ISGylation in Innate Antiviral Immunity and Pathogen Defense Responses: A Review

Abstract

The interferon-stimulating gene 15 (ISG15) protein is a ubiquitin-like protein induced by interferons or pathogens. ISG15 can exist in free form or covalently bind to the target protein through an enzymatic cascade reaction, which is called ISGylation. ISGylation has been found to play an important role in the innate immune responses induced by type I interferon, and is, thus, critical for the defense of host cells against RNA, DNA, and retroviruses. Through covalent binding with the host and viral target proteins, ISG15 inhibits the release of viral particles, hinder viral replication, and regulates the incubation period of viruses, thereby exerting strong antiviral effects. The SARS-CoV-2 papain-like protease, a virus-encoded deubiquitinating enzyme, has demonstrated activity on both ubiquitin and ISG15 chain conjugations, thus playing a suppressive role against the host antiviral innate immune response. Here we review the recent research progress in understanding ISG15-type ubiquitin-like modifications, with an emphasis on the underlying molecular mechanisms. We provide comprehensive references for further studies on the role of ISG15 in antiviral immunity, which may enable development of new antiviral drugs.

Keywords: ISG15; SARS PLpro; immune response; innate antiviral immunity; isgylation.

FIGURE 1

The conjugation of ISG15. The binding process of ISG15 and substrate is similar to the three-step enzyme cascade reaction of ubiquitination. The formation of thioester bond between E1 activating enzyme (UBE1L) and ISG15 depends on ATP, thereby activating ISG15. Next, SIG15 is transferred to the cysteine active site of E2 ligase (UBCH8). Finally, E3 ligase binds to polysomes, thereby promoting the binding of ISG15 to the nascent target protein. The process of ISGylation is reversible, and USP18 as a deubiquitinating enzyme can specifically remove ISG15 from the binding protein.

FIGURE 2

The function of ISG15 in immune response. Under pathogenic stimuli such as viral and bacterial infections, LPS, and DNA damage, monocytes, lymphocytes, neutrophils, etc. can all secrete ISG15. Intracellular ISG15 can bind to proteins related to innate immune signaling pathways, activate IRF3, STAT1, JAK1 and other proteins, or inhibit protein activity (such as RIG-I), thereby promoting or inhibiting the secretion of IFNγ. The ISG15 secreted in vitro can bind to the LFA1 receptor on the cell surface, thereby promoting the secretion of IFNγ from NK cells and T cells. It can also induce the proliferation of NK cells and the maturation of dendritic cells.

FIGURE 3

Antiviral effects of ISGylation on host and viral proteins. ISG15 affects the infection of cells by the virus through covalently binding with viral proteins and host proteins. 1. The combination of ISG15 and the viral nucleoprotein (green) can destroy the protein oligomerization and the ability of the viral nucleoprotein to inhibit virus replication. The ubiquitin-like modification formed by this combination can be cleaved by PLpro to restore the replication ability of the virus.2. The combination of ISG15 and host protein (blue) can inhibit the interaction between host protein and virus protein, thereby inhibiting the release of virus particles in the cell.