Structural insights into the species preference of the influenza B virus NS1 protein in ISG15 binding

More than Meets the ISG15: Emerging Roles in the DNA Damage Response and Beyond

Maintenance of genome stability is a crucial priority for any organism. To meet this priority, robust signalling networks exist to facilitate error-free DNA replication and repair. These signalling cascades are subject to various regulatory post-translational modifications that range from simple additions of chemical moieties to the conjugation of ubiquitin-like proteins (UBLs). Interferon Stimulated Gene 15 (ISG15) is one such UBL. While classically thought of as a component of antiviral immunity, ISG15 has recently emerged as a regulator of genome stability, with key roles in the DNA damage response (DDR) to modulate p53 signalling and error-free DNA replication. Additional proteomic analyses and cancer-focused studies hint at wider-reaching, uncharacterised functions for ISG15 in genome stability. We review these recent discoveries and highlight future perspectives to increase our understanding of this multifaceted UBL in health and disease.

How ISG15 combats viral infection

Interferon (IFN)-stimulated gene product 15 (ISG15) is a ubiquitin-like protein critical for the control of microbial infections. ISG15 appears to serve a wide variety of functions, which regulate multiple cellular responses contributing to the development of an antiviral state. ISG15 is a versatile molecule directly modulating both host and virus protein function which regulate many signaling pathways, including its own synthesis. Here we review the various roles ISG15 plays in the antiviral immune response, and examine the mechanisms by which viruses attempt to mitigate or exploit ISG15 activity.

Determining the molecular drivers of species-specific interferon-stimulated gene product 15 interactions with nairovirus ovarian tumor domain proteases

Tick-borne nairoviruses (order Bunyavirales) encode an ovarian tumor domain protease (OTU) that suppresses the innate immune response by reversing the post-translational modification of proteins by ubiquitin (Ub) and interferon-stimulated gene product 15 (ISG15). Ub is highly conserved across eukaryotes, whereas ISG15 is only present in vertebrates and shows substantial sequence diversity. Prior attempts to address the effect of ISG15 diversity on viral protein-ISG15 interactions have focused on only a single species' ISG15 or a limited selection of nairovirus OTUs. To gain a more complete perspective of OTU-ISG15 interactions, we biochemically assessed the relative activities of 14 diverse nairovirus OTUs for 12 species' ISG15 and found that ISG15 activity is predominantly restricted to particular nairovirus lineages reflecting, in general, known virus-host associations. To uncover the underlying molecular factors driving OTUs affinity for ISG15, X-ray crystal structures of Kupe virus and Ganjam virus OTUs bound to sheep ISG15 were solved and compared to complexes of Crimean-Congo hemorrhagic fever virus and Erve virus OTUs bound to human and mouse ISG15, respectively. Through mutational and structural analysis seven residues in ISG15 were identified that predominantly influence ISG15 species specificity among nairovirus OTUs. Additionally, OTU residues were identified that influence ISG15 preference, suggesting the potential for viral OTUs to adapt to different host ISG15s. These findings provide a foundation to further develop research methods to trace nairovirus-host relationships and delineate the full impact of ISG15 diversity on nairovirus infection.

Species-Specific Host-Virus Interactions: Implications for Viral Host Range and Virulence

A growing number of studies indicate that host species-specific and virus strain-specific interactions of viral molecules with the host innate immune system play a pivotal role in determining virus host range and virulence. Because interacting proteins are likely constrained in their evolution, mutations that are selected to improve virus replication in one species may, by chance, alter the ability of a viral antagonist to inhibit immune responses in hosts the virus has not yet encountered. Based on recent findings of host-species interactions of poxvirus, herpesvirus, and influenza virus proteins, we propose a model for viral fitness and host range which considers the full interactome between a specific host species and a virus, resulting from the combination of all interactions, positive and negative, that influence whether a virus can productively infect a cell and cause disease in different hosts.

ISG15: It's Complicated

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.