FRET-based imaging of transbilayer movement of pepducin in living cells by novel intracellular bioreductively activatable fluorescent probes

To elucidate the mechanisms of direct transmembrane penetration of pepducins, which are artificial lipopeptide G protein-coupled receptor (GPCR) modulators, we developed two types of FRET-based probes, Pep13-FL-SS-Dab (13) targeting the inner leaflet of the lipid bilayer and Pep13-Dab-SS-FL (14) targeting the cytosol, respectively. They are composed of a pepducin moiety and a fluorescent switch component consisting of 5(6)-carboxyfluorescein (FAM) as a fluorophore and dabcyl as a quencher connected through disulfide bond linkage. When they are internalized into the cytosol, intracellular glutathione can cleave the disulfide bond to release the quencher, which results in a turn-on fluorescence signal. Using these probes, we performed live cell imaging of transbilayer movements of pepducins on MCF-7 cells for the first time. The results suggested that the lipid moiety of the probes facilitated pepducin flipping across and tethering to the membrane. The present study raises the possibility of applying the probe architecture for direct intracellular drug delivery.