Structural basis for the specificity of USP18 towards ISG15

Basters, Anja; Geurink, Paul P.; Röcker, Annika; Witting, Katharina F.; Tadayon, Roya; Hess, Sandra; Semrau, Marta S.; Storici, Paola; Ovaa, Huib; Knobeloch, Klaus-Peter; Fritz, Günter

Protein modification by ubiquitin and ubiquitin-like modifiers (Ubls) is counteracted by ubiquitin- and Ubl-proteases collectively called DUBs. In contrast to other proteases of the ubiquitin-specific protease (USP) family, USP18 shows no reactivity towards ubiquitin but specifically deconjugates the interferon induced Ubl ISG15. To identify molecular determinants for this specificity, we solved the crystal structures of mouse USP18 and of mouse USP18 in complex with mouse ISG15. USP18 was crystallized in an open and a closed conformation revealing high flexibility of the enzyme. Structural data, biochemical and mutational analysis showed that only the C-terminal ubiquitin-like domain of ISG15 is recognized and essential for USP18 activity. A critical hydrophobic patch in USP18 interacts with a hydrophobic region unique to ISG15 providing evidence that ISG15 specificity of USP18 is mediated by a small interaction interface. Our results may provide the structural basis for the development of new drugs modulating ISGylation.