Characterization of cereblon-dependent targeted protein degrader by visualizing the spatiotemporal ternary complex formation in cells.

Targeted protein degradation (TPD) through a proteasome-dependent pathway induced by heterofunctional small molecules is initiated by the formation of a ternary complex with recruited E3 ligases. This complex formation affects the degradation ability of TPD molecules, and thus we tested for visualization of the intracellular dynamics of ternary complex formation. In this study, we applied the fluorescent-based technology detecting protein-protein interaction (Fluoppi) system, in which detectable fluorescent foci are formed when ternary complex formation induced by TPD molecules occurs in cells. We show here that cells coexpressing BRD4 and cereblon (CRBN) tagged with the Fluoppi system formed detectable foci in both live and fixed cells only when treated with BRD4-targeting degraders utilizing CRBN as an E3 ligase in dose- and time-dependent manners. Notably, the maintenance and efficacy of TPD molecule-induced foci formation correlated with the ability to degrade target proteins. Furthermore, we demonstrated that BRD4-targeting and FKBP12-targeting degraders formed ternary complexes mainly in the nucleus and cytoplasm, respectively, suggesting that TPD molecules utilize the proteasome to degrade target proteins in their corresponding localized region. Our results also suggest that the Fluoppi system is a powerful tool for characterizing TPD molecules by visualizing the spatiotemporal formation of ternary complex.