Shotgun proteomics has a number of bioinformatic tools available for identification
and quantification of peptides, and the subsequent protein inference. msstitch is a
tool to integrate a number of these tools, generating ready to use result files.
If you need support for a specific program, there is limited time but infinite gratitude :)
An example command flow would first store mzML spectra data in an SQLite file:
msstitch storespectra --spectra file1.mzML file2.mzML \
--setnames sampleset1 sampleset2 -o db.sqlite
Or, to add spectra to an existing SQLite lookup:
msstitch storespectra --dbfile lookup.sqlite --spectra file3.mzML file4.mzML \
--setnames sampleset2 sampleset3
Then store quantification data from dinosaur (MS1 precursor quant) and isobaric quantification (including precursor purities, but use centroided MS1 for this) from OpenMS together with the spectra. MS1 precursor quant features will be aligned with MS2 spectra by selecting the best m/z match (an m/z window of 2x m/z tolerance), from all features within the retention time window of 2x RT tolerance. --rttol is specified in seconds.
msstitch storequant --dbfile db.sqlite --spectra file1.mzML file2.mzML \
--mztol 10 --mztoltype ppm --rttol 10 \
--dinosaur file1.dinosaur file2.dinosaur \
--isobaric file1.consensusXML file2.consensusXML
When using Hardklor/Kronik instead of Dinosaur, you can instead use:
msstitch storequant --dbfile db.sqlite --spectra file1.mzML file2.mzML \
--kronik file1.kronik file2.kronik \
--isobaric file1.consensusXML file2.consensusXML
For both Dinosaur and Kronik, the MS1 peak sum is used which theoretically would be more correct
when having differently shaped envelopes. If you'd rather use the envelope apex, pass --apex
in the above command.
Create a decoy database where peptides are reversed between tryptic residues:
msstitch makedecoy uniprot.fasta -o decoy.fasta --scramble tryp_rev --maxshuffle 10
Or without removing peptide sequences that match to the target DB:
msstitch makedecoy uniprot.fasta -o decoy.fasta --scramble tryp_rev --ignore-target-hits
After running two samples of MSGF and percolator, we can start making a more proper set of PSM tables by adding percolator data and filtering on FDR. The following adds percolator svm-score, q-value (FDR), and posterior error as columns to the PSM table:
# Add percolator data, filter 0.01 FDR
msstitch perco2psm -i psms1.txt \
--perco percolator1.xml --mzid psms1.mzIdentML \
--filtpsm 0.01 --filtpep 0.01
msstitch perco2psm -i psms2.txt \
--perco percolator2.xml --mzid psms2.mzIdentML \
--filtpsm 0.01 --filtpep 0.01
# Combine the two sets and split to a target and decoy file
msstitch concat -i psms1.txt psms2.txt -o allpsms.txt
msstitch split -i allpsms.txt --splitcol TD
Now refine the PSM tables, using the earlier created SQLite DB, adding more information (sample name, MS1 precursor quant, isobaric quant, proteingroups, genes). In this example we set isobaric quantitation intensities to NA if the precursor purity measured is <0.3.
