SINE compounds activate exportin-1 degradation via an allosteric mechanism

SummaryThe nuclear export receptor exportin 1 (XPO1/CRM1) is often overexpressed in cancer cells, leading to the mislocalization of numerous cancer-related protein cargoes1,2. Selinexor, a covalent XPO1 inhibitor, and other Selective Inhibitor of Nuclear Export (SINEs) restore proper nuclear localization by blocking XPO1-cargo binding2-7. SINEs also induce XPO1 degradation via the Cullin-RING E3 ubiquitin ligase (CRL) substrate receptor ASB87. Here we elucidate the mechanism underlying the high-affinity engagement of CRL5ASB8 with SINE-conjugated XPO1. Cryogenic electron microscopy (cryoEM) structures reveal that ASB8 binds to a cryptic site on XPO1, which becomes accessible only upon SINE conjugation. While molecular glue degraders typically interact with both CRL and the substrate8-10, SINEs bind to XPO1 without requiring interaction with ASB8 for efficient XPO1 degradation. Instead, an allosteric mechanism facilitates high affinity XPO1-ASB8 interaction, leading to XPO1 ubiquitination and degradation. ASB8-mediated degradation is also observed upon treatment of the endogenous itaconate derivate 4-octyl itaconate, which suggests a native mechanism that is inadvertently exploited by synthesized XPO1 inhibitors. This allosteric XPO1 degradation mechanism of SINE compounds expands the known modes of targeted protein degradation beyond the well-characterized molecular glue degraders and proteolysis targeting chimeras of CRL4.