Performing glycoproteomics searches with MSFragger-Glyco
- Getting Started
- Glyco-search with MSFragger: Search Types and Parameters
- Using FragPipe to run MSFragger glyco searches
- Running MSFragger glyco searches from terminal/Linux
- Interpreting the output
1) Getting Started
MSFragger can be downloaded here
FragPipe (a graphical interface and pipeline for MSFragger on Windows) can be downloaded here.
Follow the instructions on the FragPipe Releases page to launch the program. We recommend using FragPipe for Windows users. All parameters for running glyco searches detailed below can be found in the MSFragger tab of FragPipe, and detailed instructions for running glyco searches can be found in section (3) of this tutorial.
Before you get started, make sure your LC-MS file format is compatible with the workflows you want to perform (for Thermo data,
we recommend converting .raw files to mzML).
See the MSFragger wiki for full details of system requirements.
2) Glyco-search with MSFragger: Search Types and Parameters:
The types of searches that can be performed for glycoproteomics data are detailed below, along with several key parameters and recommended settings. Details on all parameters can be found in the MSFragger documentation here. The key parameters listed here are the differences required to perform a glyco-search; all other parameters can generally be used as in a normal MSFragger search (i.e. left as default or set to appropriate values for your instrument/analysis). Default parameter files for several types of glyco-searches can be found here, or by loading a glyco workflow in FragPipe.
Mass Offset Search
This is the primary search method recommended for glycoproteomics searches with MSFragger. It uses
involves setting glycan masses of interest as mass offsets (similar to an open search, but only considering
a narrow window around each mass offset). This allows MSFragger to simultaneously search for peptides that have lost none,
part, or all of their original glycan, making it ideal for searching collisional, hybrid, and photo-activation datasets.
- labile_search_mode: (possible values: nglycan, labile, off) nglycan mode checks for N-glycosylation motif N-X-S/T (X is not P), but is otherwise identical to “labile” mode. Labile mode should be used for all labile modifications (other than N-glycans), including O-glycans. Specify the allowed residues in restrict_deltamass_to. “Off” results in a standard (non-glyco) MSFragger search, in which all mass offsets are assumed to remain intact during activation.
- restrict_deltamass_to: (overridden in nglycan mode). Specify which amino acids (single letter codes) are allowed to contain the glycan mass offsets. Allowed values are single letter amino acid codes. When labile_search_mode is ‘off’, ‘-‘ can be used to allow non-localized matches (i.e. matches to labile mods that have dissociated), by setting the value to ‘STY-‘, for example. It is not necessary to include ‘-‘ for labile and nglycan modes, as labile modifications are considered by default. Default value: ‘all’. For typical O-glycoproteomics searches, should be set to ‘ST’. NOT used in nglycan mode.
- mass_offsets: (value: 0/mass1/mass2/…) All possible glycan masses should be specified as mass offsets (separated by ‘/’). Depending on the type of data and search, the glycans considered can vary. The masses of all glycans of interest should be determined and converted to a list separated by ‘/’ (see example parameter file). An arbitrary number of mass offsets can be searched, but search speed decreases with the number of masses used. If more than a few thousand masses are being considered (e.g. in an exploratory search), consider using an open search instead.
- fragment_ion_series: fragments types of interest should be specified here depending on the activation method and data. b~/y~ refer to b/y ions + HexNAc (common for N-glycans in CID/HCD). Recommendations:
CID/HCD/IRMPD - b,y,b~,y~,Y (NGlycan or low energy) or b,y,Y (OGlycan or high energy)
hybrid(EThcD/AIETD/etc) - b,y,c,z,Y
ETD/ECD - c,z
- localize_delta_mass: (values: 0 or 1) specifies whether to search for shifted ions (i.e. peptide fragments containing the intact glycan). Recommended (1) for ETD/ECD and hybrid searches (EThcD/AIETD/etc), not recommended (0) for CID/HCD/IRMPD.
- Y_type_masses: (value: 0/mass1/mass2/…) Masses of Y-type ions (intact peptide plus partially fragmented glycan) to consider in search. Note that ALL Y masses are applied to ALL peptide searches, so including too many can reduce search performance. Can be used in non-glyco searches as well (any modification that partially fragments, leaving behind some mass can be considered).
