Materials. Tandem Mass Tag (TMT) Isobaric Reagents Were from Thermo Fisher Scientific (Waltham

Materials. Tandem mass tag (TMT) isobaric reagents were from Thermo Fisher Scientific (Waltham, MA). Water and organic solvents were from J.T. Baker (Center Valley, PA).

Media and growth. Saccharomyces cerevisae strains from the haploid MATalpha collection (BY4742 MATα his3Δ1 leu2Δ0 lys2Δ0 ura3Δ0). Cultures were grown in standard yeast-peptone-dextrose (YPD) media to an optical density (OD) of 0.6/mL and then harvested.

Cell lysis and protein digestion. Yeast cultures were harvested by centrifugation, washed twice with ice cold deionized water, and resuspended at 4°C in a buffer containing 50 mM EPPS pH 8.5, 8 M urea, and protease (complete mini, EDTA-free) inhibitors (Roche, Basel, Switzerland). Cells were lysed using the MiniBeadbeater (Biospec, Bartlesville, OK) in microcentrifuge tubes at maximum speed for three cycles of 60 s each, with 1 min pauses on ice between cycles to avoid overheating of the lysates. After centrifugation, cleared lysates were transferred to new tubes. We determined the protein concentration in the lysate using the bicinchoninic acid (BCA) protein assay (Thermo Fisher Scientific, Waltham, MA).

Proteins were subjected to disulfide reduction with 5 mM tris (2-carboxyethyl)phosphine (TCEP), (room temperature, 30 min) and alkylation with 10 mM iodoacetamide (room temperature, 30 min in the dark). Excess iodoacetamide was quenched with 10 mM dithiotreitol (room temperature, 15 min in the dark). Methanol-chloroform precipitation was performed prior to protease digestion. In brief, four parts neat methanol was added to each sample and vortexed, one-part chloroform was added to the sample and vortexed, and three parts water was added to the sample and vortexed. The sample was centrifuged at 20,000 RPM for 2 min at room temperature and after removing the aqueous and organic phases subsequently washed twice with 100% methanol, prior to air-drying.

Samples were resuspended in 8 M urea, 50 mM EPPS, pH 8.5. The protein extract was then diluted to 1 M urea with 50 mM EPPS pH 8.5 and digested at room temperature for 16 h with LysC protease at a 100:1 protein-to-protease ratio. Trypsin was then added at a 100:1 protein-to-protease ratio and the reaction was incubated 6 h at 37°C.

Tandem mass tag labeling. TMT reagents (0.8 mg) were dissolved in anhydrous acetonitrile (40 μL) of which 10 μL was added to the peptides (100 g) along with 30 μL of acetonitrile to achieve a final acetonitrile concentration of approximately 30% (v/v). Specifically, peptides from the ∆met6 strain replicates were conjugated to tags 126, 127N, 127C, the ∆pfk2 strain replicates with tags 128N, 128C, 129N, and the ∆ura2 strain replicates with tags 129C, 130N, 130C. Following incubation at room temperature for 1 h, the reaction was quenched with hydroxylamine to a final concentration of 0.3% (v/v). The TMT-labeled samples were pooled at a 1:1:1:1:1:1:1:1:1 ratio. The sample was vacuum centrifuged to near dryness and subjected to C18 solid-phase extraction (SPE) (Sep-Pak, Waters).The sample was reconstituted in 5% acetonitrile and 5% formic acid for LC-MS/MS processing. For each analysis, 0.1-1 µg of the TKO standard was loaded onto the C18 capillary column using a Proxeon NanoLC-1000 UHPLC.

Liquid chromatography and tandem mass spectrometry. Peptides were separated on a 100 μm inner diameter microcapillary column packed with ∼0.5 cm of Magic C4 resin (5 μm, 100 Å, MichromBioresources) followed by ∼35 cm of GP-18 resin (1.8 μm, 200 Å, Sepax, Newark, DE). For each analysis, we loaded 0.1-1 µg of the TKO standard onto the C18 capillary column using a Proxeon NanoLC-1000 UHPLC.Peptides were separated in-line with the mass spectrometer using a 45min gradient of 6 to 26% acetonitrile in 0.125% formic acid at a flow rate of ∼450 nL/min.

