Selective enrichment and identification of cross-linked peptides to study 3-D structures of protein complexes by mass spectrometry.

Chemical cross-linking of protein complexes combined with mass spectrometry is a powerful approach to obtain 3-D structural information by revealing amino residues that are in close spatial proximity. To increase the efficiency of mass spectrometric analysis, we have demonstrated the selective enrichment of cross-linked peptides from the 350 kDa protein complex RNA polymerase (RNAP) from Bacillus subtilis. Bis(succinimidyl)-3-azidomethyl glutarate was used as a cross-linker along with an azide-reactive cyclooctyne-conjugated resin to capture target peptides. Subsequently released peptides were fractionated by strong cation exchange chromatography and subjected to LC-MS/MS. We mapped 10 different intersubunit and 24 intrasubunit cross-links by xComb database searching supplied with stringent criteria for confirmation of the proposed structure of candidate cross-linked peptides. The cross-links fit into a homology model of RNAP. Cross-links between β lobe 1 and the β' downstream jaw, and cross-links involving the N-terminal and C-terminal parts of the α subunits suggest conformational flexibility. The analytical strategy presented here can be applied to map protein-protein interactions at the amino acid level in biological assemblies of similar complexity. Our approach enables the exploration of alternative peptide fragmentation techniques that may further facilitate cross-link analysis.