Synthesis of Fluorescent Jasplakinolide Analogues for Live-Cell STED Microscopy of Actin

Abstract

The nanometer thickness of filaments and the dynamic behavior of actin – a protein playing crucial role in cellular function and motility – makes it attractive for observation with superresolution optical microscopy. We developed the solution-phase synthesis of des-bromo-des-methyl-jasplakinolide-lysine, used as the “recognition unit” (ligand) for F-actin in living cells. The first amino acid – Fmoc-O-TIPS-beta-tyrosine – was prepared in 78% yield (2 steps in one pot). The new solution-phase synthesis involves 2-phenylisopropyl protection of the carboxyl group and does not require excesses of commercially unavailable amino acids. The overall yield of the target intermediate obtained in 9 steps is about 8%. 2-Phenylisopropyl group can be cleaved from carboxyl with 2 – 3% (v/v) of TFA in acetonitrile (0-10°C), without affecting TIPS protection of the phenolic hydroxyl in beta-tyrosine and N-Boc protection in lysine. Des-bromo-des-methyl-jasplakinolide-lysine was coupled with red-emitting fluorescent dyes 580CP and 610CP (via 6-aminohexanoate linker). Actin in living cells was labeled with 580CP and 610CP probes, and the optical resolution measured as full width at half maximum of line profiles across actin fibers was found to be 300-400 nm and 100 nm under confocal and STED conditions, respectively. The solution-phase synthesis of des-bromo-des-methyl-jasplakinolide-lysine opens a way to better fluorescent probes perspective for actin imaging.