Super Photostable Organic Dye for Single Molecule Imaging and Single Particle Tracking in Live Cells

Photobleaching of organic fluorophores limits live-cell single-molecule imaging by truncating trajectories and biasing quantitative analyses of membrane protein dynamics. We recently found that photobluing of Alexa Fluor 647 yields a super-photostable red-emitting species, which we named Phoenix Fluor 555 (PF555). This chapter describes two practical workflows for prolonged single-molecule imaging and single-particle tracking of plasma-membrane proteins using a TIRF microscope. In Protocol A, cell-surface SNAP-tag targets labeled with benzylguanine-Alexa Fluor 647 (BG-AF647) are in situ photoblued by 642 nm illumination to a super photostable species that is detected in the red channel (572-624 nm), enabling extended single-molecule acquisition without specialized imaging buffers. In Protocol B, HaloTag targets are directly labeled with Phoenix Fluor 555 chloroalkane ligand (CA(O₄)-PF555) to provide a straightforward "label-and-image" route for robust red-channel single-molecule movies. Using EGFR-based membrane constructs expressed transiently in COS-7 cells as an example, we provide step-by-step procedures for cell preparation, live-cell labeling, optional photobluing, single-molecule acquisition under physiological condition, and instructions for prolonging single particle tracking.