| CRISPR knockout screening | Gene essentiality for Fe-S cluster assembly | Identification of novel assembly factors |
| RNA-seq | Transcriptional changes in response to iron stress | Studying regulation of assembly genes |
| AP-MS | Protein-protein interactions | Mapping the assembly complex |
| Fluorescence microscopy | Subcellular localization of assembly proteins | Determining mitochondrial vs cytosolic roles |
| Isotope labeling | Sulfur transfer and cluster dynamics | Tracing sulfur flux in cells |
| Seahorse respirometry | Mitochondrial respiration | Assessing functional impact of assembly defects |
| Inductively coupled plasma mass spectrometry (ICP-MS) | Iron and sulfur content | Quantifying cluster formation in vitro |
Biochemical Assays for Fe-S Cluster Assembly
In vitro reconstitution assays using purified components (e.g., NFS1, ISCU, frataxin) can measure the kinetics of cluster assembly and transfer. These assays often use UV-visible spectroscopy to monitor the formation of [2Fe-2S] clusters, which have characteristic absorption spectra. Radioactive or stable isotope labeling of cysteine can track sulfur transfer.
Genetic and Genomic Approaches
CRISPR-Cas9 knockout screens can identify genes required for Fe-S cluster assembly and for the function of Fe-S proteins. RNA interference (RNAi) and CRISPR interference (CRISPRi) can be used to knockdown gene expression and assess the impact on Fe-S cluster-dependent processes. Transcriptomic analysis (RNA-seq) can reveal changes in gene expression in response to iron deficiency or oxidative stress.
Proteomic and Interaction Studies
Affinity purification coupled with mass spectrometry (AP-MS) can identify protein-protein interactions within the Fe-S cluster assembly machinery. Proximity labeling techniques, such as BioID, can map the interactome of assembly factors in living cells. These methods have been used to elucidate the dynamic interactions between NFS1, ISCU, and chaperones.
Imaging and Subcellular Localization
Fluorescence microscopy with GFP-tagged assembly proteins can reveal their subcellular localization and dynamics. Mitochondrial morphology and iron content can be assessed using fluorescent dyes (e.g., MitoTracker, FeRhoNox). Electron microscopy can visualize mitochondrial ultrastructure in cells with defective Fe-S cluster assembly.
How CRISPR Can Be Used to Study GO:0016226 iron-sulfur cluster assembly
Knockout
CRISPR knockout of core Fe-S cluster assembly genes such as NFS1, ISCU, or GLRX5 results in loss of Fe-S cluster formation, leading to impaired mitochondrial respiration and cell death in many cell types. Conditional knockout models can bypass embryonic lethality and allow study of tissue-specific functions. Knockout cell lines are valuable for identifying which Fe-S proteins are most sensitive to assembly defects.
Point Mutation
CRISPR point mutation knock-in can recreate patient-specific mutations in genes like FXN or ISCU to study their effects on Fe-S cluster assembly and protein function. These models help dissect the molecular mechanisms of disease-associated mutations and test potential therapeutic interventions.
Knock-in
Knock-in of epitope tags (e.g., FLAG, HA) into endogenous loci of assembly genes allows for affinity purification and proteomic analysis of the assembly machinery. Knock-in of fluorescent proteins (e.g., GFP) enables live-cell imaging of protein localization and dynamics. Knock-in of inducible promoters can provide controlled expression for rescue experiments.
Overexpression
Overexpression of Fe-S cluster assembly proteins, such as frataxin or GLRX5, can protect cells from oxidative stress and iron-induced toxicity. Overexpression models are used to study the effects of increased cluster assembly on cellular metabolism and to identify downstream targets. In cancer research, overexpression of NFS1 can support proliferation under oxidative stress conditions.
How EDITGENE Supports iron-sulfur cluster assembly Research
Researchers studying iron-sulfur cluster assembly-related genes often need to determine whether a candidate gene is causally involved in the pathway, how mutations affect protein function, and what the downstream consequences are for cellular physiology. EDITGENE provides a comprehensive suite of CRISPR-based services to address these questions, from generating knockout cell lines to creating precise point mutations and knock-in reporters.
