Irreversible inactivation of ISG15 by a viral leader protease enables alternative infection detection strategies

Swatek, Kirby N.; Aumayr, Martina; Pruneda, Jonathan N.; Visser, Linda J.; Berryman, Stephen; Kueck, Anja F.; Geurink, Paul P.; Ovaa, Huib; van Kuppeveld, Frank J. M.; Tuthill, Tobias J.; Skern, Tim; Komander, David

Lb pro is a prime example of a viral protein that contributes to successful viral replication through multifunctional roles. In addition to previously known crucial activities during replication (4, 6), we now show how it also hinders antiviral signaling through removal of ISG15 and to a lesser extent, ubiquitin from proteins (Fig. 5) . Importantly, its mechanism of incomplete cleavage irreversibly damages the modifiers, which can no longer be attached to proteins. This mechanism is conceptually similar to the activity of RavZ, a Legionella effector that hydrolyzes the C terminus of Atg8 ubiquitin-like modifiers involved in autophagy (30) . Moreover, on the substrate side, Lb pro activity precludes remodification of Lys residues, and their small GlyGly modification(s) may not alter protein function significantly (Fig. 5 ). The slight cross-reactivity with ubiquitin is likely important, since ubiquitin modifications are much more abundant, and it is hence difficult to delineate the origin of the observed GlyGly signatures. Nonetheless, these multifaceted traits highlight the importance of Lb pro as a potent virulence factor (31) . It is possible that other viruses and pathogens may use this elegant antiinflammatory strategy. While the leader proteases of other picornaviridae are highly divergent on the sequence level, the highly related apthovirus equine rhinitis A virus may also encode an enzyme that generates GlyGly epitopes, which could be tested using GlyGly epitope detection in infected samples.