Isotope signatures allow identification of chemically cross-linked peptides by mass spectrometry: a novel method to determine interresidue distances in protein structures through cross-linking.

Knowledge of protein structures and protein-protein interactions is essential for understanding of biological processes. Recent advances in protein cross-linking and mass spectrometry (MS) have shown significant potential to contribute to this area. Here we report a novel method to rapidly and accurately identify cross-linked peptides based on their unique isotope signature when digested in the presence of H(2)(18)O. This method overcomes the need for specially synthesized cross-linkers and/or multiple MS runs required by other techniques. We validated our method by performing a "blind" analysis of 5 proteins/complexes of known structure. Side chain repacking calculations using Rosetta show that 17 of our 20 positively identified cross-links fit the published atomic structures. The remaining 3 cross-links are likely due to protein aggregation. The accuracy and rapid throughput of our workflow will advance the use of protein cross-linking in structural biology.