GO:0032020 ISG15-protein conjugation: Ubiquitin-like Protein Modification, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0032020 (ISG15-protein conjugation) describes the covalent addition of ISG15, a ubiquitin-like protein, to target proteins, a process often called ISGylation.
• ISG15 conjugation is strongly induced by type I interferons and is a core effector arm of innate antiviral immunity.
• The conjugation cascade is enzymatic and sequential, involving UBA7 (E1), UBE2L6 (E2), and HERC5 or other E3 ligases, with reversible removal by USP18.
• ISG15 conjugation targets both host proteins and viral proteins, including influenza A NS1 and influenza B NS1, thereby influencing viral replication.
• Dysregulated ISG15 conjugation is implicated in cancer, autoinflammation, and susceptibility to viral infections such as influenza and SARS-CoV-2.
• CRISPR knockout, knock-in, and overexpression cell models are powerful tools to dissect causal roles of ISG15 pathway genes in disease and immunity.
Description
ISG15-protein conjugation (GO:0032020) is the biological process in which the ubiquitin-like protein ISG15 is covalently attached to target proteins, a modification commonly referred to as ISGylation. Unlike ubiquitin, ISG15 is not constitutively expressed at high levels; its expression and conjugation are strongly induced by type I interferons, making this process a central component of the innate antiviral response. The reaction is carried out by a dedicated enzymatic cascade and is reversible, allowing dynamic regulation of substrate proteins during infection and stress. Researchers study ISG15-protein conjugation because it directly modulates host defense, viral replication, and cellular signaling. For example, influenza B virus NS1 protein inhibits ISG15 conjugation to counteract interferon responses, while influenza A virus NS1 is itself a target of ISGylation. Beyond antiviral immunity, ISG15 conjugation has been linked to cancer biology, autoinflammatory conditions, and emerging viral infections such as SARS-CoV-2. Understanding which proteins are conjugated, how the conjugation machinery is regulated, and what consequences this modification has for cell fate is therefore of broad biomedical importance. The process is experimentally tractable: knockout, point-mutation, knock-in, and overexpression cell models allow precise interrogation of ISG15 pathway components. This article summarizes the definition, mechanism, key genes, disease relevance, and research methods for GO:0032020, with all factual claims supported by peer-reviewed literature.
ISG15-protein conjugation At A Glance
| GO ID | GO:0032020 |
|---|---|
| GO term | ISG15-protein conjugation |
| Ontology | biological_process |
| Synonym | None listed in QuickGO |
| Definition | The covalent addition to a protein of ISG15, a ubiquitin-like protein. |
| Major function | Covalent attachment of ISG15 to target proteins (ISGylation), a reversible post-translational modification involved in antiviral immunity and cellular stress responses. |
| Key enzymes | UBA7 (E1), UBE2L6 (E2), HERC5 and other E3 ligases, and the protease USP18 for removal. |
| Induction | Strongly induced by type I interferons. |
| Reversibility | Reversed by ISG15-specific proteases such as USP18. |
| Disease links | Cancer, autoinflammation, influenza, SARS-CoV-2, and other viral infections. |
What Is GO:0032020?
GO:0032020 (ISG15-protein conjugation) is defined as the covalent addition of ISG15, a ubiquitin-like protein, to a target protein. In practical terms, it is an enzymatic post-translational modification in which the C-terminal glycine of ISG15 forms an isopeptide bond with lysine residues on substrate proteins, analogous to ubiquitination but executed by ISG15-specific enzymes. This modification is reversible and is tightly controlled by interferon signaling and by dedicated proteases.
Why Is ISG15-protein conjugation Important in Cell Biology?
ISG15-protein conjugation is a critical effector mechanism of interferon-mediated innate immunity, directly targeting both host and viral proteins to modulate infection outcomes. Its dysregulation is associated with human diseases including cancer, autoinflammatory disorders, and severe viral infections, making it a high-value area for therapeutic and diagnostic research. Because the process is enzymatically defined and genetically tractable, it serves as a model for understanding ubiquitin-like protein conjugation in general.
• Central to antiviral defense: ISG15 conjugation restricts replication of influenza, SARS-CoV-2, and other viruses.
• Targets viral proteins directly, such as influenza A NS1, altering viral function.
• Influenza B NS1 counteracts ISG15 conjugation, illustrating host-pathogen arms races.
• Implicated in cancer biology, where ISG15 and its conjugates influence tumor progression and immune evasion.
• Linked to autoinflammatory and interferonopathies through dysregulated ISG15 pathways.
• Provides a paradigm for ubiquitin-like protein conjugation and cross-talk with ubiquitin-proteasome systems.
• Enables development of CRISPR models to test causality of ISG15 pathway genes in disease.
