Off-On Squalene Epoxidase-Specific Fluorescent Probe for Fast Imaging in Living Cells.

SQLE (squalene epoxidase) is a cell membrane-bound enzyme. It is a target of fungicides and may become a new target for cancer therapy. Therefore, monitoring the content and distribution of the key enzyme in living cells is very challenging. To achieve this goal, tetraphenyl ethylene-Ter (TPE-Ter) was first designed as a new fluorescent probe to SQLE based on its active cavity. Spectral experiments discovered that SQLE/TPE-Ter shows stronger emission with fast response time and low interference from other analytes. Molecular dynamics simulation clearly confirmed the complex structure of SQLE/TPE-Ter, and the key residues contribute to restriction of TPE-Ter single-molecular motion in the cavity. TPE-Ter-specific response to SQLE is successfully demonstrated in living cells such as LO2, HepG2, and fungi. Imaging of TPE-Ter-treated fungi indicates that it can be used to rapidly assess antifungal drug susceptibility (30 min at least). The present work provides a powerful tool to detect content and distribution of SQLE in living cells.