ISGylation

ISGylation refers to the process in which the ubiquitin-like protein ISG15 is covalently attached to target proteins. This process is similar to the ubiquitination system, but follows its own enzymatic cascade mechanism.

Type I interferons (IFN) induce the expression of >500 genes, which are collectively referred to as ISGs (IFN-stimulated genes). One of the first ISGs induced by IFN is ISG15 (interferon-stimulated gene 15). The free ISG15 protein synthesized by the ISG15 gene is post-translationally conjugated to cellular proteins and also released from cells into the extracellular milieu. ISG15 comprises two ubiquitin-like domains (UBL1 and UBL2), each of which bears a striking resemblance to ubiquitin, leading to its former name ubiquitin cross-reactive protein (UCRP).

Like ubiquitin, ISG15 also exhibits a characteristic β-Grasp fold in both UBL domains. The UBL2 domain has a conserved C-terminal Gly-Gly motif through which cellular proteins are attached via an enzymatic cascade similar to ubiquitylation, termed ISGylation.

The ISG15 protein is only minimally expressed under physiological conditions. However, its IFN-dependent expression is abnormally increased or impaired in several human diseases, including various tumor entities, neurodegenerative diseases(ataxia telangiectatica and amyotrophic lateral sclerosis), inflammatory diseases (Mendelian susceptibility to mycobacterial diseases(MSMD), bacteriopathy and viropathy). ISG15 and ISGylation have both inhibitory and stimulatory functions in the etiology and pathogenesis of human diseases. Both ISG15 and ISGylation are now seen as diagnostic/prognostic biomarkers and therapeutic targets for these diseases (Mirzalieva O et al. 2022).

Important features:

• ISG15 as a modifier: The ISG15 gene (ISG15 stands for "interferon-stimulated gene 15") is produced especially in cells that are stimulated by type I interferons (e.g. INF-α and INF-β). It acts like a small protein that can be attached to various target molecules.

Enzymatic process. The ISGylation process takes place in several steps:

1. Activation: ISG15 is activated by an E1-activating enzyme (e.g. UBE1L).
2. Conjugation: The activated ISG15 is then transferred to an E2-conjugating enzyme (e.g. UBCH8).
3. Ligation: An E3 ligase enzyme (such as HERC5) mediates the covalent binding of ISG15 to the target proteins.

Functional consequences: ISGylation can change the properties and functions of the modified proteins. These include:

• Alteration of stability and localization
• Modulation of interactions with other proteins
• Involvement in antiviral defense, as many ISG15-modified proteins play a role in signaling cascades of the immune response or in the inhibition of virus replication

Biological significance:

• This modification process is particularly active in cells that have been activated by a viral event or by the action of interferons. ISGylation thus contributes to antiviral immunity and to the degradation of viral components.

1. Kim YJ et al. (2016) Consecutive Inhibition of ISG15 Expression and ISGylation by Cytomegalovirus Regulators. PLoS Pathog 12:e1005850.
2. Liu G et al.(2021) ISG15-dependent activation of the sensor MDA5 is antagonized by the SARS-CoV-2 papain-like protease to evade host innate immunity. Nat Microbiol 6:467-478.
3. Mirzalieva O et al. (2022) ISG15 and ISGylation in Human Diseases. Cells 11(3):538.
4. Swaim CD et al.(2017) Extracellular ISG15 Signals Cytokine Secretion through the LFA-1 Integrin Receptor. Mol Cell 68:581-590.e5.
5. Zhang D et al. (2011) Interferon-stimulated gene 15 and the protein ISGylation system. J Interferon Cytokine Res 31:119-130.