• Supports biomarker and drug target discovery in infectious and inflammatory diseases.
What Happens During ISG15-protein conjugation?
Interferon-induced expression of ISG15 and conjugation machinery
In simple terms: Interferon signals tell the cell to make more ISG15 and the enzymes that attach it to other proteins.
Type I interferons strongly induce the expression of ISG15 and the enzymes required for its conjugation, establishing a primed state for antiviral defense. This induction is a hallmark of the interferon response and is required for efficient ISGylation of target proteins. Camptothecin has also been shown to enhance ISG15 conjugation in response to interferon, indicating that additional stress signals can modulate the pathway.
Enzymatic cascade: E1, E2, and E3 activities
In simple terms: A relay of three types of enzymes passes ISG15 down a line until it is attached to a target protein.
ISG15 conjugation proceeds through a sequential enzymatic cascade analogous to ubiquitination: the E1 activating enzyme UBA7 activates ISG15, the E2 conjugating enzyme UBE2L6 transfers it, and E3 ligases such as HERC5 mediate attachment to substrate lysines. This cascade ensures specificity and is tightly regulated. Proteasome activity can modulate conjugation to the ubiquitin-like protein ISG15, indicating cross-talk with the ubiquitin-proteasome system.
Substrate targeting and modification
In simple terms: Once attached, ISG15 changes how the target protein behaves, often tagging it for antiviral or signaling functions.
ISG15 is covalently attached to lysine residues on target proteins, forming an isopeptide bond. Substrates include host proteins involved in immunity and signaling, as well as viral proteins such as influenza A NS1. The functional consequences depend on the substrate and context, ranging from altered protein stability to modulation of protein-protein interactions.
Reversal by ISG15-specific proteases
In simple terms: Enzymes can cut ISG15 off target proteins, making the modification reversible.
ISG15 conjugation is reversible; the protease USP18 removes ISG15 from conjugated proteins, allowing dynamic regulation. This reversibility is essential for preventing excessive or prolonged ISGylation, which can be detrimental. The balance between conjugation and deconjugation determines the steady-state level of ISGylated proteins.
Recognition and downstream effects
In simple terms: Cells have sensor proteins that recognize ISG15 tags and trigger antimicrobial responses.
ISGylated proteins can be recognized by dedicated sensor proteins, such as ring finger protein 213 (RNF213), which assembles into a sensor for ISGylated proteins with antimicrobial activity. This recognition links ISG15 conjugation to downstream effector functions, including restriction of intracellular pathogens. Thus, ISGylation is not merely a tag but a platform for assembling antimicrobial complexes.
Key Genes Involved in GO:0032020 ISG15-protein conjugation
The following genes and proteins are central to ISG15-protein conjugation, based on published literature.
| Gene | Major Role | Research Relevance |
|---|---|---|
| ISG15 | Ubiquitin-like protein conjugated to targets | Core substrate of the pathway; induced by interferon; linked to antiviral immunity and cancer. |
| UBA7 | E1 activating enzyme for ISG15 | Initiates conjugation cascade; knockout abolishes ISGylation. |
| UBE2L6 | E2 conjugating enzyme for ISG15 | Transfers ISG15 to E3 ligases; essential for conjugation. |
| HERC5 | E3 ligase for ISG15 | Mediates substrate specificity; major ISG15 E3 in human cells. |
| USP18 | ISG15-specific protease | Removes ISG15 from conjugates; regulates interferon signaling. |
| RNF213 | Sensor for ISGylated proteins | Recognizes ISG15 conjugates and exerts antimicrobial activity. |
| NS1 (influenza A) | Viral protein targeted by ISGylation | ISG15 conjugation targets NS1, affecting viral replication. |
| NS1 (influenza B) | Viral inhibitor of ISG15 conjugation | Blocks ISGylation to evade interferon responses. |
| IFNAR1 | Type I interferon receptor subunit | Mediates interferon-induced ISG15 expression. |
| STAT1 | Interferon signaling transcription factor | Drives ISG15 and conjugation machinery expression. |
| STAT2 | Interferon signaling transcription factor | Part of ISGF3 complex inducing ISG15. |
| IRF9 | Interferon regulatory factor | Forms ISGF3 with STAT1/2 to induce ISG15. |
| UBE2L6 | E2 enzyme | Polymorphisms linked to immune responses. |
| HERC6 | E3 ligase (paralog of HERC5) | Contributes to ISG15 conjugation in some contexts. |
| TRIM25 | E3 ligase and interferon effector | Interplays with ISG15 conjugation in antiviral signaling. |
| USP18 | Deconjugating enzyme | Also regulates interferon receptor stability. |
| ISG15 | Cytokine-like extracellular form | Secreted ISG15 can act on immune cells. |
| RNF213 | Antimicrobial sensor | Mutations linked to vascular and immune disorders. |
How Is ISG15-protein conjugation Regulated?
