Ubiquitin-like protein that is conjugated to intracellular target proteins after IFN-alpha or IFN-beta stimulation. Its enzymatic pathway is partially distinct from that of ubiquitin, differing in substrate specificity and interaction with ligating enzymes. ISG15 conjugation pathway uses a dedicated E1 enzyme, but seems to converge with the Ub conjugation pathway at the level of a specific E2 enzyme. Targets include STAT1, SERPINA3G/SPI2A, JAK1, MAPK3/ERK1, PLCG1, EIF2AK2/PKR, MX1/MxA, and RIG-1. Deconjugated by USP18/UBP43. Shows specific chemotactic activity towards neutrophils and activates them to induce release of eosinophil chemotactic factors. May serve as a trans-acting binding factor directing the association of ligated target proteins to intermediate filaments. May also be involved in autocrine, paracrine and endocrine mechanisms, as in cell-to-cell signaling, possibly partly by inducing IFN-gamma secretion by monocytes and macrophages. Seems to display antiviral activity during viral infections. Interacts with, and is conjugated to its targets by the UBE1L (E1 enzyme) and UBE2E2 (E2 enzyme) (Probable). Interaction with influenza B NS1 protein inhibits this conjugation. By type I interferons. Detected in lymphoid cells, striated and smooth muscle, several epithelia and neurons. Note: This description may include information from UniProtKB.
Biological Process: ISG15-protein conjugation; modification-dependent protein catabolic process; cell-cell signaling; negative regulation of viral genome replication; regulation of interferon-gamma production; cytokine and chemokine mediated signaling pathway; virus-host interaction; defense response to bacterium; innate immune response; negative regulation of protein ubiquitination; defense response to virus; negative regulation of interferon type I production
SS: The number of records in which this modification site was determined using site-specific methods. SS methods include amino acid sequencing, site-directed mutagenesis, modification site-specific antibodies, specific MS strategies, etc.