Downsizing a BTK PROTAC yields a G-loop independent molecular glue that blurs the boundary between degrader classes

Abstract Targeted protein degradation (TPD) represents a novel and rapidly evolving therapeutic strategy that harnesses the cell's own proteolytic systems to selectively eliminate disease-associated proteins. In this study, we serendipitously discovered and characterized the first Bruton's tyrosine kinase (BTK)-targeting molecular glue, PS-10. PS-10 efficiently degrades BTK with a DC 50 in the single-digit nanomolar range and a D max exceeding 95%, despite displaying negligible binary binding affinity for the kinase itself. Cryo-EM and computational modeling revealed that PS-10 induces a protein-protein interaction (PPI) interface between CRBN and BTK stabilized by an induced-fit rotation of the BTK N-lobe, with extensive PPIs distinct from previously known CRBN-substrate complexes. Crucially, global proteomics and mutational mapping across other kinases (CSK, AURKA, LIMK1, and TEC) demonstrated that this degradation mechanism extends beyond BTK and relies on a diverse set of PPI residues. These findings underscore a remarkable structural plasticity, indicating that productive E3-neosubstrate PPIs can be dynamically achieved via distinct structural trajectories. Ultimately, this demonstrates that rigid, pre-existing degrons are not mandatory; rather, the existence of even a weak target binder presents a distinct opportunity to exploit plastic PPI interfaces to maximize cooperativity, establishing a novel and generalizable blueprint for rational molecular glue design.