Structural mechanism of CRL4-instructed STAT2 degradation via a novel cytomegaloviral DCAF receptor

Targeted protein degradation via intramolecular bivalent glues

Targeted protein degradation is a pharmacological modality that is based on the induced proximity of an E3 ubiquitin ligase and a target protein to promote target ubiquitination and proteasomal degradation. This has been achieved either via proteolysis-targeting chimeras (PROTACs)-bifunctional compounds composed of two separate moieties that individually bind the target and E3 ligase, or via molecular glues that monovalently bind either the ligase or the target1-4. Here, using orthogonal genetic screening, biophysical characterization and structural reconstitution, we investigate the mechanism of action of bifunctional degraders of BRD2 and BRD4, termed intramolecular bivalent glues (IBGs), and find that instead of connecting target and ligase in trans as PROTACs do, they simultaneously engage and connect two adjacent domains of the target protein in cis. This conformational change 'glues' BRD4 to the E3 ligases DCAF11 or DCAF16, leveraging intrinsic target-ligase affinities that do not translate to BRD4 degradation in the absence of compound. Structural insights into the ternary BRD4-IBG1-DCAF16 complex guided the rational design of improved degraders of low picomolar potency. We thus introduce a new modality in targeted protein degradation, which works by bridging protein domains in cis to enhance surface complementarity with E3 ligases for productive ubiquitination and degradation.

A conformational selection mechanism of flavivirus NS5 for species-specific STAT2 inhibition

Flaviviruses, including Zika virus (ZIKV) and Dengue virus (DENV), rely on their non-structural protein 5 (NS5) for both replication of viral genome and suppression of host IFN signaling. DENV and ZIKV NS5s were shown to facilitate proteosome-mediated protein degradation of human STAT2 (hSTAT2). However, how flavivirus NS5s have evolved for species-specific IFN-suppression remains unclear. Here we report structure-function characterization of the DENV serotype 2 (DENV2) NS5-hSTAT2 complex. The MTase and RdRP domains of DENV2 NS5 form an extended conformation to interact with the coiled-coil and N-terminal domains of hSTAT2, thereby promoting hSTAT2 degradation in cells. Disruption of the extended conformation of DENV2/ZIKV NS5, but not the alternative compact state, impaired their hSTAT2 binding. Our comparative structural analysis of flavivirus NS5s further reveals a conserved protein-interaction platform with subtle amino-acid variations likely underpinning diverse IFN-suppression mechanisms. Together, this study uncovers a conformational selection mechanism underlying species-specific hSTAT2 inhibition by flavivirus NS5.

Unlocking DCAFs To Catalyze Degrader Development: An Arena for Innovative Approaches

Chemically induced proximity-based targeted protein degradation (TPD) has become a prominent paradigm in drug discovery. With the clinical benefit demonstrated by certain small-molecule protein degraders that target the cullin-RING E3 ubiquitin ligases (CRLs), the field has proactively strategized to tackle anticipated drug resistance by harnessing additional E3 ubiquitin ligases to enrich the arsenal of this therapeutic approach. Here, we endeavor to explore the collaborative efforts involved in unlocking a broad range of CRL4DCAF for degrader drug development. Throughout the discussion, we also highlight how both conventional and innovative approaches in drug discovery can be taken to realize this objective. Moving ahead, we expect a greater allocation of resources in TPD to pursue these high-hanging fruits.