HaloTag-based reporters for fluorescence imaging and biosensing.

Visualizing the structure and dynamics of biomolecules is critical to understand biological function, and requires methods to fluorescently label targets of interest in their cellular context. Self-labelling proteins, which combine a genetically encoded tag with a small-molecule fluorophore, have attracted considerable attention for this purpose, as they can overcome limitations of fluorescent proteins. Among them, the HaloTag protein is the most broadly used, showing fast specific labelling with a small, easy to functionalize and cell-permeant ligand. Synthetic chemistry and protein engineering have provided a portfolio of powerful imaging tools exploiting HaloTag, along with general methods to optimize and adapt them to specific applications. Here, we provide an overview of fluorescent reporters based on the HaloTag protein for imaging and biosensing, highlighting engineering strategies and general applications.