ISG15-protein conjugation is primarily regulated at the level of interferon signaling, which induces expression of ISG15 and the conjugation machinery. The process is reversible through USP18-mediated deconjugation, and cross-talk with the ubiquitin-proteasome system can modulate conjugation efficiency. Viral proteins such as influenza B NS1 can inhibit conjugation, providing a pathogen-driven regulatory layer. Additionally, stress signals like camptothecin can enhance ISG15 conjugation in response to interferon.
ISG15-protein conjugation and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| ISG15 | Viral susceptibility, autoinflammation | ISG15 knockout and knock-in cell lines. |
| UBA7 | Antiviral immunity | UBA7 knockout cells to abolish ISGylation. |
| USP18 | Interferonopathy, viral infection | USP18 knockout or point-mutation models. |
| RNF213 | Vascular and immune disorders | RNF213 knockout and tagged knock-in cells. |
| NS1 (influenza) | Viral evasion | Infection models with NS1 mutants. |
ISG15 conjugation in viral infections
ISG15 conjugation is a key antiviral mechanism. Influenza B virus NS1 protein inhibits ISG15 conjugation to evade host immunity, while influenza A NS1 is a target of ISGylation. SARS-CoV-2 and other viral infections also interact with the ISG15 pathway, highlighting its broad relevance. Dysregulation of ISG15 conjugation can lead to increased viral susceptibility.
ISG15 conjugation in cancer
ISG15 and its conjugation pathway have been implicated in cancer biology beyond their role as ubiquitin-like proteins. ISGylation can influence tumor cell proliferation, immune evasion, and response to therapy. The pathway is therefore being explored as a potential biomarker and therapeutic target in oncology.
ISG15 conjugation in autoinflammation and interferonopathies
Dysregulated ISG15 conjugation is associated with autoinflammatory conditions and interferonopathies. Loss or gain of function in ISG15 pathway genes can alter interferon signaling and immune homeostasis. Understanding these links may inform diagnosis and treatment of rare inflammatory diseases.
From ISG15-protein conjugation-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of ISG15 conjugation affect antiviral response? | ISG15 or UBA7 knockout cell lines. |
| What is the role of a specific ISG15 substrate lysine? | Point-mutation knock-in of substrate. |
| How does USP18 deconjugation regulate interferon signaling? | USP18 knockout or catalytic-dead knock-in. |
| Can tagged ISG15 track conjugation dynamics? | Tagged ISG15 knock-in (e.g., HA or FLAG). |
| Does overexpression of HERC5 enhance ISGylation? | HERC5 overexpression cell lines. |
| What is the impact of RNF213 on ISGylated protein sensing? | RNF213 knockout and overexpression models. |
How to Study the ISG15-protein conjugation Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Mass spectrometry | ISG15-conjugated proteins and sites | Global substrate discovery. |
| Western blot | Free and conjugated ISG15 levels | Validation of conjugation. |
| Immunoprecipitation | Specific ISGylated proteins | Target validation. |
| CRISPR knockout screen | Genes required for ISGylation | Regulator discovery. |
| RNA-seq | Transcriptional changes upon ISGylation | Pathway analysis. |
| Proximity labeling | Interactors of ISG15 machinery | Complex mapping. |
| Fluorescence microscopy | Localization of ISG15 conjugates | Live-cell imaging. |
| Luciferase reporter | Interferon pathway activity | Functional assays. |
Proteomic detection of ISG15 conjugates
Mass spectrometry-based proteomics can identify ISG15-conjugated proteins and map modification sites, providing a global view of ISGylation. This approach is useful for discovering novel substrates and understanding pathway dynamics.
Immunoblotting and immunoprecipitation
Western blotting with anti-ISG15 antibodies detects free ISG15 and conjugated species, while immunoprecipitation can isolate specific ISGylated proteins. These methods are standard for validating conjugation events.
CRISPR-based genetic screens
CRISPR knockout screens can identify genes required for ISG15 conjugation or for cellular responses to ISGylation. Such screens are powerful for uncovering novel regulators.
Imaging and reporter assays
Fluorescently tagged ISG15 or reporter systems can visualize conjugation in live cells and tissues. These tools help study spatiotemporal dynamics of ISGylation.
How CRISPR Can Be Used to Study GO:0032020 ISG15-protein conjugation
Knockout
CRISPR knockout of ISG15, UBA7, UBE2L6, or HERC5 abolishes or reduces ISG15 conjugation, enabling loss-of-function studies in antiviral and cancer models. Knockout cell lines are essential for testing causality of specific pathway components.
