Targeted Protein Degradation by Electrophilic PROTACs that Stereoselectively and Site-Specifically Engage DCAF1

Targeted
protein degradation induced by heterobifunctional
compounds
and molecular glues presents an exciting avenue for chemical probe
and drug discovery. To date, small-molecule ligands have been discovered
for only a limited number of E3 ligases, which is an important limiting
factor for realizing the full potential of targeted protein degradation.
We report herein the discovery by chemical proteomics of azetidine
acrylamides that stereoselectively and site-specifically react with
a cysteine (C1113) in the E3 ligase substrate receptor DCAF1. We demonstrate
that the azetidine acrylamide ligands for DCAF1 can be developed into
electrophilic proteolysis-targeting chimeras (PROTACs) that mediated
targeted protein degradation in human cells. We show that this process
is stereoselective and does not occur in cells expressing a C1113A
mutant of DCAF1. Mechanistic studies indicate that only low fractional
engagement of DCAF1 is required to support protein degradation by
electrophilic PROTACs. These findings, taken together, demonstrate
how the chemical proteomic analysis of stereochemically defined electrophilic
compound sets can uncover ligandable sites on E3 ligases that support
targeted protein degradation.