An aptly positioned azido group in the spacer of a protein cross-linker for facile mapping of lysines in close proximity

Cross-links between amino acid residues in close proximity can provide distance constraints for the validation of models of the 3D structure proteins. The mapping of cross-links by the identification of linked peptides in proteolytic digests is facilitated by cleavable cross-linkers that enable isolation of the cleavage products while preserving information about the linkage. We present an amine-specific cross-linker, bis(succinimidyl)-3-azidomethyl glutarate (BAMG), that fulfils these requirements. Two parallel reaction pathways are induced by tris(carboxyethyl)phosphine (TCEP) in cross-linked peptides from BAMG-treated cytochrome c. One pathway leads to cleavage of the cross-linked species, while in the other the azido group of BAMG is reduced to an amino group without cleavage. Cross-linked peptides and peptides modified by partially hydrolysed BAMG yield distinct sets of TCEP-induced reaction products. These can be isolated by reversed-phase diagonal chromatography and identified by mass spectrometry to reveal the identity of the parent compounds. The ease with which cross-link-derived reaction products can be isolated and identified indicates that the mapping of cross-links in complex biological assemblies and mixtures of protein complexes might become feasible in the near future.