The ISG15(M21786:9606) protein is a ubiquitin-like modifier induced by interferon, which induces modification and somehow inhibits cellular and viral proteins . Like ubiquitin(AB062071:9606) , ISG15(M21786:9606) is conjugated to lysines on numerous target proteins through its conserved C-terminal region. This reaction is mediated by specific E1-activating, E2-conjugating, and E3-ligase enzymes, all of which are also IFN-inducible.
Viruses escape from the antiviral activity of ISG15(M21786:9606) by using different mechanisms. The influenza B virus NS1 protein(P03502) for instance blocks the covalent linkage of ISG15(M21786:9606) to its target proteins by directly interacting with ISG15(M21786:9606). The papain-like protease(P0C6X7) from the severe acute respiratory syndrome coronavirus cleaves ISG15(M21786:9606) derivatives.