Figure 1. HaloTag7 and dye-containing substrates.

(a) Crystal structure of HaloTag7 covalently reacted with substrate chloroalkane-tetramethylrhodamine (CA-TMR, PDB: 6Y7A).¹⁶ The model highlights how the catalytic residue, D106, is at the bottom of a ~15Å hydrophobic channel. This model also highlights how interactions between the rhodamine dye and the surface helices of HaloTag7 drive binding, illustrating why non-rhodamine substrates often have slower kinetics. (b) Structures of fluorogenic dyes, Bz-1, Bz-2 and Bz-3, and structures of other common HaloTag7 ligands, CA-TMR and CA-JF₆₃₅.