For SPS-MS3 analysis with the Orbitrap Fusion Lumos mass spectrometer, the scan sequence began with an MS1 spectrum (Orbitrap analysis; resolution 120,000; mass range 400−1400 m/z; automatic gain control (AGC) target 5 × 105; maximum injection time 250 ms). Precursors for MS2/MS3 analysis were selected using a Top10 method. MS2 analysis consisted of collision-induced dissociation (CID); AGC 1.8 × 104; normalized collision energy (NCE) 35; maximum injection time 120 ms; and isolation window of 0.4 Da. Following acquisition of each MS2 spectrum, we collected an MS3 spectrum using our recently described method in which multiple MS2 fragment ions were captured in the MS3 precursor population using isolation waveforms with multiple frequency notches [1].MS3 precursors were fragmented by high energy collision-induced dissociation (HCD) and analyzed using the Orbitrap (NCE 55; AGC 1.2 × 105; maximum injection time 120 ms, resolution was 60,000 at 400 Th).

For SPS-MS3 analysis with the Orbitrap Fusion Classic mass spectrometer, the scan sequence began with an MS1 spectrum (Orbitrap analysis; resolution 120,000; mass range 400−1400 m/z; automatic gain control (AGC) target 5 × 105; maximum injection time 250 ms). Precursors for MS2/MS3 analysis were selected using a Top10 method. MS2 analysis consisted of collision-induced dissociation (CID); AGC 1.8 × 104; normalized collision energy (NCE) 35; maximum injection time 120 ms; and isolation window of 0.7 Da. Following acquisition of each MS2 spectrum, we collected an MS3 spectrum using our recently described method in which multiple MS2 fragment ions were captured in the MS3 precursor population using isolation waveforms with multiple frequency notches [23]. MS3 precursors were fragmented by high energy collision-induced dissociation (HCD) and analyzed using the Orbitrap (NCE 55; AGC 1.0 × 105; maximum injection time 120 ms, resolution was 60,000 at 400 Th).

For hrMS2 analysis,with the Orbitrap Fusion Lumos mass spectrometer, the scan sequence began with an MS1 spectrum (Orbitrap analysis; resolution 120,000; mass range 400−1400 m/z; automatic gain control (AGC) target 5 × 105; maximum injection time 250 ms). Precursors for MS2/MS3 analysis were selected using a TopSpeed method of 3 sec. MS2 analysis consisted of collision-induced dissociation (HCD) with Orbitrap-based mass analysis (AGC 1.0 × 105; normalized collision energy (NCE) 35; maximum injection time 150 ms; resolution was 60,000 at 400 Th;and isolation window of 0.7 Da).

Data analysis. Mass spectra were processed using a SEQUEST-based in-house software pipeline [2]. Spectra were converted to mzXML using a modified version of ReAdW.exe. Database searching included all entries from the yeast SGD (Saccharomyces Genome Database) (March 20, 2015). This database was concatenated with one composed of all protein sequences in the reversed order. Searches were performed using a 50 ppm precursor ion tolerance for total protein level analysis. The product ion tolerance was set to 0.9 Da. These wide mass tolerance windows were chosen to maximize sensitivity in conjunction with Sequest searches and linear discriminant analysis [2, 3]. TMT tags on lysine residues and peptide N termini (+229.163 Da) and carbamidomethylation of cysteine residues (+57.021 Da) were set as static modifications, while oxidation of methionine residues (+15.995 Da) was set as a variable modification.

Peptide-spectrum matches (PSMs) were adjusted to a 1% false discovery rate (FDR) [4, 5]. PSM filtering was performed using a linear discriminant analysis, as described previously [2], while considering the following parameters: XCorr, ΔCn, missed cleavages, peptide length, charge state, and precursor mass accuracy. For TMT-based reporter ion quantitation, we extracted the signal-to-noise (S:N) ratio for each TMT channel and found the closest matching centroid to the expected mass of the TMT reporter ion. PSMs were identified, quantified, and collapsed to a 1% peptide false discovery rate (FDR) and then collapsed furtherto a final protein-level FDR of 1%. Moreover, protein assembly was guided by principles of parsimony to produce the smallest set of proteins necessary to account for all observed peptides.

Peptide intensities were quantified by summing reporter ion counts across all matching PSMs using in-house software so as to give greater weight to more intense ions, as described previously [1, 6]. PSMs with poor quality, MS3 spectra with TMT reporter summed signal-to-noise ratio that is less than 100, or no MS3 spectra were excluded from quantitation, and isolation specificity of ≥ 0.7 was required [6]. For individual proteins, we normalized across each channel so that the sum across all 10 channels for a single protein is 100. This normalization allows us to compare, for example the expression level of protein A from the 126 channel in replicate 1 with the protein A level from the 126 channel in replicate 2, as both will be a fraction of the total signal (for all reporter ions) for that particular protein. Protein quantitation values were exported for further analysisin Microsoft Excel,GraphPad Prism, JMP, and BoxPlotR[7].

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[7] Krzywinski, M., Altman, N., Visualizing samples with box plots. Nat Methods 2014, 11, 119-120.