A Selective, Protein-Based Fluorescent Sensor with Picomolar Affinity for Rare Earth Elements

Sensitive yet rapid methods for detection of rare earth elements (REEs), including lanthanides (Lns), would facilitate mining and recycling of these elements. Here we report a highly selective, genetically encoded fluorescent sensor for Lns, LaMP1, based on the recently characterized protein, lanmodulin. LaMP1 displays a 7-fold ratiometric response to all Ln ^III s, with apparent K _d s of 10-50 pM but only weak response to other common divalent and trivalent metal ions. We use LaMP1 to demonstrate for the first time that a Ln-utilizing bacterium, Methylobacterium extorquens, selectively transports early Lns (La ^III -Nd ^III ) into its cytosol, a surprising observation as the only Ln-proteins identified to date are periplasmic. Finally, we apply LaMP1 to suggest the existence of a Ln ^III uptake system utilizing a secreted metal chelator, akin to siderophore-mediated Fe ^III acquisition. LaMP1 not only sheds light on Ln biology but also may be a useful technology for detecting and quantifying REEs in environmental and industrial samples.