Targeted protein degradation (TPD) is an emerging therapeutic modality with the potential to tackle disease-causing proteins that have historically been highly challenging to target with conventional small molecules. In the 20 years since the concept of a proteolysis-targeting chimera (PROTAC) molecule harnessing the ubiquitin–proteasome system to degrade a target protein was reported, TPD has moved from academia to industry, where numerous companies have disclosed programmes in preclinical and early clinical development. With clinical proof-of-concept for PROTAC molecules against two well-established cancer targets provided in 2020, the field is poised to pursue targets that were previously considered ‘undruggable’. In this Review, we summarize the first two decades of PROTAC discovery and assess the current landscape, with a focus on industry activity. We then discuss key areas for the future of TPD, including establishing the target classes for which TPD is most suitable, expanding the use of ubiquitin ligases to enable precision medicine and extending the modality beyond oncology.

Viral papain-like cysteine protease (PLpro, NSP3) is essential for SARS-CoV-2 replication and represents a promising target for the development of antiviral drugs. Here, we used a combinatorial substrate library and performed comprehensive activity profiling of SARS-CoV-2 PLpro. On the scaffold of the best hits from positional scanning, we designed optimal fluorogenic substrates and irreversible inhibitors with a high degree of selectivity for SARS PLpro. We determined crystal structures of two of these inhibitors in complex with SARS-CoV-2 PLpro that reveals their inhibitory mechanisms and provides a molecular basis for the observed substrate specificity profiles. Last, we demonstrate that SARS-CoV-2 PLpro harbors deISGylating activity similar to SARSCoV-1 PLpro but its ability to hydrolyze K48-linked Ub chains is diminished, which our sequence and structure analysis provides a basis for. Together, this work has revealed the molecular rules governing PLpro substrate specificity and provides a framework for development of inhibitors with potential therapeutic value or drug repurposing.

In December 2019, the first cases of a novel coronavirus infection causing COVID-19 were diagnosed in Wuhan, China. Viral Papain-Like cysteine protease (PLpro, NSP3) is essential for SARS-CoV-2 replication and represents a promising target for the development of antiviral drugs which would be facilitated by an understanding of its substrate specificity. Here, we used a combinatorial substrate library containing natural and a wide variety of nonproteinogenic amino acids and performed comprehensive activity profiling of SARS-CoV-2-PLpro. We found that the P2 site of SARS-CoV-2-PLpro is highly specific for Gly, the P3 site exhibits a high degree of promiscuity, and the P4 site exhibits a preference for amino acids with hydrophobic side chains. We also demonstrate that SARS-CoV-2-PLpro harbors deubiquitinating activity. Both the substrate binding profile and deubiquitinating activity are shared with the highly related SARS-CoV-PLpro which harbors near identical S4-S2 binding pockets. On the scaffold of best hits from positional scanning we have designed optimal fluorogenic substrates and irreversible inhibitors with a high degree of selectivity for SARS PLpro variants versus other proteases. Altogether this work has revealed the molecular rules governing PLpro substrate specificity and provides a framework for development of inhibitors with potential therapeutic value or drug repositioning.

SUMMARY Deubiquitinating enzymes (DUBs) recognize and cleave linkage-specific poly-ubiquitin (polyUb) chains, but mechanisms underlying specificity remain elusive in many cases. The Severe Acute Respiratory Syndrome (SARS) coronavirus papain-like protease (PLpro) is a DUB that cleaves ISG15, a two domain Ub-like protein, and Lys48-linked polyUb chains, releasing diUbLys48 products. To elucidate this specificity, we report the 2.85 Å crystal structure of SARS PLpro bound to a diUbLys48 activity-based probe. SARS PLpro binds diUbLys48 in an extended conformation via two contact sites, S1 and S2, which are proximal and distal to the active site, respectively. We show that specificity for polyUbLys48 chains is predicated on contacts in the S2 site and enhanced by an S1–S1′ preference for a Lys48 linkage across the active site. In contrast, ISG15 specificity is dominated by contacts in the S1 site. Determinants revealed for polyUbLys48 specificity should prove useful in understanding PLpro deubiquitinating activities in coronavirus infections.