Implicit Biology in Peptide Spectral Libraries

Mass spectral libraries are collections of mass spectra curated specifically to facilitate the identification of small molecules, metabolites, and short peptides. One of the most comprehensive peptide spectral libraries is curated by NIST and contains upward of half a million annotated spectra dominated by human and model organisms including budding yeast and mouse. While motivated primarily by the technological goal of increasing sensitivity and specificity in spectral identification, we have found that the NIST spectral library constitutes a surprisingly rich source of biological knowledge. In this Article, we show that data-mining of these published libraries while applying strict empirical thresholds yields many characteristics of protein biology. In particular, we demonstrate that the size and increasingly comprehensive nature of these libraries, generated from whole-proteome digests, enables inference from the presence but crucially also from the absence of spectra for individual peptides. We illustrate implicit biological trends that lead to significant absence of spectra accounted for by complex post-translational modifications and overlooked proteolytic sites. We conclude that many subtle biological signatures such as genetic variants, regulated proteolysis, and post-translational modifications are exposed through the systematic mining of spectral collections originally compiled as general-purpose, technology-oriented resources

Delayed Fragmentation and Optimized Isolation Width Settings for Improvement of Protein Identification and Accuracy of Isobaric Mass Tag Quantification on Orbitrap-Type Mass Spectrometers

Fragmentation of multiple peptides in a single tandem mass scan impairs accuracy of isobaric mass tag based quantification. Consequently, practitioners aim at fragmenting peptide ions with the highest possible purity without compromising on sensitivity and coverage achieved in the experiment. Here we report the first systematic study optimizing delayed fragmentation options on Orbitrap instruments. We demonstrate that by delaying peptide fragmentation to occur closer to the apex of the chromatographic peak in liquid chromatography–tandem mass spectrometry (LC–MS/MS) experiments cofragmentation is reduced by 2-fold and peptides are fragmented with 2.8-fold better signal-to-noise ratios. This results in significantly improved accuracy of isobaric mass tag quantification. Further, we measured cofragmentation dependence on isolation width. In comparison to Orbitrap XL instruments the reduced space charging in the Orbitrap Velos enables isolation widths as narrow as 1 Th without impairing coverage, thus substantially reducing cofragmentation. When delayed peptide fragmentation and narrow isolation width settings were both applied, cofragmentation-induced ratio compression could be reduced by 32% on a log2 scale under otherwise identical conditions

Proteomics in whole brain homogenate.

Proteomics was performed in surgically resected brain tissue from Active and Control participants. A) PCA in brain tissue indicated no segregation of Active and Control participants in PCA1 (p = 0.73) and with some segregation in PCA2 (p = 0.016). Clinical diagnoses (FCD, TSC) are noted as well. B) Differential expression analysis in brain tissue indicated that there were 11 significantly altered proteins in Active versus Control participants at an FDR S1 Table and S1 Fig. There were no significant proteins at FDR C) WGCNA of brain proteomics and phospho-S6 evaluated in total brain homogenate by western blot, regardless of everolimus and clinical diagnoses. There were 1109 proteins that correlated with phospho-S6 levels (P235/236, p −6, R2 = 0.87) in the M-Brown module. D) The top significantly correlated protein with P240/244 was a negative correlation with ANK2 (p = 7.92 x 10−5, R2 = 0.74) in the M-Brown module. E) Module trait correlation identified 5 significantly associated modules with phospho-S6 (p < 0.05). Modules were clustered by eigenprotein adjacency (relatedness to other modules) on the left. Name of module is indicated by “M-color.” P values are indicated for those modules with p < 0.05 correlation. Positive correlation is indicated in red and negative correlation in blue. Top module GO annotations are noted on the right (FDR < 5%, at least 5 proteins/annotation). Several modules did not have a significant GO annotation (“n.s.”).</p

Metabolomics raw data in brain.

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LC-MS/MS proteomics in brain.

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Ribosomal S6 phosphorylation in whole brain homogenate.

A) Representative western blot images from surgically resected brain tissue of Active participants after taking 4.5 mg/m2 everolimus for 7 days and controls. Total S6, phospho-S6 (Ser235/236), phospho-S6 (Ser240/244), and beta-actin as loading control were evaluated in 14 participants. TSC participants are indicated “*”, while all others are FCD participants. B) Quantification of total S6 relative to actin indicates variability in baseline levels from sampled brain tissue, as well as one patient with increased expression of a smaller detected band (NYU-002). C) Percentage of phospho-S6 (Ser235/236) relative to total S6 indicates an average 1.19-fold decrease when comparing all Active participants to controls (p = 0.67). The highest level was seen in NYU-004. D) Percentage of phospho-S6 (Ser240/244) relative to total S6 indicates an average 1.15-fold decrease when comparing all Active participants to controls (p = 0.66). The highest level was seen in NYU-004. Error bars indicate SEM.</p

LC-MS/MS plasma sample DIA settings.

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Altered brain proteins at FDR<15% in Active versus Control participants.

A-K) Expression of the 11 altered brain proteins at FDRFig 3A. Error bars indicate SEM. L) The 11 altered proteins were associated with 2 signaling pathways having a p value of overlap 2. The same four proteins (FGA, FGB, FGG, PLG) were associated with activation of the coagulation system (p value of overlap = 1.45 x 10−9, z = 2.00) and acute phase response signaling (p value of overlap = 1.23 x 10−6, z = 2.00). (TIF)</p

LC-MS/MS brain IPA pathways.

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Whole western blot images for phospho-S6 (Ser235/236).

A) All cases (n = 14) were evaluated by western blot for phospho-S6 (Ser235/236) quantification on 2 blots, as depicted in Fig 1A. Bands were visualized after ECL on a BioRad ChemiDoc. Quantification was performed on bands in the red outlined box in Fiji ImageJ. One sample (NYUC001) was included on both gels to allow for normalization across blots for all samples. A’) Corresponding ladder for panel A is shown in the colorometric brightfield image. B) On the same blots, actin was evaluated after stripping the phospho-S6 (Ser235/236). B’) Corresponding ladder for panel B is shown in the colorometric brightfield image. C) On the same blots, total S6 was evaluated with no stripping (both actin and total S6 are present). C’) Corresponding ladder for panel C is shown in the colorometric brightfield image. (PDF)</p