Faster scanning capabilities of high resolution mass spectrometers have expanded their functionality beyond data-dependent acquisition (DDA) to targeted proteomics with higher precision. Transitioning from discovery workflows to targeted peptide quantitation assays on a single high resolution LC-MS system provides an opportunity to rapidly developing targeted assays with high multiplexing by taking advantage of retention time scheduling. We therefore investigated the feasibility of implementing highly multiplexed peptide quantitation assays using scheduled, high resolution multiple reaction monitoring (sMRM-HR) derived from discovery data sets on a single orthogonal quadrupole time-of flight (QqTOF) TripleTOF 5600 LC-MS system. We assessed the selectivity and reproducibility of MRM-HR, also referred to as parallel reaction monitoring (PRM), by measuring standard peptide concentration curves and system suitability assays. Evaluating up to 500 peptides per LC-MS run, the robustness and accuracy of MRM-HR assays were compared to traditional SRM workflows on triple quadrupole instruments. The high resolution and mass accuracy of full scan MS/MS spectra resulted in sufficient selectivity to monitor 6-10 MS/MS fragment ions per precursor ion and provided flexibility for post-acquisition assay refinement and optimization. We demonstrate the applicability of this workflow to complex biological samples in a yeast lysate repeatability study monitoring 532 precursor ions, and by quantitatively profiling 466 peptide precursor ions in whole cell lysates from wild-type and mutant E. coli strains with sMRM-HR to validate a previously generated candidate list of differentially expressed proteins. These results establish a robust sMRM-HR workflow to rapidly transition from discovery analysis to highly multiplexed, targeted peptide quantitation.

MRM-HR and scheduled sMRM-HR experiments acquired on a TripleTOF 5600 (SCIEX). i) MRM-HR Response curve of 6 Protein Mix in complex matrix (C. elegans lysate), acquisition of 3 reproducibility test injections, and 2 response curve replicates; ii) MRM-HR System Suitability Study using 6 Protein Mix (10 replicate acquisitions, 3x blanks); iii) scheduled sMRM-HR Reproducibility study of proteins from yeast whole cell lysate (3 replicates); iv) scheduled sMRM-HR - Differential expression of proteins comparing E. coli wild type and ackA mutant strains (3 replicates each for WT and ackA mutant).

i) Response curve for spiked digested six protein mix in complex matrix (C. elegans whole cell lysate). A mixture of six pre-digested proteins (‘six protein mix’) was spiked into digested C. elegans whole cell lysate (1 ug on column) at 8 concentrations points spanning from 15 attomoles to 62.5 femtomoles (blank, 0.015, 0.061, 0.244, 0.975, 3.9, 15.6, and 62.5 fmol). Two replicate concentration curves, each with injections from lowest to highest spike concentration were acquired on the TripleTOF 5600 (MRM-HR mode). ii) Predigested Six Protein Mix was purchased from Michrom for the System Suitability Study following an Acquisition Protocol as described by Abbatiello et al. (Mol. Cell. Proteomics, 2014). iii) Digested whole cell lysate from yeast - reproducibility assessment and highly multiplexed scheduled sMRM-HR experiments. BY4743 yeast strain samples were grown at 30˚C in synthetic complete media, 0.67% yeast nitrogen base with ammonium sulfate (Sigma), plus required amino acids, supplemented with 2% glucose until the OD600 reached between 0.5 and 1.0. Subsequently, 1x1e08 cells were harvested, washed twice with water, pelleted, and frozen. Cell pellets were defrosted and re-suspended in 100 µl yeast lysis buffer (25 mM HEPES pH 7.5, 5 mM MgCl2, 50 mM KCl, 10% glycerol, Complete Mini Protease Inhibitors (Roche) and 1 volume acid-washed beads. Cells were lysed with three 1 min cycles of beating and icing using a Biospec Mini Beadbeater-8. Cell lysate were separated from beads, transferred to a new tube, and centrifuged at 15,800 g for 5 min at 4 ˚C. Finally, the supernatant was drawn off and used for downstream proteomic analysis. Samples were suspended and denatured in a final solution of 6 M urea, 100 mM Tris and tryptic digestion was performed. iv) Whole cell lysates from E. coli mutant (ackA) and WT strains – scheduled MRM-HR, differential protein expression. Briefly, E. coli WT and isogenic mutant strains cells were grown at 37˚C in TB7 [1% (w/v) tryptone buffered at pH 7.0 with potassium phosphate (100 mM)] supplemented with 0.4% glucose. Cell pellets were suspended in 6 mL of PBS and centrifuged at 4 °C, 15,000 g for 20 min. The cell pellet was collected, re-suspended, and denatured in a final solution of 6 M urea, 100 mM Tris, 75 mM NaCl. Samples were sonicated on ice (5x each), cellular debris removed, and the supernatant of each sample further processed for tryptic digestion.