Comprehensive Characterization of Swine Cardiac Troponin T Proteoforms by Top-Down Mass Spectrometry

Cardiac troponin T (cTnT) regulates the Ca²⁺-mediated interaction between myosin thick filaments and actin thin filaments during cardiac contraction and relaxation. cTnT is released into the blood following injury, and increased serum levels of the protein are used clinically as a biomarker for myocardial infarction. Moreover, mutations in cTnT are causative in a number of familial cardiomyopathies. With the increasing use of large animal (swine) model to recapitulate human diseases, it is essential to characterize species-dependent protein sequence variants, alternative RNA splicing, and post-translational modifications (PTMs), but challenges remain due to the incomplete database and lack of validation of the predicted splicing isoforms. Herein, we integrated top-down mass spectrometry (MS) with online liquid chromatography (LC) and immunoaffinity purification to comprehensively characterize miniature swine cTnT proteoforms, including those arising from alternative RNA splicing and PTMs. A total of seven alternative splicing isoforms of cTnT were identified by LC/MS from swine left ventricular tissue, with each isoform containing un-phosphorylated and mono-phosphorylated proteoforms. The phosphorylation site was localized to Ser1 for the mono-phosphorylated proteoforms of cTnT1, 3, 4, and 6 by online MS/MS combining collisionally activated dissociation (CAD) and electron transfer dissociation (ETD). Offline MS/MS on Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer with CAD and electron capture dissociation (ECD) was then utilized to achieve deep sequencing of mono-phosphorylated cTnT1 (35.2 kDa) with a high sequence coverage of 87%. Taken together, this study demonstrated the unique advantage of top-down MS in the comprehensive characterization of protein alternative splicing isoforms together with PTMs. Graphical Abstract ᅟ.

Analyzing Glycopeptide Isomers by Combining Differential Mobility Spectrometry with Electron- and Collision-Based Tandem Mass Spectrometry

Differential mobility spectrometry (DMS) has been employed to separate isomeric species in several studies. Under the right conditions, factors such as separation voltage, temperature, the presence of chemical modifiers, and residence time can combine to provide unique signal channels for isomeric species. In this study, we examined a set of glycopeptide isomers, MUC5AC-3 and MUC5AC-13, which bear an N-acetyl-galactosamine (GalNAc) group on either threonine-3 or threonine-13. When analyzed as a mixture, the resulting MS and MS/MS spectra yield fragmentation patterns that cannot discern these convolved species. However, when DMS is implemented during the analysis of this mixture, two features emerge in the DMS ionogram representing the two glycopeptide isomers. In addition, by locking in DMS parameters at each feature, we could observe several low intensity CID fragments that contain the GalNAc functionality-specific amino acid residues - identifying the DMS separation of each isomer without standards. Besides conventional CID MS/MS, we also implemented electron-capture dissociation (ECD) after DMS separation, and clearly resolved both isomers with this fragmentation method, as well. The electron energy used in these ECD experiments could be tuned to obtain maximum sequence coverage for these glycopeptides; this was critical as these ions were present as doubly protonated species, which are much more difficult to fragment efficiently via electron-transfer dissociation (ETD). Overall, the combination of DMS with electron- or collision-based MS/MS methods provided enhanced separation and sequence coverage for these glycopeptide isomers. Graphical Abstract ᅟ.

Characterization of the iron-binding properties of pyoverdine using electron-capture dissociation-tandem mass spectrometry

Pyoverdines (PVD) are a group of siderophores produced by fluorescent Pseudomonads. Identification of PVD variants mostly relies on liquid chromatography-tandem mass spectrometry (LC-MS/MS) using collision-induced dissociation (CID). Here, both CID and the novel dissociation technique electron-capture dissociation (ECD) were applied to characterize PVD succinamide and its Fe(III)-chelated complex. The results clearly showed that ECD produced diagnostic side chain fragmentation of the PVD peptide chain and preserved the labile Fe(III) binding to the chromophore in contrast to CID. The ECD technique is therefore expected to support the understanding of strain-specific Fe(III) transport processes of PVDs.