|
Advertisement | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Molecular & Cellular Proteomics 4:1826-1830, 2005.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ABSTRACT |
|---|
|
|
|---|
Recently we described a new approach (10) to proteomic analysis that uses preparative IEF by free flow electrophoresis (FFE)1 (11, 12) for a first dimension fractionation of complex peptide mixtures. The use of FFE not only provides a high resolution peptide separation, but also it adds a constraint of peptide pI information to the determination of peptide sequence matches in the sequence database search of the MS/MS data, significantly improving the confidence of the peptide sequence matches and effectively increasing the number of high confidence protein identifications (10, 1315).
The goal of this study was to use peptide separation by FFE coupled with a linear ion trap mass spectrometer to comprehensively identify proteins in whole human saliva. We identified 437 proteins with high confidence, providing the largest catalogue of proteins from a single saliva sample to date. The protein catalogue provides new information on the composition of this bodily fluid and its potential utility in disease diagnostics. The statistically validated and transparently presented dataset (shown in the supplemental table) provides a model for presenting large, mass spectrometry-based proteomic data that should provide improved dissemination and comparison of datasets in this clinically important biological fluid.
| EXPERIMENTAL PROCEDURES |
|---|
|
|
|---|
FFE Fractionation of Peptides and Sample Processing
Preparative IEF of the peptide mixture was performed using a commercially available Pro Team free flow electrophoresis system (BD Biosciences) (11, 12). The saliva peptides were dissolved in 250 µl of FFE separation buffer and fractionated by FFE into a 96-well microtiter plate as described previously (10). Immediately after FFE separation, the pH of each FFE fraction was measured using a microelectrode (Accument combination microelectrode, Fisher). A 50-µl aliquot (of
500 µl total) was taken from each of the microtiter plate wells and processed as described previously (10) prior to mass spectrometric analysis.
µLC-ESI MS/MS Analysis
All µLC separations were done on an automated Paradigm MS4 system (Michrom Bioresources, Inc., Auburn, CA). Each processed FFE fraction was automatically loaded across a Paradigm Platinum Peptide Nanotrap (Michrom Bioresources, Inc.) precolumn (0.15 x 50 mm, 400-µl volume) for sample concentrating and desalting at a flow rate of 50 µl/min in HPLC buffer A. The in-line analytical capillary column (75 µm x 12 cm) was home-packed using C18 resin (5-µm, 200-Å Magic C18AG, Michrom Bioresources, Inc.) and Picofrit capillary tubing (New Objective, Cambridge, MA). Peptides were eluted using a linear gradient of 1035% buffer B over 60 min followed by isocratic elution at 80% buffer B for 5 min with a flow rate of 0.25 µl/min across the column.
Peptides were analyzed by MS/MS using a linear ion trap mass spectrometer system (LTQ, Thermo Electron Corp., San Jose, CA). The electrospray voltage was set to 2.0 kV using a collision energy setting of 29% and a data-dependent procedure that alternated between one MS scan (over the m/z range of 4001800) followed by four MS/MS scans for the four most abundant precursor ions in the MS survey scan. Both the MS and MS/MS spectra were acquired using a single microscan with a maximum fill time of 50 ms in the ion trap. m/z values selected for MS/MS were dynamically excluded for 30 s.
Sequence Database Searching and Peptide Sequence Match Filtering
The MS/MS spectra were sequence database-searched using TurboSEQUEST (17) (Thermo Finnigan, San Jose, CA). The MS/MS spectra were searched against the non-redundant human International Protein Index database (18) containing
50,000 protein sequences with a reverse version of the same database attached at the end of the forward version. The search parameters used included a precursor ion mass accuracy tolerance of 2.0 with methionine oxidation specified as a differential modification. Tryptic cleavage sites were specified as described below. The peptide sequence match results were organized and viewed using the software tool Interact (19). False positive rates were calculated as described previously (10, 20). The predicted pI of peptide sequences was calculated according to Shimura et al. (21) using an automated script, and peptide pI values were automatically inputted into the Interact results file. For FFE fractions in the pH range of 6.58.0 (fraction numbers 4658), the average peptide pI value was used rather than the measured fraction pH for filtering peptide sequence matches in steps one and two (see "Results"). The MS/MS spectra were first searched against the database with the enzyme trypsin specified, allowing up to two missed cleavage sites in the peptide sequence match. To identify non-tryptic peptides derived from proline-rich proteins, as have been found in other proteomic studies of saliva (7, 8), the MS/MS data were also searched with no enzyme specified, and the peptide matches were filtered by peptide pI and FFE fraction pH. This resulted in the identification of eight additional proteins, which were added to the protein results from the first filtering step described under "Results."
