Covalent Molecular Glue Locks Kalirin-Rac1 Complex to Halt GTPase Activation
- Researchers combined XChem crystallographic fragment screening with in silico screening, then merged fragment hits into covalent leads targeting the Kalirin-Rac1 GEF-GTPase complex.
- Instead of directly competing with GTP/GDP for Rac1's binding site, the covalent compounds act as molecular glues, locking Kalirin and Rac1 into an inactive ternary complex that blocks nucleotide exchange.
- Two resulting inhibitors, RS-009 and MC-278, reduce Kalirin's GEF activity through this dual action: partial nucleotide competition plus stabilization of the non-productive complex.
- Biochemical selectivity panels and cellular activity-based protein profiling show the compounds can distinguish among different GEF-GTPase pairs rather than acting indiscriminately.
- The authors frame this as an alternative to direct GTPase inhibition that could offer finer spatiotemporal control, positioning RS-009/MC-278 as chemical probe candidates.
Locking down the non-productive Kalirin-Rac1 complex rather than competing head-on for the nucleotide pocket is the smarter play — GTP/GDP affinity is sub-nanomolar, and pan-Rac1 nucleotide-pocket tools like NSC23766 have never been clean. It's the SOS1-KRAS playbook applied to Rho biology, not a new idea, but a good one to borrow. Selectivity across GEF-GTPase pairs is the actual prize, not a burden — Rho-family homology is brutal, so an interface-driven selectivity handle beats trying to differentiate active-site chemistry. What I'd interrogate: fragment merging from XChem hits into covalent leads has a high attrition rate, so two validated compounds (RS-009, MC-278) is a real result, not a given. Before calling these clean probes I want warhead kinetics and full-proteome ABPP, not just the GEF-GTPase panel.
Discovery of a molecular glue inhibitor that stabilises a non-productive Kalirin-Rac1 complex ↗