Structural signatures of the class III lasso peptide BI-32169 and the branched-cyclic topoisomers using trapped ion mobility spectrometry-mass spectrometry and tandem mass spectrometry

Lasso peptides are a class of bioactive ribosomally synthesized and post-translationally modified peptides (RiPPs) characterized by a mechanically interlocked topology, where the C-terminal tail of the peptide is threaded and trapped within an N-terminal macrolactam ring. BI-32169 is a class III lasso peptide containing one disulfide bond that further stabilizes the lasso structure. In contrast to its branched-cyclic analog, BI-32169 has higher stability and is known to exert a potent inhibitory activity against the human glucagon receptor. In the present work, tandem mass spectrometry, using collision-induced dissociation (CID) and electron capture dissociation (ECD), and trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) experiments were carried out to evidence specific structural signatures of the two topologies. CID experiments showed similar fragmentation patterns for the two topoisomers, where a part of the C-terminal tail remains covalently linked to the macrolactam ring by the disulfide bond, which cannot clearly constitute a signature of the lasso topology. ECD experiments of BI-32169 showed an increase of hydrogen migration events in the loop region when compared with those of its branched-cyclic topoisomer evidencing specific structural signatures for the lasso topology. The high mobility resolving power of TIMS resulted in the identification of multiple conformations for the protonated species but did not allow the clear differentiation of the two topologies in mixture. When in complex with cesium metal ions, a reduced number of conformations led to a clear identification of the two structures. Experiments reducing and alkylating the disulfide bond of BI-32169 showed that the lasso structure is preserved and heat stable and the associated conformational changes provide new insights about the role of the disulfide bond in the inhibitory activity against the human glucagon receptor.