| RESULTS |
|---|
|
|
|---|
6.58.0. The reason for this discrepancy is unknown and needs further investigation, although it may reflect an inaccuracy in the pI prediction algorithm as it has been observed regardless of the method used for IEF of peptide mixtures (10, 13, 14). The bottom line in the plot shows the distribution of matched peptide sequence across each FFE fraction. The majority of the peptides cluster in the pH ranges 3.55.0 with very few peptides detected in fractions with neutral pH values (pH
78), similar to the distribution of tryptic peptides in other studies using preparative IEF (10, 13, 14).
|
pH) between the calculated peptide pI value for the matched sequence and the measured pH value of the FFE fraction from which the peptide was identified. True peptide sequence matches should have pI values very close to the measured fraction pH value, whereas false matches are expected to have random pI values and be eliminated when using the
pH filter (10, 15). The first step initially filtered the peptide sequence matches using a
pH tolerance of ±0.5, which we have shown to be the optimal
pH tolerance based upon the IEF resolution using FFE (10). This filtering step allows for the p score threshold to be reduced while still maintaining a false positive rate below 1% (10). The optimal p score threshold using
pH filtering will be different for each dataset being analyzed. As Fig. 1B shows, for this particular dataset the p score could be reduced to 0.76 when applying the
pH filter, decreased from the p score threshold of 0.96 needed to achieve the same confidence without considering peptide pI. The second step filtered the peptide sequence matches using a low stringency p score threshold of 0.2 and peptide pI, again using a
pH value of ±0.5, with the added proviso that a protein would be added to the catalogue only if it was matched by two or more unique peptide sequences. This step is based upon the assumption that when combined with the peptide pI constraint, multiple peptide sequence matches provide added confidence to protein identification even when the matches have a low p score. Indeed using these criteria the calculated false positive rate for this filtering step was also below 1%. Each filtering step added to the catalogue. The first step identified 433 proteins from peptide matches with a p score at or above the 0.76 threshold; each was added to the catalogue. 181 of these proteins had at least two peptide sequence matches, and the remainder had one peptide match. The second step identified and added to the catalogue another four proteins. At least one additional peptide sequence match was also added to 101 proteins (as indicated in the supplemental table) already in the catalogue, increasing the proteins identified by two or more peptide sequence matches to 221 of 437 total proteins. The supplemental table provides detailed information on this dataset, including all peptide sequence matches and the known biochemical functions and localizations of the identified proteins.
| DISCUSSION |
|---|
|
|
|---|
Our approach identified 437 proteins with high confidence (false positive rate below 1%). We compared our catalogue to those from other proteomic studies of saliva attempting to comprehensively identify proteins in saliva using non-gel electrophoresis-based strategies. One recent study using multidimensional liquid chromatography and tandem mass spectrometry identified 102 proteins in whole human saliva (8). These protein matches were statistically validated using reversed database searching, providing an estimated false positive rate below 1%. Most of their catalogues proteins are contained in ours but not vice versa. Another recent report used both liquid chromatography-based separations and also two-dimensional gel separations to identify a combined 309 proteins from saliva (7). The overlap between their catalogue and ours was relatively small with most of the common proteins between the studies being those that have also been found in other proteomic studies, most likely indicative of their high abundance and housekeeping functions in saliva. By comparison with these other studies, our catalogue of proteins is the largest obtained from a single saliva sample to date, thereby providing new information on its composition.
Comparison of other catalogues with ours highlights an ongoing problem in the proteomics community: a lack of standards in publishing mass spectrometry-derived proteomic datasets (23, 24). For example, in the case of the study described in Ref. 7, the dataset was non-transparently presented with little information on the criteria for determining correct peptide sequence matches provided and no estimate of false positive rates or detailed information on the scoring of peptide sequence matches. Furthermore the protein sequence database used outputted protein accession numbers for identified proteins from a variety of proteomic and genomic databases as opposed to non-redundant sequence databases such as the International Protein Index database (18) used in our present study that provide consistent accession number formats (e.g. Uniprot) for identified proteins. Collectively these factors make comparison of these large proteomic datasets difficult. As such, we hope that the dataset of saliva proteins we present here will serve as a model for publishing large scale proteomic data to the growing number of research groups investigating this clinically important bodily fluid, helping the dissemination and comparison of proteomic datasets obtained from future studies.
