ISG15 and immune diseases

Abstract

ISG15, the product of interferon (IFN)-stimulated gene 15, is the first identified ubiquitin-like protein, consisting of two ubiquitin-like domains. ISG15 is synthesized as a precursor in certain mammals and, therefore, needs to be processed to expose the C-terminal glycine residue before conjugation to target proteins. A set of three-step cascade enzymes, an E1 enzyme (UBE1L), an E2 enzyme (UbcH8), and one of several E3 ligases (e.g., EFP and HERC5), catalyzes ISG15 conjugation (ISGylation) of a specific protein. These enzymes are unique among the cascade enzymes for ubiquitin and other ubiquitin-like proteins in that all of them are induced by type I IFNs or other stimuli, such as exposure to viruses and lipopolysaccharide. Mass spectrometric analysis has led to the identification of several hundreds of candidate proteins that can be conjugated by ISG15. Some of them are type I IFN-induced proteins, such as PKR and RIG-I, and some are the key regulators that are involved in IFN signaling, such as JAK1 and STAT1, implicating the role of ISG15 and its conjugates in type I IFN-mediated innate immune responses. However, relatively little is known about the functional significance of ISG15 induction due to the lack of information on the consequences of its conjugation to target proteins. Here, we describe the recent progress made in exploring the biological function of ISG15 and its reversible modification of target proteins and thus in their implication in immune diseases.

Fig. 1

Protein modification by ISG15. ISG15-specific proteases cleave off the C-terminal extensions from ISG15 precursors to generate matured ISG15 molecules. ISG15 is activated by UBE1L (E1) at the expense of ATP and is subsequently linked to the activating enzyme via thiosester bond. ISG15 linked to UBE1L is transferred to UbcH8 (E2) and then to a target protein with the aid of an ISG15 E3 ligase, such as EFP and HERC5. UBP43 functions in the reversal of the ISGylation process by cleaving off ISG15 molecules that are conjugated to the substrate proteins via isopeptide bonds.

Fig. 2

Comparison of the amino acid sequence of ISG15 with that of ubiquitin and FAT10. (A) The amino acid sequences of ubiquitin, ISG15, and FAT10 are aligned. Similar amino acids are indicated by dark blue and the identical amino acids are by light blue. (B) Schematic diagram shows the N- and C-terminal ubiquitin-like domains that are flanked by a flexible linker region containing a Pro residue. (C) The amino acid sequences of ISG15 from various species are aligned. Note that ISG15 molecules from human, mouse, and rat have the C-terminal extensions of 5–8 amino acids, whereas ISG15 molecules from bovine and fish do not.

Fig. 3

Comparison of the tertiary structure of ISG15 with that of ubiquitin. (A) ISG15 alone. (B) Ubiquitin alone. (C) Ubiquitin is overlapped to the N- and C-terminal domains of ISG15. (D) All of ubiquitin and the N- and C-terminal domains of ISG15 are overlapped. Information for structures was from the protein data bank (PDB): ubiquitin from UCH-L3-Ub complex (PDB code: 1XD3) and ISG15 (PDB code: 1Z2M). The photographic images were made by using the PyMol program.

Fig. 4

Viral immune-evading proteins. NS3/4A is involved in the cleavage of IPS-1, thus blocking the RIG-I signaling for IRF-3-mediated induction of type I IFN as well as of ISG15. NS1B inhibits the thioesterification of UBE1L and thereby the generation of ISG15 conjugates that are required for antiviral response. PLpro, OTU protease, and viral E3 protein mediate the removal of ISG15 from its conjugates.