Interferon-stimulated gene 15 (ISG15) is a 17 kDA secreted
protein that in humans is encoded by the ISG15gene.[5][6] ISG15 is induced by
type I interferon (IFN) and serves many functions, acting both as an extracellular
cytokine and an intracellular protein modifier. The precise functions are diverse and vary among species but include potentiation of
Interferon gamma (IFN-II) production in
lymphocytes,
ubiquitin-like conjugation to newly-synthesized proteins and negative regulation of the IFN-I response.[7]
Structure
The ISG15 gene consists of two
exons and encodes for a 17 kDa polypeptide. The immature polypeptide is cleaved at its carboxy terminus, generating a mature 15 kDa product that terminates with a LRLRGG
motif, as found in
ubiquitin. The tertiary structure of ISG15 also resembles
ubiquitin, despite only ~30%
sequence identity. Specifically, this structure consists of two
ubiquitin-like domains connected by a polypeptide ‘hinge.’ Of note, ISG15 shows substantial sequence variation among species, with homology as low as 30% between
orthologs.[8]
Function
After induction by
type I interferon, ISG15 can be found in three forms, each with unique functions:
Extracellular cytokine
ISG15 is secreted from the cell and can be detected in
supernatant or
blood plasma.[9][10] ISG15 binds the LFA-1 integrin receptor on NK- and T-cells to potentiate their production of IFN-II,[11][12] which is essential for
mycobacterial immunity.
Intracellular conjugate: ISGylation
In a ubiquitin-like fashion, ISG15 is covalently linked by its C-terminal LRLRGG motif to lysine residues on newly synthesized proteins. This process, termed ISGylation, is catalyzed by a series of conjugating enzymes. The activating
E1 enzyme (UBE1L) charges ISG15 by forming a high-energy thiolester intermediate and transfers it to the UbcH8
E2 enzyme. UbcH8 has been identified as the major E2 for ISGylation, although it also functions in ubiquitination. The E2 protein subsequently transfers the ISG15 to specific
E3 ligases (Herc5[13]) and relevant intracellular substrates. Only one deconjugating protease with specificity to ISG15 has been identified to date:
USP18 (a member of the USP family) cleaves ISG15-peptide fusions and also removes ISG15 (deISGylation) from native conjugates.[14] The effects of ISGylation are incompletely understood and involve both activation and inhibition of antiviral immunity.
Free intracellular molecule
Unconjugated ISG15 negatively regulates IFN-I signaling by preventing the
SKP2-mediated
proteasomal degradation of
USP18, a direct inhibitor of the
IFN-I receptor.[15] Absence of ISG15 leads to persistent IFN-I signaling in human, but not mouse, systems.[16]
ISG15 was originally identified in the late 1970s as a 15-kDa protein produced in response to type I interferon, a potent class of antiviral cytokines.[19] Given the molecular weight, it was originally termed ‘a 15-kDa protein’, but later renamed interferon-stimulated-gene-15 when the cassette of
interferon-stimulated genes were recognized.[20][21] In 1987 it was identified that ISG15
cross-reacts with anti-
ubiquitin antibodies, and subsequent experiments uncovered the ubiquitin-like conjugation of ISG15 to other cellular proteins, coined ‘ISGylation’.[22][23] Given its inducibility by IFN-I, studies in the following decades focused on the antiviral activity of ISG15. These studies were carried out predominantly with
in vitro systems and mouse models, and ascribed several antiviral functions to ISGylation. During this time, it was also discovered that ISG15 could be detected outside of cells.[9] and in human serum samples.[10] This free form of ISG15 could stimulate IFN-II production in lymphocytes.[11] Finally, ISG15 could also be detected as an un-conjugated intracellular molecule with functions independent of ISGylation.[24]
The discovery of humans deficient in ISG15 elucidated the importance of these functions in human biology. ISG15-deficient patients were first identified by their susceptibly to BCG-strain
mycobacteria, owing to the essential function of free ISG15 to potentiate the IFN-gamma /
Interleukin-12 axis[12] Surprisingly, despite the IFN-inducible nature of ISG15 and the previously-ascribed antiviral functions in mice, ISG15-deficient patients showed no susceptibility to viral infections.[12] In fact, follow-up studies uncovered enhanced type I IFN signatures, manifesting as basal ganglia calcifications akin to
TORCH infection but without an infectious etiology.[15] This persistent, low-level inflammation was later shown to confer enhanced resistance to a wide array of viruses.[16] This phenotype results from a previously-unrecognized function of ISG15 to negatively regulate IFN signaling, which is absent in murine systems. Other higher-order mammals (e.g. pig and dog), however, have achieved this negative regulatory function of ISG15, seemingly by convergent evolution.[25]
Andersen JB, Hassel BA (December 2006). "The interferon regulated ubiquitin-like protein, ISG15, in tumorigenesis: friend or foe?". Cytokine & Growth Factor Reviews. 17 (6): 411–21.
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10.1016/j.cytogfr.2006.10.001.
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Clauss IM, Wathelet MG, Szpirer J, Content J, Islam MQ, Levan G, et al. (1990). "Chromosomal localization of two human genes inducible by interferons, double-stranded RNA, and viruses". Cytogenetics and Cell Genetics. 53 (2–3): 166–8.
doi:
10.1159/000132920.
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Feltham N, Hillman M, Cordova B, Fahey D, Larsen B, Blomstrom D, Knight E (October 1989). "A 15-kD interferon-induced protein and its 17-kD precursor: expression in Escherichia coli, purification, and characterization". Journal of Interferon Research. 9 (5): 493–507.
doi:
10.1089/jir.1989.9.493.
PMID2477469.
Lowe J, McDermott H, Loeb K, Landon M, Haas AL, Mayer RJ (October 1995). "Immunohistochemical localization of ubiquitin cross-reactive protein in human tissues". The Journal of Pathology. 177 (2): 163–9.
doi:
10.1002/path.1711770210.
PMID7490683.
S2CID10420781.
Smith JK, Siddiqui AA, Krishnaswamy GA, Dykes R, Berk SL, Magee M, et al. (August 1999). "Oral use of interferon-alpha stimulates ISG-15 transcription and production by human buccal epithelial cells". Journal of Interferon & Cytokine Research. 19 (8): 923–8.
doi:
10.1089/107999099313460.
PMID10476939.
Bebington C, Doherty FJ, Fleming SD (October 1999). "Ubiquitin cross-reactive protein gene expression is increased in decidualized endometrial stromal cells at the initiation of pregnancy". Molecular Human Reproduction. 5 (10): 966–72.
doi:
10.1093/molehr/5.10.966.
PMID10508226.
Padovan E, Terracciano L, Certa U, Jacobs B, Reschner A, Bolli M, et al. (June 2002). "Interferon stimulated gene 15 constitutively produced by melanoma cells induces e-cadherin expression on human dendritic cells". Cancer Research. 62 (12): 3453–8.
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External links
Overview of all the structural information available in the
PDB for
UniProt: P05161 (Human Ubiquitin-like protein ISG15) at the
PDBe-KB.
Overview of all the structural information available in the
PDB for
UniProt: Q64339 (Mouse Ubiquitin-like protein ISG15) at the
PDBe-KB.
1z2m: Crystal Structure of ISG15, the Interferon-Induced Ubiquitin Cross Reactive Protein
2hj8: Solution NMR structure of the C-terminal domain of the interferon alpha-inducible ISG15 protein from Homo sapiens. Northeast Structural Genomics target HR2873B