| ACKNOWLEDGMENTS |
|---|
| FOOTNOTES |
|---|
Published, MCP Papers in Press, August 15, 2005, DOI 10.1074/mcp.D500008-MCP200
1 The abbreviations used are: FFE, free flow electrophoresis; µLC, microcapillary LC. ![]()
* This work was supported in part by funding from the Minnesota Medical Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. ![]()
S The on-line version of this article (available at http://www.mcponline.org) contains supplemental material. ![]()
** To whom correspondence should be addressed: Dept. of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, 6-155 Jackson Hall, 321 Church St. S.E., Minneapolis, MN 55455. Tel.: 612-624-5249; Fax: 612-624-0432; E-mail: tgriffin{at}umn.edu
| REFERENCES |
|---|
|
|
|---|
B-dependent cytokines: TNF-
, IL-1-
, IL-6, and IL-8 in different oral fluids from oral lichen planus patients.
Clin. Immunol.
114, 278
283[Medline]This article has been cited by other articles:
![]() |
C. Planque, V. Kulasingam, C. R. Smith, K. Reckamp, L. Goodglick, and E. P. Diamandis Identification of Five Candidate Lung Cancer Biomarkers by Proteomics Analysis of Conditioned Media of Four Lung Cancer Cell Lines Mol. Cell. Proteomics, December 1, 2009; 8(12): 2746 - 2758. [Abstract] [Full Text] [PDF] |
||||
![]() |
M. Kesimer, S. Kirkham, R. J. Pickles, A. G. Henderson, N. E. Alexis, G. DeMaria, D. Knight, D. J. Thornton, and J. K. Sheehan Tracheobronchial air-liquid interface cell culture: a model for innate mucosal defense of the upper airways? Am J Physiol Lung Cell Mol Physiol, January 1, 2009; 296(1): L92 - L100. [Abstract] [Full Text] [PDF] |
||||
![]() |
S.-J. Li, M. Peng, H. Li, B.-S. Liu, C. Wang, J.-R. Wu, Y.-X. Li, and R. Zeng Sys-BodyFluid: a systematical database for human body fluid proteome research Nucleic Acids Res., January 1, 2009; 37(suppl_1): D907 - D912. [Abstract] [Full Text] [PDF] |
||||
![]() |
J. S. Gnanandarajah, C. M. T. Dvorak, C. R. Johnson, and M. P. Murtaugh Presence of free haptoglobin alpha 1S-subunit in acute porcine reproductive and respiratory syndrome virus infection J. Gen. Virol., November 1, 2008; 89(11): 2746 - 2753. [Abstract] [Full Text] [PDF] |
||||
![]() |
H. Xie, G. Onsongo, J. Popko, E. P. de Jong, J. Cao, J. V. Carlis, R. J. Griffin, N. L. Rhodus, and T. J. Griffin Proteomics Analysis of Cells in Whole Saliva from Oral Cancer Patients via Value-added Three-dimensional Peptide Fractionation and Tandem Mass Spectrometry Mol. Cell. Proteomics, March 1, 2008; 7(3): 486 - 498. [Abstract] [Full Text] [PDF] |
||||
![]() |
P. A. Grimsrud, M. J. Picklo Sr., T. J. Griffin, and D. A. Bernlohr Carbonylation of Adipose Proteins in Obesity and Insulin Resistance: Identification of Adipocyte Fatty Acid-binding Protein as a Cellular Target of 4-Hydroxynonenal Mol. Cell. Proteomics, April 1, 2007; 6(4): 624 - 637. [Abstract] [Full Text] [PDF] |
||||
![]() |
Y. Andersch-Bjorkman, K. A. Thomsson, J. M. Holmen Larsson, E. Ekerhovd, and G. C. Hansson Large Scale Identification of Proteins, Mucins, and Their O-Glycosylation in the Endocervical Mucus during the Menstrual Cycle Mol. Cell. Proteomics, April 1, 2007; 6(4): 708 - 716. [Abstract] [Full Text] [PDF] |
||||
![]() |
D. Soo-Quee Koh and G. Choon-Huat Koh The use of salivary biomarkers in occupational and environmental medicine Occup. Environ. Med., March 1, 2007; 64(3): 202 - 210. [Full Text] [PDF] |
||||
![]() |
W.-J. Qian, J. M. Jacobs, T. Liu, D. G. Camp II, and R. D. Smith Advances and Challenges in Liquid Chromatography-Mass Spectrometry-based Proteomics Profiling for Clinical Applications Mol. Cell. Proteomics, October 1, 2006; 5(10): 1727 - 1744. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| All ASBMB Journals | Journal of Biological Chemistry |
| Journal of Lipid Research | ASBMB Today |