Isotope-coded and affinity-tagged cross-linking (ICATXL): an efficient strategy to probe protein interaction surfaces.

Chemical cross-linking followed by identification of the cross-linked residues by mass spectrometry provides structural information on protein interaction surfaces. Nevertheless, accurate analysis of the digested, cross-linked proteins is often challenging. Herein, we describe a novel strategy that relies on the use of affinity-tagged cross-linkers and isotope coding on the cross-linker-modified species. Incorporation of O16 or O18 during the hydrolysis of the cross-linkers results in a characteristic "doublet" for the undesired products of a half-cross-linking reaction. Therefore, genuine cross-linked peptides are readily distinguished for further structural analysis. This strategy permits a sensitive and facile analysis on a dimeric protease inhibitor, ecotin, showing general applicability to other protein assemblies.