In-vivo crosslinking of membrane protein complexes. The PIR strategy uses specially designed reagents that cross biological membranes to access membrane proteins in their native environment. Lability of the linker during ionization is exploited to reveal low-abundance productive crosslinked pairs of peptides. Reproduced from ‘Cross-linking Measurements of In Vivo Protein Complex Topologies. Chunxiang Zheng, Li Yang, Michael R. Hoopmann, Jimmy K. Eng, Xiaoting Tang, Chad R. Weisbrod, James E. Bruce. Mol. Cell. Proteomics. 10(10):M110.006841. 2011’. Reprinted by permission American Society for Biochemistry and Molecular Biology 2011.