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Protein ID With MS/MS Data

Introduction:  We have already talked about how peptide-mass fingerprinting uses the intact masses of the peptides produced by an enzyme digest to get a best fit match to a protein in a sequence database.  MS/MS spectral matching uses the un-interpreted peaks in a peptide fragment spectrum to match to a theoretical fragment spectrum in a sequence database.  This MS/MS fragment spectrum is the result of a collision induced dissociation, (CID), occurring within a mass spectrometer. The fragmentations are produced either in a collision cell in a tandem mass spectrometer or within an ion trap.  In an MS/MS based protein ID experiment multiple peptides are usually found and all of their fragment spectrums are used to correlate to a protein.  In both PMF and MS/MS ID the larger the number of peptides identified the greater the confidence in the protein correlation.  One can see that the correlation made with multiple MS/MS spectra can easily be superior to the identification made in a PMF experiment.  In an extreme example a single peptide mass in a PMF experiment can never really be correlated to a protein, however the heterogeneity imparted to a single peptide in an MS/MS experiment can deliver enough amino acid sequence to correlate to a protein.  Why is this?  Some may argue that a single peptide mass in a PMF experiment can be correlated if one has an accurate enough mass.  However, that mass could be anything, any carbon based thing, there is a lack of heterogeneity in a single mass and a lack of information.  With MS/MS data the heterogeneity produced is rich and a number of constraints can be placed.  The primary constraint that might be overlooked is that it is a peptide, remember we have amino acid sequence evidence.  Once we know this we can place the most important constraint, enzyme specificity.  With enzyme specificity, intact peptide mass, and a partial amino acid sequence, the peptide correlation can be very strong even with a single peptide. Below is a step by step simplification of how sequence database search programs accomplish peptide and protein ID.  
A Step by Step General Scheme for MS/MS Protein ID 

Step 1.  Step one usually begins with placing an enzyme specificity constraint.  Some programs pre-index the sequence database based on the enzyme specificity.  This makes the search go much faster as for example tryptic peptides can be indexed and mass lists can be pre-made.  The downside is a separate database needs to be indexed for each enzyme or each time a  potential modification is changed.
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Step 2.  Step two involves matching the parent mass of the intact peptide to the peptides in the database.  Generally the narrower you can set the parent mass constraint the faster the search will go because fewer peptides will need to be correlated in the next step.  One needs to be certain that the accuracy of the mass spectrometer is not exceeded when setting the parent mass window in the search.   In this step a short list of peptides are selected for the fragment mass correlation in the next step.
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Step 3.
Step three takes the list of peptides identified by parent mass in step two and compares the theoretical fragment masses of these peptides to the experimentally derived fragment spectra.  
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Step 4. 
Peptide Ranking. Hits are ranked by how many of the fragment masses match the theoretical fragment masses in the sequence database.

Step 5.  If more than one peptide is searched all of the peptides found are correlated to their prospective proteins.  The protein with the greatest number of well correlated peptides is usually the most significant hit.
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Step 6. 
Some programs go a bit further using probability to back-up the proposed match.  Most users like to have a real probability number, this gives them some comfort in the designation.  Probability takes the onus off of the user.  While probability comforts us, we find for the most important individual protein hit we still go in and manually validate the spectral match. 

 

Simple MS/MS Protein ID Exercise: 

Use the MS/MS spectrum in Figure1 below to search a sequence database.  For this example we will be using the web version of the database searching program Phenyx, this program is made available by GeneBio.  

 

Figure 1

 

Figure 1.  This is a peptide fragment spectrum collected on an LCQ ion trap mass spectrometer.  Note the parent m/z value 690.39 in the upper left hand header of the spectrum.

Exercise Procedure:

You can use the dta file that we have already created for you, right click on this dta link, and save this file to your computer, choose save target as.  When saving the file to your computer, during the saving dialog  you may need to select "all files" as the file type and you may also need to add the suffix .dta to the "msms" name, in order for your computer to get the file type correct.   A dta file is a mass / intensity pair list that is a representation of the original MS/MS spectrum, the dta file is a Thermo Finnigan file format. The dta file is very small, a few KB, once you have it downloaded, if you want, you can open it with notepad to inspect the simple file format. 

Steps to follow on the Phenyx submission page:

1.) Follow this link to the Phenyx web portal.

2.) Click on the "
Log In" button on the Phenyx page to log in as a guest. (We recommend that you create your own account, so that you can keep your future data submissions private and so that you can keep track of past and future submissions.)

3.) Then click on the
Submission button.  The main submission page will come up, do the following:

        A.) Change taxonomy to "
root." You will need to scroll up to the top of the list to find root.
               (root means that the program will search all species.) 
        B.) Change the algorithm to
LCQ. (this is the instrument we used)
        C.) Click on the "
Browse" button and browse to and load the msms.dta file that you just downloaded.
        D.) Look for the file format field, and change the file format to
dta.
        E.) Click on the
Submit button.

Wait a few seconds until the "link to result" says, "
available",  then click on the link below it.  If you do not have pop ups blocked, like we do,  the results page would have appeared automatically.  

Inspect the results page:

What peptide/protein was identified? How many peaks were identified? What peaks went unidentified and why? How long did the search take? If you had 2000 spectra how long do you think it would take? 
 

Conclusion:

Mass spectrometry based protein identification is too easy right?  So far we have used three MS sequence database searching techniques to identify a single protein.  Continue on to the final tutorial to learn how to identify many proteins in a somewhat typical  proteomics experiment.

 

References:

  1. Jimmy K. Eng, Ashley L. McCormack and John R. Yates, III An approach to correlate tandem mass spectral data of peptides with amino acid sequences in a protein database JASMS, Volume 5, Issue 11, November 1994, Pages 976-989

  2. Yates JR 3rd, Eng JK, McCormack AL, Schieltz D. Method to correlate tandem mass spectra of modified peptides to amino acid sequences in the protein database.
    Anal Chem. 1995 Apr 15;67(8):1426-36. PMID: 7741214 

  3. Yates JR 3rd, Eng JK, McCormack AL. Mining genomes: correlating tandem mass spectra of modified and unmodified peptides to sequences in nucleotide databases.
    Anal Chem. 1995 Sep 15;67(18):3202-10. PMID: 8686885

  4. Lim H, Eng J, Yates JR 3rd, Tollaksen SL, Giometti CS, Holden JF, Adams MW, Reich CI, Olsen GJ, Hays LG. Identification of 2D-gel proteins: a comparison of MALDI/TOF peptide mass mapping to mu LC-ESI tandem mass spectrometry.  J Am Soc Mass Spectrom. 2003 Sep;14(9):957-70. PMID: 12954164

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