Cellular Validation of Chemically Improved Inhibitor Identifies Monoubiquitination on OTUB2

Ubiquitin thioesterase OTUB2, a cysteine protease from the ovarian tumor (OTU) deubiquitinase superfamily, is associated with enhanced expression during tumor progression and metastasis. Development of OTUB2 inhibitors is therefore believed to be therapeutically important, yet potent and selective small-molecule inhibitors targeting OTUB2 are scarce. Here, we describe the development of an improved OTUB2 inhibitor, LN5P45, comprising a chloroacethydrazide moiety that covalently reacts to the active-site cysteine residue. When added to cells, LN5P45 shows outstanding target engagement and proteome-wide selectivity. Importantly, we found that LN5P45 and the other OTUB2 inhibitors strongly induce monoubiquitination of OTUB2 via lysine 31. Thus, our findings provide novel insights for the design of future OTUB2-related therapeutics and open new questions regarding the understanding of OTUB2 regulation at the post-translational modification level.