USP18 - a multifunctional component in the interferon response

Abstract

Ubiquitin-specific proteases (USPs) represent the largest family of deubiquitinating enzymes (DUB). These proteases cleave the isopeptide bond between ubiquitin and a lysine residue of a ubiquitin-modified protein. USP18 is a special member of the USP family as it only deconjugates the ubiquitin-like protein ISG15 (interferon-stimulated gene (ISG) 15) from target proteins but is not active towards ubiquitin. Independent of its protease activity, USP18 functions as a major negative regulator of the type I interferon response showing that USP18 is - at least - a bifunctional protein. In this review, we summarise our current knowledge of protease-dependent and -independent functions of USP18 and discuss the structural basis of its dual activity.

Keywords: ISG15; USP18; interferons; protease; ubiquitin like modifier proteins.

Figure 1. Enzymatic and non-enzymatic functions of USP18

Type I interferons (IFN) bind to a dimeric receptor (IFNAR1 and IFNAR2) on the cell surface to activate the intracellular kinases TYK2 and JAK1. These kinases recruit and phosphorylate the transcription factors STAT1 and STAT2 which subsequently bind the transcription factor IRF9 and translocate into the nucleus. The trimeric complex binds to the ISRE element in promoters of interferon-stimulated genes (ISGs) and induces the expression of ISGs including USP18, ISG15 and the E1, E2 and E3 enzymes that catalyse ISGylation of substrate proteins. USP18 deISGylates target proteins by cleaving the isopeptide bond between the C-terminus of ISG15 and a lysine residue of the substrate protein. USP18 also interacts with IFNAR2 and STAT2 to block type I interferon signalling in a protease-independent manner. Abbreviations: IRF9, IFN-regulatory factor 9, JAK1, Janus activated kinase 1; STAT, signal transducer and activator of transcription; TYK2, tyrosine kinase 1.

Figure 2. Structure of free and ISG15-bound USP18

(A) Overall structure of USP18 (pdb code 5cht) showing the three-domain architecture with finger, thumb and palm domains. The finger domain co-ordinates a Zn ion shown as sphere. Superposition of the two chains present in the asymmetric unit revealed that the enzyme had crystallised in two conformations that mainly differ in the orientation of the finger domain. (B) Structure of the USP18–ISG15 complex (pdb code 5chv). Only the C-terminal Ubl domain of ISG15 makes extensive contacts with all three domains of USP18 (blue) and the C-terminal tail of ISG15 lies in the cleft between the palm and the thumb domains where it reaches the catalytic triad. The two ISG15 molecules present in the asymmetric unit differ in the relative orientation of N- and C-terminal Ubl domains whereas the two USP18 molecules are virtually identical. The ISG15 molecules are shown in yellow and orange, USP18 in blue. (C) Surface representation of USP18 in the active conformation (ISG15-bound). The surface area with which USP18 recognises ISG15 was calculated by the Eppic server (software version 3.0.4) [64] and is shown in red. The residues that form ISG15-binding box1 (IBB-1) are depicted in yellow and shown as a close-up view below. (D) The suggested surface areas with which USP18 binds to STAT2 and IFNAR2 are depicted in blue and green, respectively. These surface areas were defined using deletion constructs of human USP18 [22,58] and the respective residues of mouse USP18 are shown. For IFNAR2 an additional interface was mapped in the N-terminus of USP18 [58] which is not present in the structure. A small surface area on USP18 interacts with ISG15 and was also suggested to bind to IFNAR2. This area is shown in purple.