Integrative Proteomics and Ubiquitomics Reveal on-Targets and off-Targets of PROTAC dBET1.

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional molecules that induce selective degradation of target proteins by hijacking the ubiquitin-proteasome system (UPS). Despite their transformative potential in eliminating disease-associated proteins, comprehensively identifying off-target degradation events remains technically challenging. Here, we employed an integrated proteomic and ubiquitinomic strategy to systematically profile the degradation landscape of the PROTAC molecule dBET1 in Jurkat T cells. By capturing the upstream ubiquitination events─which serve as earlier and more sensitive indicators than total protein abundance─our approach enabled the identification of previously overlooked off-target candidates. While dBET1 efficiently degraded its canonical BET family targets, our data also revealed the mitochondrial outer membrane protein VDAC1 as a putative off-target, evidenced by its depletion and increased multisite ubiquitination. Notably, our analysis framework enabled site-specific resolution of degradation events within BRD3, revealing preferential ubiquitination at functionally essential bromodomains, suggesting that degron-enriched regions may underlie domain-selective degradation. Additionally, dBET1 treatment was associated with mitochondrial depolarization and calcium homeostasis disruption, defects that we hypothesize may be functionally linked to the observed VDAC1 depletion. Together, this study demonstrates that integrating ubiquitomics provides a superior sensitivity layer for PROTAC safety assessment, capable of uncovering mechanism-based liabilities that escape conventional global proteomic screening.