The HaloTag: A Novel Technology for Cell Imaging and Protein Analysis

The ability to specifically label proteins with a wide range of optical properties and functionalities can help reveal information about protein functions and dynamics in living cells. Here, we describe a technology for covalent tethering of organic probes directly to a specially designed reporting protein expressed in live cells. The reporting protein can be used in a manner similar to green fluorescent protein, except that the fluorophore might be interchanged among a variety of standard dyes. This allows living cells to be imaged at different wavelengths without requiring changes to the underlying genetic constructs, and the colors can be rapidly switched to allow temporal analysis of protein fate. The stability of the bond permits imaging of live cells during long time periods, imaging of fixed cells, and multiplexing with different cell/protein analysis techniques. The dyes can also be exchanged with other functional molecules, such as biotin to serve as an affinity handle, or even solid supports for direct covalent immobilization. The technology complements existing methods and provides new options for cell imaging and protein analysis.