Point Mutation
Point mutations can be introduced into ISG15 (e.g., C-terminal glycine) or into substrate lysine residues to dissect conjugation chemistry and substrate specificity. Such models help distinguish conjugation-dependent from independent functions.
Knock-in
Knock-in of tagged ISG15 (e.g., HA, FLAG) or of disease-associated variants allows tracking and functional analysis of the pathway in a physiological context. Knock-in models are valuable for studying autoinflammatory mutations.
Overexpression
Overexpression of ISG15, HERC5, or other pathway genes can enhance ISGylation and amplify downstream effects, useful for gain-of-function studies. Overexpression models complement knockout approaches.
How EDITGENE Supports ISG15-protein conjugation Research
Researchers studying ISG15-protein conjugation-related genes often need to determine whether a candidate gene is causally involved in antiviral immunity, cancer, or inflammatory disease. EDITGENE provides CRISPR-based cell model services to enable such causal studies with high precision and reproducibility.
Contact EDITGENE today to design your custom CRISPR model for ISG15-protein conjugation research.
Frequently Asked Questions About ISG15-protein conjugation
What is ISG15-protein conjugation?
ISG15-protein conjugation (GO:0032020) is the covalent addition of ISG15, a ubiquitin-like protein, to target proteins, a reversible modification involved in antiviral immunity and cellular stress responses.
What genes are involved in ISG15-protein conjugation?
Key genes include ISG15, UBA7, UBE2L6, HERC5, USP18, and RNF213, which together mediate conjugation, deconjugation, and recognition.
How is ISG15 conjugation induced?
It is strongly induced by type I interferons, which upregulate ISG15 and the conjugation machinery.
What is the role of ISG15 conjugation in viral infections?
It restricts viral replication by modifying viral and host proteins; viruses like influenza B have evolved inhibitors such as NS1 to counteract it.
Is ISG15 conjugation reversible?
Yes, USP18 removes ISG15 from conjugated proteins, making the modification reversible.
What diseases are linked to ISG15 conjugation?
It is linked to cancer, autoinflammatory conditions, and susceptibility to viral infections including influenza and SARS-CoV-2.
How can I study ISG15 conjugation in the lab?
Common methods include mass spectrometry, western blot, immunoprecipitation, and CRISPR knockout screens.
What is the difference between ISG15 and ubiquitin?
ISG15 is a ubiquitin-like protein with distinct conjugation enzymes and targets, though both form covalent attachments to lysines.
Can CRISPR be used to study ISG15 conjugation?
Yes, CRISPR knockout, knock-in, and overexpression models are widely used to dissect the pathway.
What is the GO ID for ISG15-protein conjugation?
The GO ID is GO:0032020, under the biological_process ontology.
Conclusion
ISG15-protein conjugation (GO:0032020) is a central interferon-induced post-translational modification that shapes antiviral immunity, cancer biology, and inflammatory disease. Its enzymatic cascade, reversibility, and expanding list of substrates make it a rich area for mechanistic and translational research. CRISPR-based cell models and proteomic methods are accelerating discovery in this field, offering new opportunities for therapeutic intervention.
References
- 1. Mirzalieva O et al.. 2022. ISG15 and ISGylation in Human Diseases.. Cells 11(3) PMID: 35159348
- 2. Han HG et al.. 2018. ISG15 in cancer: Beyond ubiquitin-like protein.. Cancer Lett 438:52-62 PMID: 30213559
- 3. Liu M et al.. 2003. Proteasomes modulate conjugation to the ubiquitin-like protein, ISG15.. J Biol Chem 278(3):1594-602 PMID: 12426315
- 4. Zhao C et al.. 2010. ISG15 conjugation system targets the viral NS1 protein in influenza A virus-infected cells.. Proc Natl Acad Sci U S A 107(5):2253-8 PMID: 20133869
- 5. Liu M et al.. 2004. Camptothecin induces the ubiquitin-like protein, ISG15, and enhances ISG15 conjugation in response to interferon.. J Interferon Cytokine Res 24(11):647-54 PMID: 15684817
- 6. Yuan W et al.. 2001. Influenza B virus NS1 protein inhibits conjugation of the interferon (IFN)-induced ubiquitin-like ISG15 protein.. EMBO J 20(3):362-71 PMID: 11157743
- 7. Sarkar L et al.. 2023. ISG15: its roles in SARS-CoV-2 and other viral infections.. Trends Microbiol 31(12):1262-1275 PMID: 37573184
- 8. Thery F et al.. 2021. Ring finger protein 213 assembles into a sensor for ISGylated proteins with antimicrobial activity.. Nat Commun 12(1):5772 PMID: 34599178
