Met-lead(1.44.M1) is a genetically encoded fluorescent Pb[2+] indicator. It is a fusion protein consisting of a cyan fluorescent protein (ECFP ΔC11), the lead (Pb[2+]) binding domain (PbDB) (alpha helices 3-6) encoded by the PbrR gene from the bacterium Cupriavidus metallidurans CH34, and a yellow fluorescent protein (a circularly permuted form of Venus). The M1 variant has an additional flexible linker (ASSGSSG) inserted in the middle of helix 5. The PbBDs form homodimers and thereby generate one pair of ion-binding sites at the two distant edges of the structure unit. Binding of Pb[2+] to the PbDBs results in significant conformational changes and enhances the fluorescence resonance energy transfer (FRET) between the two fluorescent proteins. (PMID:32892119)