Intramolecular energy transfer to S-nitrosylated Cys residues is at the basis of fluorescence sensitivity to nitric oxide in the blue fluorescent protein mTagBFP2.

Fluorescent protein (FP) variants have recently emerged as promising intracellular nitric oxide (NO) sensors based on NO-induced fluorescence loss due to cysteine S-nitrosylation - the covalent addition of a nitroso group to a cysteine thiol within a protein to form an S-nitrosothiol. Here, we investigate the mechanisms underlying this fluorescence loss using a combined experimental and computational approach. We focus on mTagBFP2, a blue fluorescent protein that undergoes a 70% reduction in fluorescence quantum yield and lifetime upon exposure to micromolar NO concentrations. We discriminate, through mutagenesis, the contributions of two key cysteine residues and propose an unprecedented excitation energy transfer (EET) from the mTagBFP2 chromophore to the S-nitroso groups as a potential quenching mechanism. Our EET efficiency calculations incorporate full couplings, the effects of the surrounding protein and solvent, and molecular dynamics-based configurational flexibility. The computed EET efficiencies broadly align with experimental observations, with remaining discrepancies for which we advance potential explanations. Our findings establish a mechanistic basis for NO-induced fluorescence loss in mTagBFP2, providing guidelines for the rational design of next-generation NO-sensitive FPs. Moreover, they suggest that analogous S-nitrosylation-driven quenching mechanisms could be operative in other FPs with exposed cysteine residues, underscoring the risk of artefacts in cellular imaging under physiological NO levels.