- diagnostic_fragments: (value: mass1/mass2/…) m/z values of diagnostic fragment ions (e.g. Oxonium ions) that appear in spectra of peptides containing a mod of interest. If diagnostic_intensity_filter > 0, at least one of the masses provided here must be found at sufficient intensity for any mass offset to be searched for the spectrum. To disable this checking, change diagnostic_intensity_filter to 0.
- diagnostic_intensity_filter: Minimum relative intensity (relative to base peak height) for the SUM of intensities of all diagnostic fragment ions found in the spectrum to consider this a potential glyco (or other labile mod) spectrum. Set to 0 to disable. A value of 0.1 means summed intensity must be 10% the height of the base peak in the MS/MS spectrum to be considered.
- deisotope: (values: 0 or 1) glycopeptides tend to be larger than tryptic peptides, making deisotoping very helpful. Recommended setting ‘1’ for glyco data.
- mass_diff_to_variable_mod: (values: 0, 1, or 2) For delta masses that are
successfully localized, places the theoretical mass offset (i.e. glycan mass) as an “assigned modification” in the peptide sequence that can be read by quantification
tools. “0” is off. “1” assigns the modification and subtracts the theoretical modification mass from the delta mass. NOTE: this prevents extended mass modeling in
PeptideProphet and is not recommended for glyco searches. “2” assigns the modification but does not change the delta mass. This makes the output table incompatible
with PTM-Shepherd, but is the recommended setting for quantification. Not recommended for open searches. NOTE: for N-glycan searches, if localization does not succeed,
the delta mass will still be placed on the first sequon in the peptide. For peptides containing multiple sequons, reported assigned position may not be correct. Post-search
localization is strongly recommended in these cases.
Open searches can be performed on glycoproteomics data, for example to determine what glycans are present in the data.
Open searches function similarly to the mass offset search in that delta masses (mass offsets) between precursor and sequence masses are used to define glycans
and other modifications, but with a few key differences. Because any mass shift (within the provided tolerance)
is allowed, not all mass shifts correspond to glycans. For this reason, oxonium filtering is currently disabled for open searches.
However, glycopeptide-specific fragments ions (Y, b~, y~) can still be generated for peptides containing
the sequon/residue(s) of interest to effectively search for glycopeptides. If glycans of interest are known, a mass offset
search will likely be more sensitive.
- labile_search_mode, fragment_ion_series, Y_type_masses, restrict_deltamass_to, diagnostic_fragments, and diagnostic_intensity_filter, and deisotope should be used as in mass offset glyco-searches (see above).
- localize_delta_mass: shifted ions are not necessary for fully labile modifications (like glycosylation in CID/HCD searches), but can be useful for determining the presence of other types of modifications. If interested only in glycans in CID/HCD data, this can be set to 0; otherwise, and if interested in other modifications as well as glycans, set to 1.
- mass_offsets, isotope_error: set to 0
- precursor_mass_lower, precursor_mass_upper: these define the range of the open search. For example, precursor_mass_lower = -200, precursor_mass_upper = 2500 would allow any mass shift between -200 and 2500 Da, accounting for many types of glycans (and many other peptide modifications).
Variable Modification Search
Recommended only for ETD/ECD data. Enter glycans of interest as variable modifications (see the
for formatting details) on specified residue(s). It is not recommended to search more than a dozen or so
modifications at once.
- labile_search_mode: . Variable modification searches should not be used for labile modifications (set to ‘off’). If performing an ETD/ECD glyco-search, set to ‘off’ (even though it is a glyco-search, the glycans are not labile in ETD/ECD). If set to off, labile fragment ions (Y, b~,y~) will not be generated
- fragment_ion_series: Y, b~, and y~ ion types are NOT recommended for ETD/ECD data (as they should not occur). All other ion types function as normal
- mass_offsets: not used. (mass_offsets = 0)
3) Performing a glycoproteomics search with FragPipe
FragPipe can be downloaded here. Follow the instructions on that same Releases page to launch the program.
