On-Line LC−MS Approach Combining Collision-Induced Dissociation (CID), Electron-Transfer Dissociation (ETD), and CID of an Isolated Charge-Reduced Species for the Trace-Level Characterization of Proteins with Post-Translational Modifications

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Abstract
We have expanded our recent on-line LC−MS platform for large peptide analysis to combine collision-induced dissociation (CID), electron-transfer dissociation (ETD), and CID of an isolated charge-reduced (CRCID) species derived from ETD to determine sites of phosphorylation and glycosylation modifications, as well as the sequence of large peptide fragments (i.e., 2000−10 000 Da) from complex proteins, such as β-casein, epidermal growth factor receptor (EGFR), and tissue plasminogen activator (t-PA) at the low femtomol level. The incorporation of an additional CID activation step for a charge-reduced species, isolated from ETD fragment ions, improved ETD fragmentation when precursor ions with high m/z (approximately >1000) were automatically selected for fragmentation. Specifically, the identification of the exact phosphorylation sites was strengthened by the extensive coverage of the peptide sequence with a near-continuous product ion series. The identification of N-linked glycosylation sites in EGFR and an O-linked glycosylation site in t-PA were also improved through the enhanced identification of the peptide backbone sequence of the glycosylated precursors. The new strategy is a good starting survey scan to characterize enzymatic peptide mixtures over a broad range of masses using LC−MS with data-dependent acquisition, as the three activation steps can provide complementary information to each other. In general, large peptides can be extensively characterized by the ETD and CRCID steps, including sites of modification from the generated, near-continuous product ion series, supplemented by the CID−MS2 step. At the same time, small peptides (e.g., ≤2+ ions), which lack extensive ETD or CRCID fragmentation, can be characterized by the CID−MS2 step. A more targeted approach can then be followed in subsequent LC−MS runs to obtain additional information, if needed. Overall, the recently introduced ETD not only provides useful structural information, but also enhances the confidence of all assignments. The sensitivity of this new approach on the chromatographic time scale is similar to the previous Extended Range Proteomic Analysis (ERPA) using CID−MS2 and CID−MS3. The new LC−MS platform can be anticipated to be a useful approach for the comprehensive characterization of complex proteins. Keywords: ETD • CRCID • PTM • ERPA • LC−MS

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