cp db.sqlite decoy_db.sqlite
msstitch psmtable -i target.tsv -o target_psmtable.txt --fasta uniprot.fasta \
--dbfile db.sqlite --addmiscleav --addbioset --ms1quant --isobaric \
--min-precursor-purity 0.3 --proteingroup --genes
msstitch psmtable -i decoy.tsv -o decoy_psmtable.txt --fasta decoy.fasta \
--dbfile decoy_db.sqlite --proteingroup --genes --addbioset
If necessary (e.g. multiple TMT sample sets), split the table before making protein/peptide tables:
msstitch split -i target_psmtable.txt --splitcol bioset
Create a peptide table, with summarized median isobaric quant ratios, highest MS1 intensity PSM as the peptide MS1 quant intensity, and an additional linear-modeled q-value column:
msstitch peptides -i set1_target_psms.txt -o set1_target_peptides.txt \
--scorecolpattern svm --modelqvals --ms1quant \
--isobquantcolpattern tmt10plex --denompatterns _126 _127C
The same peptide table can also be made using median sweeping, which takes the median intensity channel for each PSM as a denominator. Here is also exemplified how to do channel-median centering of the ratios to normalize and use log2 intensity values before calculating ratios:
msstitch peptides -i set1_target_psms.txt -o set1_target_peptides.txt \
--scorecolpattern svm --modelqvals --ms1quant \
--isobquantcolpattern tmt10plex --mediansweep --logisoquant --median-normalize
Or, if you only want the median PSM intensity per peptide summarized, use --medianintensity
Here is also illustrated that you can use the --keep-psms-na-quant flag to NOT
throw out the PSMs which have isobaric intensity below the mininum intensity
(default 0, here 100) IN ANY channel:
msstitch peptides -i set1_target_psms.txt -o set1_target_peptides.txt \
--scorecolpattern svm --modelqvals --ms1quant \
--isobquantcolpattern tmt10plex --medianintensity \
--minint 100 --keep-psms-na-quant
In case of analyzing peptides with PTMs, you may want to process a subset of
PSMs (those with the PTMs) to create a separate peptide table from. In
that case, there is an option to divide (or subtract for log2 data) isobaric
quant values to a protein (or gene) table from a non-PTM search, often done on
another non-enriched sample. This allows discerning PTM-peptide
differential expression from its respective protein differential expression in the sample.
The protein/gene table should obviously contain the same samples/channel,
and for example be from an msstitch proteins or msstitch isosummarize command,
using --median-normalize to get median centered ratios for the proteins or genes.
After that, create a peptide table from PTM-PSMs as follows:
msstitch peptides -i set1_ptm_psms.txt -o set1_ptm_peptides.txt \
--scorecolpattern svm --isobquantcolpattern tmt10plex --denompatterns _126 _127C \
--logisoquant --totalproteome set1_proteins.txt
For proper normalizing of this table (as it would otherwise be impacted by sample differences per channel), you may want to median-center. In the case of small and possibly noisy PTM tables, it can be advisable to use another table from a global search (or e.g. the full peptide or protein table from the PTM search) for determining channel medians. This is possible by specifying the above command plus:
--median-normalize --normalization-factors-table /path/to/set1_global_proteins.txt
To create a protein table, with isobaric quantification as for peptides, the
average of the top-3 highest intensity peptides for MS1 quantification.
For all of these, summarizing isobaric PSM data to peptide, protein, gene features
is done using medians of log2 PSM quantification values per feature (e.g. a protein). If you'd
rather use averages, use --summarize-average as below, where we also show log2
transformation of intensities before summarizing and subsequent median-centering.
FDR (q-values) for the protein table is here calculated 'classically', by ranking
target and decoy proteins, taking their best scoring peptide's q-value as a score.
--logscore is used since q-value is used (higher is better when comparing peptides).
msstitch proteins -i set1_target_peptides.txt --decoyfn set1_decoy_peptides \
--psmtable set1_target_psms.txt \
-o set1_proteins.txt \
--scorecolpattern '^q-value' --logscore \
--ms1quant \
--isobquantcolpattern tmt10plex --denompatterns _126 _127C \
--summarize-average --logisoquant --median-normalize
Or the analogous process for genes, using median sweeping to get intensity ratios instead of denominators: As for peptides above, one can use the --keep-psms-na-quant flag to NOT throw out the PSMs which have isobaric intensity below the mininum intensity (default 0 used here) in any channel. Here we use picked FDR (Savitski et al. 2015 MCP) to define q-values for the genes, for which you need a target and decoy fasta to form pairs. The fasta files need to be analogous, i.e. the same order for matching T/D pair genes.
