Protein labeling with FlAsH and ReAsH.

The ability to image biochemical and phenotypical changes in living cells has become crucial for the investigation and understanding of the molecular mechanisms that govern all physiological cellular functions in health and disease. Genetically encoded reporters derived from fluorescent proteins (FPs) have proved to be extremely useful for localization and interaction studies in living cells. However, the large size and spectral properties of FP impose certain limitations for their use. The recently developed Fluorescein Arsenical Hairpin (FlAsH/tetracysteine) binder technology emerged as a promising alternative to FP for protein labeling and cellular localization studies. The combination of a small genetically encoded peptide tag with a small molecule detection reagent makes this technology particularly suitable for the investigation of biochemical changes in living cells that are difficult to approach with fluorescent proteins as molecular tags. We describe the practical application of this technology to image protein dynamics in living cells.