Efflux pump insensitive rhodamine-jasplakinolide conjugates for G- and F-actin imaging in living cells.

The actin cytoskeleton is crucial for endocytosis, intracellular trafficking, cell shape maintenance and a wide range of other cellular functions. Recently introduced cell-permeable fluorescent actin probes, such as SiR-actin, suffer from poor membrane permeability and stain some cell populations inhomogeneously due to the active efflux by the plasma membrane pumps. We analyzed a series of new probes composed of jasplakinolide and modified rhodamine fluorophores and found that rhodamine positional isomerism has a profound effect on probe performance. The probes based on the 6'-carboxy-carbopyronine scaffold are considerably less susceptible to efflux and allow efficient staining without efflux pump inhibitors. They can be used for 2D and 3D fluorescence nanoscopy at high nanomolar concentrations without significant cytotoxicity. We show that jasplakinolide-based fluorescent probes bind not only to actin filaments, but also to G-actin, which enables imaging highly dynamic actin structures. We demonstrate an excellent performance of the new probes in multiple organisms and cell types: human cell lines, frog erythrocytes, fruit fly tissues and primary neurons.