Contact EDITGENE today to design your custom CRISPR model for iron-sulfur cluster assembly research.
Frequently Asked Questions About iron-sulfur cluster assembly
What is iron-sulfur cluster assembly?
Iron-sulfur cluster assembly (GO:0016226) is the biological process that incorporates iron and sulfur into metallo-sulfur clusters, which are essential cofactors for many proteins.
What genes are involved in iron-sulfur cluster assembly?
Key genes include NFS1, ISCU, HSPA9, GLRX5, FXN, CIAO1, and MMS19, among others.
Why is iron-sulfur cluster assembly important?
It is essential for the function of proteins involved in respiration, DNA repair, and metabolism, and its dysfunction causes severe human diseases.
Where does iron-sulfur cluster assembly occur in cells?
In eukaryotes, it occurs primarily in mitochondria, with a cytosolic branch that depends on mitochondrial export.
What diseases are associated with defects in iron-sulfur cluster assembly?
Diseases include Friedreich ataxia, sideroblastic anemia, mitochondrial myopathies, and some cancers.
How can I study iron-sulfur cluster assembly using CRISPR?
CRISPR knockout, point mutation knock-in, and overexpression models can be used to dissect gene function and disease mechanisms.
What are the main steps of iron-sulfur cluster assembly?
The main steps are sulfur mobilization from cysteine, iron delivery, cluster formation on scaffold proteins, and transfer to target proteins.
What is the role of frataxin in iron-sulfur cluster assembly?
Frataxin (FXN) is a mitochondrial protein that regulates iron availability and interacts with the assembly machinery; its deficiency causes Friedreich ataxia.
Can iron-sulfur cluster assembly be targeted for cancer therapy?
Yes, inhibiting NFS1 or other assembly factors is being explored as a strategy to target cancer cells with high iron demand.
What methods are used to measure iron-sulfur cluster assembly?
Methods include UV-visible spectroscopy, radioactive sulfur labeling, CRISPR screens, and proteomics.
Conclusion
Iron-sulfur cluster assembly (GO:0016226) is a fundamental biological process that underpins the function of numerous essential proteins. Its intricate molecular machinery, conserved from bacteria to humans, ensures the proper incorporation of iron and sulfur into metallo-sulfur clusters. Defects in this pathway lead to a spectrum of human diseases, making it a critical area of research. Advances in CRISPR-based models and biochemical assays continue to unravel the mechanistic details and therapeutic potential of targeting this pathway.
References
- 1. López-López A et al.. 2022. Maturation and Assembly of Iron-Sulfur Cluster-Containing Subunits in the Mitochondrial Complex I From Plants.. Front Plant Sci 13:916948 PMID: 35677241
- 2. Ye H et al.. 2010. Human iron-sulfur cluster assembly, cellular iron homeostasis, and disease.. Biochemistry 49(24):4945-56 PMID: 20481466
- 3. Steinhilper R et al.. 2024. Two-stage binding of mitochondrial ferredoxin-2 to the core iron-sulfur cluster assembly complex.. Nat Commun 15(1):10559 PMID: 39632806
- 4. Pedroletti L et al.. 2023. Assembly, transfer, and fate of mitochondrial iron-sulfur clusters.. J Exp Bot 74(11):3328-3344 PMID: 36846908
- 5. Fan X et al.. 2022. Iron-regulated assembly of the cytosolic iron-sulfur cluster biogenesis machinery.. J Biol Chem 298(7):102094 PMID: 35654137
- 6. Gao F. 2020. Iron-Sulfur Cluster Biogenesis and Iron Homeostasis in Cyanobacteria.. Front Microbiol 11:165 PMID: 32184761
- 7. Bandyopadhyay S et al.. 2008. Iron-sulfur cluster biosynthesis.. Biochem Soc Trans 36(Pt 6):1112-9 PMID: 19021507
- 8. Ding H. 2025. Iron-sulfur cluster biogenesis and regulation of intracellular iron homeostasis in Escherichia coli.. Metallomics 17(12) PMID: 41263481