Please use the main FragPipe tutorial to learn the basics of FragPipe. This tutorial will cover only the differences needed for glycoproteomics searches from the basic workflow outlined there
FragPipe includes a number of pre-packaged workflows, including several for glycoproteomics searches. We recommend using one of these workflows as a starting point, but parameters can be entered manually as well.
To load a workflow:
Navigate to the workflow tab, select a glycoproteomics workflow from the dropdown menu, and press the “Load” button. All default glycoproteomics workflows begin with “glyco-“, followed by the type of glycan (N-linked or O-linked), and activation type (HCD or Hybrid). “HCD” is optimized for beam-type CID fragmentation but can be used as a starting point for any vibrational activation (trap CID, IRMPD, etc). “Hybrid” refers to combined electronic (e.g. ETD) and vibrational activation, for example, EThcD or ETciD.
The workflow will initialize default parameters on several of the subsequent tabs.
Editing glyco-relevant parameters
See the FragPipe tutorial for information on how to load a database file. No differences are present for glycoproteomics searches.
The vast majority of differences between regular and glycoproteomics searches can be found on
MSFragger tab. The key areas are highlighted here. Standard parameters are modified as in
a typical search - see the main FragPipe tutorial for details about the parameters on this page.
Open searches are performed by changing the precursor mass range and units here.
Most of the glyco-search options can be found in the advanced section.
(1) For mass offset glyco searches,
input all glycan masses of interest in the mass offset box. Open searches leave this box empty.
(2) Adjust the options in the Glyco/Labile mods box.
- Search Mode: distinguishes between types of mass offset and open searches. Options are:
nglycan: for N-linked glycans. Uses consensus sequon N-X-S/T (X is not P) for glycosites
labile: for all other labile modifications, including O-linked glycans
off: for modifications that do not dissociate. Should not be used for glyco search.
- Allowed Resides: note: NOT used in nglycan mode. Determines the allowed residues for mass offsets to be considered. Input is single letter amino acid codes (see above for details). For O-linked glycan searches, default is ST.
- Diagnostic Ion Minimum Intensity: Used to filter spectra considered for searching glycans based on the presence of oxonium ions. If set to 0, filter will not be used. Otherwise, value is the minimum for the summed relative intensity of all detected oxonium ions for the spectrum to be considered for glyco searching. Default value is 0.1, meaning that the sum of all oxonium ion intensities must be at least 10% of the base peak in the spectrum to pass the check.
- Y ion masses: If searching for Y ions (intact peptide with partial glycan), enter the list of masses to use here. Masses should be the mass of sugar residues only, no charging proton or terminal groups. Note: these ions will be searched for ALL glycans, so keeping the list short and including only ions that appear frequently will improve performance. These values are only used if “Y” is included in the fragment ion series (see below)
- Diagnostic Fragment Masses: Enter masses of oxonium ions to search for potential glyco-spectra. This list can include rare ions without negatively impacting performance. Masses should be entered as the observed m/z value of the oxonium ion, i.e. [M+H]+
(3) Enter the fragment ion types appropriate to your analysis in the “Fragment ion series” box. Recommendations are as follows, but should be adjusted based
the types of glycans being studied and/or acquisition parameters:
N-glycan CID/HCD: b,y,b~,y~,Y
N-glycan hybrid (EThcD, etc): b,y,c,z,Y
O-glycan CID/HCD: b,y
O-glycan hybrid (EThcD, etc): b,y,c,z
N- or O-glycan ETD/ECD: c,z (NOTE: this search should be done with glycans as variable modifications, NOT mass offsets, as glycans are not expected to dissociate in this mode.)
For highly enriched N-glycopeptides (at least 10-20% of all spectra are of glycopeptides), setting the “–glyc” tag in the PeptideProphet options can help by modeling peptides with the consensus sequon (N-X-S/T) separately from peptides without. Otherwise, there are no differences on this page between a glyco-search and a standard mass offset or open search.
All other tabs:
No difference between a regular MSFragger search and a glyco search. See the main FragPipe tutorial linked at the beginning of this document for details and instructions.
