Protein complexes are key macromolecular machines of the cell, but their description remains incomplete. We and others previously reported an experimental strategy for global characterization of native protein assemblies based on chromatographic fractionation of biological extracts coupled to precision mass spectrometry analysis (chromatographic fractionation–mass spectrometry, CF–MS), but the resulting data are challenging to process and interpret. Here, we describe EPIC (elution profile-based inference of complexes), a software toolkit for automated scoring of large-scale CF–MS data to define high-confidence multi-component macromolecules from diverse biological specimens. As a case study, we used EPIC to map the global interactome of Caenorhabditis elegans, defining 612 putative worm protein complexes linked to diverse biological processes. These included novel subunits and assemblies unique to nematodes that we validated using orthogonal methods. The open source EPIC software is freely available as a Jupyter notebook packaged in a Docker container (https://hub.docker.com/r/baderlab/bio-epic/).
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The supporting co-fractionation data are available via ProteomeXchange with the identifier PXD011182. The entire WormMap network (Cytoscape format) is available on GitHub (https://github.com/BaderLab/EPIC/tree/master/WormMap) and has been submitted to the BioGRID database. Source Data for Fig. 2 are available online.
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This study was supported by a Foundation Grant (FDN no. 148399) from the Canadian Institute of Health Research (CIHR, to A.E.), and US National Institutes of Health grants (nos. P41 GM103504, GM070743 to G.D.B.) L.Z.M.H. was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Mass Spectrometry-Enabled Science and Engineering (MS-ESE) program. C. elegans strain access was supported by the NIH Office of Research Infrastructure Programs (P40 OD010440).
The authors declare no competing interests.
Peer review information: Allison Doerr was the primary editor on this article and managed its editorial process and peer review in collaboration with the rest of the editorial team.
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Integrated supplementary information
a) Schematic workflow of bead-based sample pre-enrichment. b) Venn diagram showing improved proteome coverage by pre-enrichment. c) Bar chart showing improved detection of low abundance proteins. d) Bar chart showing improved detection of small (low molecular mass) proteins. e) Bar chart showing the distribution of identified proteins across top 8 biological processes in GO. f) Bar chart showing the distribution of identified proteins across top 13 cellular localizations in GO. g) Bar chart showing distribution of identified proteins across top 13 molecular functions in GO.
Previously reported protein complexes, collected from the CORUM, GO and Intact curation databases, are first mapped to a target species protein complexes based on InParanoid orthology predictions. Redundancy is minimized to generate a final set of reference assemblies.
Plots showing the Pearson correlation coefficients (distribution density curves) obtained for a representative worm protein co-fractionation experiment; positive (CORUM derived; blue) and negative (randomized; orange) co-complex interactions, as well as the positive/negative ratio (green), are shown.
Histogram of maximal correlation scores of positive protein-protein interaction pairs among all seven different correlation metrics across all 16 co-fractionation experiments. The red line indicates the cutoff chosen for EPIC.
Supplementary Figure 5 Composite score comparison for original and optimized features integrated with different sources functional evidence.
Composite score analysis demonstrates that for predicting complexes, based on EPIC analysis of CF/MS data, integration of functional associations from WormNet outperforms STRING and GeneMANIA evidence. The analysis also shows an optimized set of EPIC-derived co-elution scores better predicts protein complex memberships than were reported previously.
Supplementary Figure 6 ROC curve and Precision-recall curve for co-complex PPI prediction from different input data.
The plot demonstrates that the best co-complex interaction predictions were obtained after integrating experimental data with supporting functional evidence data (that is WormNet).
Pie chart showing overlap of predicted co-complex interactions with PPIs from BioGRID, iRefIndex and our previously reported conserved metazoan complex map.
Detailed overview of the EPIC computational pipeline.
a) Number of Peptides before and after removing ‘one-hit-wonders’ for each used searching tools identified in one co-fractionation experiment. There are 16 co-fractionation experiments (n = 16). b) Percentage of one-hit-wonders for each search engine. There are 16 co-fractionation experiments (n = 16). In each box plot, the red line is the median, the lower and upper line of the box indicates the first and the third quartile. The upper and lower whiskers extend to the largest value less than the third quartile plus 1.5 times the interquartile range (IQR) and smallest value greater than first quartile minus 1.5 times the IQR, respectively. All data points beyond the whiskers are plotted as individual points.
Precision-recall (PR) curves (a) and Receiver-operating-characteristic (ROC) curves (b) for different iterations of Poisson noise added in Pearson correlation coefficients feature.
Precision-recall (PR) curves (a) and Receiver-operating-characteristic (ROC) curves (b) for different Bayes correlation priors: uniform (Bayes1), Dirichlet-marginalized (Bayes2) and zero count-motivated (Bayes3).
(a). Boxplot showing the complex prediction performance (composite score) from two different machine-learning classifiers (random forest n = 1014 vs. support vector machine n = 945). (b). Boxplot showing the complex prediction performance (composite score) based on the 234 results from each four different protein search/quantification tool. (c). Boxplot showing the relationship between different numbers of correlation scores and complex prediction performance (that is composite score). n = 28, 110, 224, 280, 224, 112, 32 and 4 are the number of composite score results with various correlation scores used (from 1 to 8). Red arrow indicates the set of (five) correlation scores producing the highest composite score. In each box plot, the red line is the median, the lower and upper line of the box indicates the first and the third quartile. The upper and lower whiskers extend to the largest value less than the third quartile plus 1.5 times the interquartile range (IQR) and smallest value greater than first quartile minus 1.5 times the IQR, respectively. All data points beyond the whiskers are plotted as individual points.
(a). Line plot of the number of experiments and corresponding averaged composite score. (b). Line plot of the number of experiments and the corresponding averaged value of composite score times the number of predicted protein complexes.
Supplementary Figs. 1–13 and Supplementary Tables 1, 8 and 9
Complete list of predicted worm PPIs.
Complete list of predicted worm protein complexes in WormMap.
Results of AP–MS validation experiments.
Functional (GO term) enrichment on assemblies in WormMap.
Phenotypic enrichment analysis of complexes in WormMap.
Disease enrichment for human orthologs of worm protein macromolecules in WormMap.