Arylfluorosulfates Inactivate Intracellular Lipid Binding Protein(s) through Chemoselective SuFEx Reaction with a Binding Site Tyr Residue

Arylfluorosulfates
have appeared only rarely in the literature
and have not been explored as probes for covalent conjugation to proteins,
possibly because they were assumed to possess high reactivity, as
with other sulfur­(VI) halides. However, we find that arylfluorosulfates
become reactive only under certain circumstances, e.g., when fluoride
displacement by a nucleophile is facilitated. Herein, we explore the
reactivity of structurally simple arylfluorosulfates toward the proteome
of human cells. We demonstrate that the protein reactivity of arylfluorosulfates
is lower than that of the corresponding aryl sulfonyl fluorides, which
are better characterized with regard to proteome reactivity. We discovered
that simple hydrophobic arylfluorosulfates selectively react with
a few members of the intracellular lipid binding protein (iLBP) family.
A central function of iLBPs is to deliver small-molecule ligands to
nuclear hormone receptors. Arylfluorosulfate probe 1 reacts
with a conserved tyrosine residue in the ligand-binding site of a
subset of iLBPs. Arylfluorosulfate probes 3 and 4, featuring a biphenyl core, very selectively and efficiently
modify cellular retinoic acid binding protein 2 (CRABP2), both in
vitro and in living cells. The X-ray crystal structure of the CRABP2–4 conjugate, when considered together with binding site mutagenesis
experiments, provides insight into how CRABP2 might activate arylfluorosulfates
toward site-specific reaction. Treatment of breast cancer cells with
probe 4 attenuates nuclear hormone receptor activity
mediated by retinoic acid, an endogenous client lipid of CRABP2. Our
findings demonstrate that arylfluorosulfates can selectively target
single iLBPs, making them useful for understanding iLBP function.