4) Running MSFragger glyco searches from terminal/Linux
On Windows, we strongly recommend using FragPipe (above) to run MSFragger, but the linux commands below can be adapted to run via command line if desired.
On Linux, the following shell script includes commands to run MSFragger and downstream processing with Philosopher for glyco-searches. These commands are identical to standard MSFragger operation, the only difference for a glyco-search is in the MSFragger parameters file.
Changes from defaults
- If you are searching N-glycans and have highly enriched data (at least ~20% of PSMs are glycopeptides), adding the –glyc tag in the PeptideProphet step can help with modeling.
- When using mass offset or open searches, make sure the –masswidth provided to PeptideProphet is large enough to include the largest of your glycan masses (otherwise those PSMs will not be modeled).
Linux shell script:
# Specify paths of tools and files to be analyzed.
# Run MSFragger. Change the -Xmx value according to your computer’s memory.
java -Xmx32G -jar $msfraggerPath $fraggerParamsPath $dataDirPath/<spectral files ending with .mzML (required for quantification) or .raw>
# Move pepXML files to current directory.
mv $dataDirPath/*.pepXML ./
# Move MSFragger tsv files to current directory.
mv $dataDirPath/*.tsv ./ # Comment this line if localize_delta_mass = 0 in your fragger.params file.
# Run PeptideProphet, ProteinProphet, and FDR filtering with Philosopher
$philosopherPath workspace –clean
$philosopherPath workspace –init
$philosopherPath database –annotate $fastaPath –prefix $decoyPrefix
# Pick one from the following commands and comment the other.
$philosopherPath peptideprophet –nonparam –expectscore –decoyprobs –ppm –accmass –decoy $decoyPrefix –database $fastaPath ./*.pepXML # closed search
$philosopherPath peptideprophet –nonparam –expectscore –decoyprobs –masswidth 4000.0 –clevel -2 –decoy $decoyPrefix –combine –database $fastaPath ./*.pepXML # Open or Mass Offset search
$philosopherPath proteinprophet –maxppmdiff 20000000 ./*.pep.xml
# Pick one from the following commands and comment the other one.
$philosopherPath filter –sequential –razor –mapmods –tag $decoyPrefix –pepxml ./ –protxml ./interact.prot.xml # closed or non-specific closed search
$philosopherPath filter –sequential –razor –mapmods –tag $decoyPrefix –pepxml ./interact.pep.xml –protxml ./interact.prot.xml # Open search
# Perform quantification.
$philosopherPath freequant –dir $dataDirPath
# Make reports.
5) Interpreting the output
Mass offset and open searches will generate PSMs with delta mass values corresponding to glycan masses. These delta mass values do not contain localization information in the output PSM table (by default) and so will not be rolled up to the peptide and protein.tsv output tables. This can be changed with the mass_diff_to_variable_mod parameter (see above), which can be used to make the output compatible with quantification tools. Note, however, that this prevents re-localization with PTM-Shepherd, which should be disabled when using.
- PTM-Shepherd (available in FragPipe) can be used on the output of mass offset or open glyco searches. It aggregates observed delta mass values into modifications (matching against entries in Unimod), which can be helpful in distinguishing glycans of similar mass. Full support for glyco data is being developed and will be included in a future release.
- If localize_delta_mass=1 was set in the MSFragger search, a .tsv file of localization information is generated in the output directory and can be used to determine glycosylation sites. The localization is calculated using all fragment ions searched in MSFragger, so the inclusion of b~/y~ ions (for example) can enhance localization results.
- The MSFragger option “mass_diff_to_variable_mod” can be used to convert delta masses to variable mods, allowing them to be propagated and used in quantification (for example). Delta masses that are successfully localized will be placed as variable mods. If localization is ambiguous, for N-glycan mode searches, the delta mass will be placed on the first sequon. NOTE: for peptides containing multiple sequons, this means the reported assigned position may not be correct if localization did not succeed. For peptides containing multiple sequons, follow-up localization after the search is strongly recommended. For all other searches, the modification will remain as a delta masses if not localized.