ISG15: It's Complicated

Abstract

Interferon-stimulated gene product 15 (ISG15) is a key component of host responses to microbial infection. Despite having been known for four decades, grasping the functions and features of ISG15 has been a slow and elusive process. Substantial work over the past two decades has greatly enhanced this understanding, revealing the complex and variable nature of this protein. This has unveiled multiple mechanisms of action that are only now beginning to be understood. In addition, it has uncovered diversity not only between how ISG15 affects different pathogens but also between the function and structure of ISG15 itself between different host species. Here we review the complexity of ISG15 within the context of viral infection, focusing primarily on its antiviral function and the mechanisms viruses employ to thwart its effects. We highlight what is known regarding the impact of ISG15 sequence and structural diversity on these interactions and discuss the aspects presenting the next frontier toward elucidating a more complete picture of ISG15 function.

Keywords: DUB; ISGylation; RIG-I; USP18; interferon.

Graphical abstract

Fig. 1

ISG15 is conjugated to a wide range of viral and cellular proteins, influencing immune responses. (a) The infected host cell senses viral RNA via RIG-I, which induces signaling leading to the expression and secretion of type I IFN. Type I interferon binding to IFN receptor (IFNR) result in the expression of IFN-stimulated genes (ISGs), including ISG15 and its conjugating enzymes (Ube1L, UbcH8, and Herc5). Herc5 association to ribosomes results in the ISGylation of newly synthesized host and viral proteins including several ISGs (white). ISG15 conjugation can inhibit (red) or enhance (green) the activation state of innate immune signaling proteins. USP18 is a negative regulator of IFNR signaling by limiting STAT activation. USP18 protected from degradation by ISG15 binding, thereby enhancing the inhibition of IFNR signaling. Extracellular ISG15 binds to LFA-1 receptors of leukocytes and induces the expression and secretion of IFN-ɣ. (b) Viral proteins can counter ISGylation by sequestering ISG15/ISGylated proteins, or by deconjugating ISG15. (c) The 2A protease of Coxsackie B3 virus induces host translational shutoff by cleaving eIF4G. ISGylation of 2A restores host translation by preventing eIF4G cleavage. (d) ISGylation of viral proteins interferes with virus replication. ISGylation of Influenza B virus NP reduces the oligomerization and formation of viral ribonucleoprotein complexes (RNPs). NS1B counters ISGylation by sequestering ISG15 and ISGylated NP.

Fig. 2

Structure of human ISG15 (PDB entry 1Z2M). The secondary structure is denoted with helices and loops shown in red and sheets shown in gold. The C-terminal domain is denoted by a green background, hinge region by a blue background, and the N-terminal domain by a purple background. A sequence alignment of human ISG15 (hISG15), mouse ISG15 (mISG15), canine ISG15 (caISG15), sheep ISG15 (shISG15), bovine ISG15 (boISG15), camel ISG15 (cISG15), vesper bat ISG15 (bISG15), fish ISG15 (fISG15), and Ub is shown with the domain architecture indicated with colored bars. The residues of ISG15 known to interact with the influenza B NS1 protein, coronavirus PLPs, and nairovirus OTUs are indicated. The sequence alignment graphic was generated using the ESPript server .

Fig. 3

ISG15 structural diversity. (a) The C-terminal domains of ISG15 chains from human ISG15 (red, , , , and PDB entries 3R66 and 6BI8), murine ISG15 (gold, , , and PDB entries 5CHF and 5CHW), bat ISG15 (blue, [66]), bovine ISG15 (cyan green, [65]), and canine ISG15 (dark teal, [65]) were overlaid to identify the trends in domain–domain orientations between species. Representative chains were selected for each species. For murine ISG15, two structures were included to show the wider range of conformations that have been observed. Colored wedges show the approximate spatial range occupied by the N-terminal domain of ISG15 from each species. (b) Surface rendering of ISG15 showing the relative conservation among > 100 species (left). Calculations and renderings were performed using the ConSurf server and PyMOL , , , , . Surface of ISG15 known to interact with viral proteins based on x-ray crystal complex structures (right), with the interface with the influenza B NS1 protein shown in green, nairovirus OTU interface in yellow, and coronavirus PLP in brown. The template ISG15 structure for surface renderings was human ISG15 (PDB entry 1Z2M).