Single-Molecule Localization Microscopy and Tracking with a Fluorescent Mechanosensitive Probe.

A milestone in optical imaging of mechanical forces in cells has been the development of the family of flipper fluorescent probes able to report membrane tension noninvasively in living cells through their fluorescence lifetime. The specifically designed Flipper-CF probe with an engineered inherent blinking mechanism was recently introduced for super-resolution fluorescence microscopy of lipid ordered membranes but was too dim to be detected in lipid disordered membranes at the single-molecule level (García-Calvo, J. 2020, 142(28), 12034-12038). We show here that the original and commercially available probe Flipper-TR is compatible with single-molecule based super-resolution imaging and resolves both liquid ordered and liquid disordered membranes of giant unilamellar vesicles below the diffraction limit. Single probe molecules were additionally tracked in lipid bilayers, enabling to distinguish membranes of varying composition from the diffusion coefficient of the probe. Differences in brightness between Flipper-CF and Flipper-TR originate in their steady-state absorption and fluorescence properties. The general compatibility of the Flipper-TR scaffold with single-molecule detection is further shown in super-resolution experiments with targetable Flipper-TR derivatives.