A biocompatible condensation reaction for the labeling of terminal cysteine residues on proteins.

This communication describes a novel protein labeling method that uses a single amino-acid tag — N-terminal cysteine residue — and small-molecule probes carrying the cyanobenzothiazole unit for specific labeling of proteins in vitro and at the surface of live cells. This simple ligation reaction proceeds with a high degree of specificity in physiological conditions, and should offer an important alternative to currently available protein labeling methods.