FMN-Based Fluorescent Proteins as Heavy Metal Sensors Against Mercury Ions

Bacterial light-oxygen-voltage-sensing photoreceptor-derived flavin mononucleotide (FMN)-based fluorescent proteins act as a promising distinct class of fluorescent proteins utilized for various biomedical and biotechnological applications. The key property of its independency towards oxygen for its chromophore maturation has greatly helped this protein to outperform the other fluorescent proteins such as GFP and DsRed for anaerobic applications. Here, we describe the feasibility of FMN-containing fluorescent protein FbFP as a metal-sensing probe by measuring the fluorescence emission changes of a protein with respect to the concentration of metal ions. In the present study, we demonstrated the mercury-sensing ability of FbFP protein and the possible amino acids responsible for metal binding. A ratiometric approach was employed here in order to exploit the fluorescence changes observed at two different emission maxima with respect to Hg²⁺ at micromolar concentration. The engineered variant FbFP_C56I showed high sensitivity towards Hg²⁺ and followed a good linear relationship from 0.1 to 3 μM of Hg²⁺. Thus, further engineering with a rational approach would enable the FbFP to be developed as a novel and highly selective and sensitive biosensor for other toxic heavy metal ions as well.