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.