The best of both worlds: Chemigenetic fluorescent sensors for biological imaging.

Synthetic-based fluorescent chemosensors and protein-based fluorescent biosensors are two well-established classes of tools for visualizing and monitoring biological processes in living tissues. Chemigenetic sensors, created using a combination of both synthetic parts and protein parts, are an emerging class of tools that aims to combine the strengths, and overcome the drawbacks, of traditional chemosensors and biosensors. This review will survey the landscape of strategies used for fluorescent chemigenetic sensor design. These strategies include: attachment of synthetic elements to proteins using in vitro protein conjugation; attachment of synthetic elements to proteins using autonomous protein labeling; and translational incorporation of unnatural amino acids.

Chemigenetic indicators based on synthetic chelators and green fluorescent protein.

Molecular fluorescent indicators are versatile tools for dynamic imaging of biological systems. We now report a class of indicators that are based on the chemigenetic combination of a synthetic ion-recognition motif and a protein-based fluorophore. Specifically, we have developed a calcium ion (Ca2+) indicator that is based on genetic insertion of circularly permuted green fluorescent protein into HaloTag protein self-labeled with a ligand containing the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. We have demonstrated the versatility of this design by also developing a sodium ion (Na+) indicator using a crown-ether-containing ligand. This approach affords bright and sensitive ion indicators that can be applicable to cell imaging. This design can enable the development of chemigenetic indicators with ion or molecular specificities that have not been realized with fully protein-based indicators.