Development of BromoTag: A “Bump-and-Hole”–PROTAC System to Induce Potent, Rapid, and Selective Degradation of Tagged Target Proteins

Small-molecule-induced
protein depletion technologies, also called
inducible degrons, allow degradation of genetically engineered target
proteins within cells and animals. Here, we design and develop the
BromoTag, a new inducible degron system comprising a Brd4 bromodomain
L387A variant as a degron tag that allows direct recruitment by heterobifunctional
bumped proteolysis targeting chimeras (PROTACs) to hijack the VHL
E3 ligase. We describe extensive optimization and structure–activity
relationships of our bump-and-hole–PROTACs using a CRISPR knock-in
cell line expressing model target BromoTag-Brd2 at endogenous levels.
Collectively, our cellular and mechanistic data qualifies bumped PROTAC
AGB1 as a potent, fast, and selective degrader of BromoTagged proteins,
with a favorable pharmacokinetic profile in mice. The BromoTag adds
to the arsenal of chemical genetic degradation tools allowing us to
manipulate protein levels to interrogate the biological function and
therapeutic potential in cells and in vivo.