msstitch genes -i set1_target_peptides.txt --decoyfn set1_decoy_peptides \
--psmtable set1_target_psms.txt \
-o set1_genes.txt \
--scorecolpattern '^q-value' --logscore \
--fdrtype picked --targetfasta tdb.fa --decoyfasta decoy.fa \
--ms1quant \
--isobquantcolpattern tmt10plex --mediansweep \
--keep-psms-na-quant
Or when there are ENSEMBL entries in the fasta search database, even for ENSG, here with summarized median PSM intensity per ENSG:
msstitch ensg -i set1_target_peptides.txt --decoyfn set1_decoy_peptides \
--psmtable set1_target_psms.txt \
-o set1_ensg.txt \
--scorecolpattern '^q-value' --logscore \
--ms1quant \
--isobquantcolpattern tmt10plex --medianintensity \
--median-normalize
Finally, merge multiple sets of proteins (or genes/ENSG) into a single output. Here we set an cutoff so that features with FDR > 0.01 are set to NA for the respective sample set.
msstitch merge -i set1_proteins.txt set2_proteins.txt -o protein_table.txt \
--setnames sampleset1 sampleset2 \
--dbfile db.sqlite \
--fdrcolpattern 'q-value' --mergecutoff 0.01 \
--ms1quantcolpattern area --isobquantcolpattern plex
Trypsinize a fasta file (minimum retained peptide length, do cut K/RP, allow 1 missed cleavage)
msstitch trypsinize -i uniprot.fasta -o tryp_up.fasta --minlen 7 \
--cutproline --miscleav 1
Create an SQLite file with tryptic sequences for filtering out e.g. known-sequence data. Options as for trypsinization, --insourcefrag builds lookup with support for in-source fragmented peptides that have lost some N-terminal residues:
msstitch storeseq -i canonical.fa --cutproline --minlen 7 \
--miscleav 1 --insourcefrag
Filter a percolator output file using the created SQLite, removing sequences that match those stored in the SQLite. The below also removes sequences in the sample which are deamidated (i.e. D -> N), and sequences that have lost at most 2 N-terminal amino acids due to in-source fragmentation (DB must have been built with support for that).
msstitch filterperco -i perco.xml --dbfile tryptic.sqlite \
--insourcefrag 2 --deamidate -o filtered.xml
Create an SQLite file with full-protein sequences for filtering any peptide of a minimum length specified that matches to those. Slower than filtering tryptic sequences but more comprehensive:
msstitch storeseq -i canonical.fa --fullprotein --minlen 7
Filter a percolator output file on protein sequences using the SQLite, removing sequences in sample which match to anywhere in the protein. Sequences may be deamidated, and minimum length parameter must match the one the database is built with.
msstitch filterperco -i perco.xml --dbfile proteins.sqlite \
--fullprotein --deamidate --minlen 7 -o filtered.xml
Create an isobaric ratio table median-summarizing the PSMs by any column number you want in a PSM table. E.g. you have added a column with exons. The following uses average of two channels as denominator, outputs a new table with first column the features found in column nr.20 of the PSM table:
msstitch isosummarize -i psm_table.txt --featcol 20 \
--isobquantcolpattern tmt10plex --denompatterns 126 127C
Re-use an earlier PSM table and add PSMs from searched spectra files of a new or re-searched sample. Saves time so you won't have to re-search all the spectra in case of a big analysis. In the example below, new PSMs are the result of a sample set that has been re-searched, (e.g. when MS reruns are done in case of bad spectra), so we delete the existing sample set before continuing. Protein grouping is done after regenerating the PSM table, to illustrate you can do protein grouping on the entire table instead of only on the sample set. Since the new table is the one which supplies the header, the columns not supplied in the command (here protein groups) will be removed from the final result. This function assumes all PSMs presented are in the same order in the table, so they should not have been inserted in parallel, safest is to not generate the lookup table by hand.
msstitch deletesets -i old_psmtable.txt -o cleaned_psmtable.txt \
--dbfile db.sqlite --setnames bad_set
msstitch psmtable -i rerun_target.tsv --oldpsms cleaned_psmtable.txt \
-o new_almost_done_psmtable.txt --fasta uniprot.fasta \
--dbfile db.sqlite --addmiscleav --addbioset --ms1quant --isobaric
msstitch psmtable -i new_almost_done_psmtable.txt -o new_target_psms.txt \
--proteingroup
It is also possible to only pass a PSM table to deletesets:
msstitch deletesets -i old_psmtable.txt -o cleaned_psmtable.txt \
--setnames bad_set