Dysregulation of protein–protein interactions
(PPIs) commonly
leads to disease. PPI stabilization has only recently been systematically
explored for drug discovery despite being a powerful approach to selectively
target intrinsically disordered proteins and hub proteins, like 14-3-3,
with multiple interaction partners. Disulfide tethering is a site-directed
fragment-based drug discovery (FBDD) methodology for identifying reversibly
covalent small molecules. We explored the scope of disulfide tethering
for the discovery of selective PPI stabilizers (molecular glues) using
the hub protein 14-3-3σ. We screened complexes of 14-3-3 with
5 biologically and structurally diverse phosphopeptides derived from
the 14-3-3 client proteins ERα, FOXO1, C-RAF, USP8, and SOS1.
Stabilizing fragments were found for 4/5 client complexes. Structural
elucidation of these complexes revealed the ability of some peptides
to conformationally adapt to make productive interactions with the
tethered fragments. We validated eight fragment stabilizers, six of
which showed selectivity for one phosphopeptide client, and structurally
characterized two nonselective hits and four fragments that selectively
stabilized C-RAF or FOXO1. The most efficacious fragment increased
14-3-3σ/C-RAF phosphopeptide affinity by 430-fold. Disulfide
tethering to the wildtype C38 in 14-3-3σ provided diverse structures
for future optimization of 14-3-3/client stabilizers and highlighted
a systematic method to discover molecular glues.