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Low pH Solid Phase Amino-Labeling of Complex Peptide Digests with TMTs Improves Peptide Identification Rates for Multiplexed Global Phosphopeptide Analysis

ABSTRACT: We present a novel Tandem Mass Tag Solid Phase Amino Labeling (TMT-SPAL) protocol using reversible immobilization of peptides onto octadecyl-derivatised (C18) solid supports. This method can reduce the number of steps required in complex protocols saving time and potentially reducing sample losses. In our global phosphopeptide profiling workflow (SysQuant) we can cut 24 hours from the protocol while increasing peptide identifications (20%) and reducing side-reactions. Solid phase labeling with TMTs does require some modification to typical labeling conditions particularly pH. It has been found that complete labeling equivalent to 2 standard basic pH solution phase labeling for small and large samples can be achieved on C18 resins using slightly acidic buffer conditions. Improved labeling behaviour on C18 compared to standard basic pH solution phase labeling is demonstrated. We analysed our samples for histidine, serine, threonine and tyrosine-labeling to determine the degree of over-labeling and observed higher than expected levels (25% of all Peptide Spectral Matches (PSMs)) of overlabeling at all of these amino acids (predominantly at tyrosine and serine) in our standard solution phase labeling protocol. Over-labeling at all these sites is greatly reduced (four-fold to 7% of all PSMs) by the low pH conditions used in the TMT-SPAL protocol. Over-labeling seems to represent a so-far overlooked mechanism causing reductions in peptide identification rates with NHS-activated TMT-labeling compared to label-free methods. Our results also highlight the importance of searching data for over-labeling when labeling methods are used.

INSTRUMENT(S): LTQ Orbitrap

ORGANISM(S): Homo Sapiens (human)

TISSUE(S): Primary Cell Line Cell, Cell Culture

DISEASE(S): Breast Cancer

SUBMITTER: Vikram Mitra

LAB HEAD: Andrew H. Thompson

PROVIDER: PXD002072 | Pride | 2015-04-21

REPOSITORIES: Pride

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Project description:Cells were treated with DMSO (biological triplicate) or degrader at indicated dose and time (Meta Treatment Table) and cells were harvested by centrifugation. Lysis buffer (8 M Urea, 50 mM NaCl, 50 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (EPPS) pH 8.5, Protease and Phosphatase inhibitors from Roche) was added to the cell pellets and homogenized by 20 passes through a 21 gauge (1.25 in. long) needle to achieve a cell lysate with a protein concentration between 1 - 4 mg/mL. A micro-BCA assay (Pierce) was used to determine the final protein concentration in the cell lysate. 200 µg of protein for each sample were reduced and alkylated as previously described1. Proteins were precipitated using methanol/chloroform. In brief, four volumes of methanol were added to the cell lysate, followed by one volume of chloroform, and finally three volumes of water. The mixture was vortexed and centrifuged to separate the chloroform phase from the aqueous phase. The precipitated protein was washed with three volumes of methanol, centrifuged and the resulting washed precipitated protein was allowed to air dry. Precipitated protein was resuspended in 4 M Urea, 50 mM HEPES pH 7.4, followed by dilution to 1 M urea with the addition of 200 mM EPPS, pH 8. Proteins were first digested with LysC (1:50; enzyme:protein) for 12 hours at room temperature. The LysC digestion was diluted to 0.5 M Urea with 200 mM EPPS pH 8 followed by digestion with trypsin (1:50; enzyme:protein) for 6 hours at 37 °C. Tandem mass tag (TMT) reagents (Thermo Fisher Scientific) were dissolved in anhydrous acetonitrile (ACN) according to manufacturer’s instructions. Anhydrous ACN was added to each peptide sample to a final concentration of 30% v/v, and labeling was induced with the addition of TMT reagent to each sample at a ratio of 1:4 peptide:TMT label. The 11 or 16-plex labeling reactions were performed for 1.5 hours at room temperature and the reaction quenched by the addition of hydroxylamine to a final concentration of 0.3% for 15 minutes at room temperature. Each of the sample channels were combined in a 1:1 ratio, desalted using C18 solid phase extraction cartridges (Waters) and analyzed by LC-MS for channel ratio comparison. Samples were then combined using the adjusted volumes determined in the channel ratio analysis and dried down in a speed vacuum. The combined sample was then resuspended in 1% formic acid and acidified (pH 2 - 3) before being subjected to desalting with C18 SPE (Sep-Pak, Waters). Samples were then offline fractionated into 96 fractions by high pH reverse-phase HPLC (Agilent LC1260) through an aeris peptide xb-c18 column (phenomenex) with mobile phase A containing 5% acetonitrile and 10 mM NH4HCO3 in LC-MS grade H2O, and mobile phase B containing 90% acetonitrile and 10 mM NH4HCO3 in LC-MS grade H2O (both pH 8.0). The 96 resulting fractions were then pooled in a non-continuous manner into 24 fractions and these fractions were used for subsequent mass spectrometry analysis. Data were collected using an Orbitrap Fusion Lumos mass spectrometer (Thermo Fisher Scientific, San Jose, CA, USA) coupled with a Proxeon EASY-nLC 1200 LC pump (Thermo Fisher Scientific) or an Orbitrap Eclipse Tribrid mass spectrometer (Thermo Fisher Scientific, San Jose, CA, USA) coupled with an UltiMate 3000 RSLCnano System. Peptides were separated on an EasySpray ES803a/ES803a.rev2 75 μm inner diameter microcapillary column (Thermo Fisher Scientific) or a 100 µm inner diameter microcapillary column packed with ~ 50 cm of Accucore C18 resin (2.6 µM, 100 Å, Thermo Fisher Scientific). Peptides were separated using a 190 min gradient of 6 - 27% acetonitrile in 1.0% formic acid with a flow rate of 300 or 350 nL/min. Each analysis used an MS3-based TMT method as described previously. The data were acquired using a mass range of m/z 340 – 1350, resolution 120,000, AGC target 5 x 105, maximum injection time 100 ms, dynamic exclusion of 120 seconds for the peptide measurements in the Orbitrap. Data dependent MS2 spectra were acquired in the ion trap with a normalized collision energy (NCE) set at 35%, AGC target set to 1.8 x 104 and a maximum injection time of 120 ms. MS3 scans were acquired in the Orbitrap with a HCD collision energy set to 55%, AGC target set to 2 x 105, maximum injection time of 150 ms, resolution at 50,000 and with a maximum synchronous precursor selection (SPS) precursors set to 10.

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Low pH Solid Phase Amino-Labeling of Complex Peptide Digests with TMTs Improves Peptide Identification Rates for Multiplexed Global Phosphopeptide Analysis

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