Rational Design of the beta-Bulge Gate in a Green Fluorescent Protein Accelerates the Kinetics of Sulfate Sensing

Detection of anions in complex aqueous media is a fundamental challenge with practical utility that can be addressed by supramolecular chemistry. Biomolecular hosts such as proteins can be used and adapted as an alternative to synthetic hosts. Here, we report how the mutagenesis of the beta-bulge residues (D137 and W138) in mNeonGreen, a bright, monomeric fluorescent protein, unlocks and tunes the anion preference at physiological pH for sulfate, resulting in the turn-off sensor SulfOFF-1. This unprecedented sensing arises from an enhancement in the kinetics of binding, largely driven by position 138. In line with these data, molecular dynamics (MD) simulations capture how the coordinated entry and gating of sulfate into the beta-barrel is eliminated upon mutagenesis to facilitate binding and fluorescence quenching.