Wang, N. et al. J. Am. Chem. Soc. 140, 4995–4999 (2018).

Biocompatible, bio-orthogonal chemical reactions such as the Staudinger ligation have proven to be useful methods for biochemical and imaging studies in living cells, as well as for drug discovery. Wang et al. now add sulfur–fluoride exchange (SuFEx) chemistry to this growing toolbox. An unnatural amino acid, fluorosulfate-l-tyrosine (FSY), can be incorporated into proteins in bacterial and mammalian cells via expression of a novel, specifically evolved tRNA synthetase–tRNA pair. Incorporated FSY selectively reacts with lysine, histidine and tyrosine residues in close proximity, generating a covalent cross-link either within or between proteins. Importantly, FSY has low cytotoxicity, which the authors attribute to the low reactivity of aryl fluorosulfates inside cells. They used FSY and SuFEx chemistry to capture and analyze the protein complex between Escherichia coli Afb and Z protein, as well as the complex between PAPS reductase and thioredoxin.