Shotgun proteomics refers to the use of bottom-up proteomics techniques in identifying proteins in complex mixtures using a combination of high performance liquid chromatography combined with mass spectrometry. The name is derived from shotgun sequencing of DNA which is itself named after the rapidly expanding, quasi-random firing pattern of a shotgun. The most common method of shotgun proteomics starts with the proteins in the mixture being digested and the resulting peptides are separated by liquid chromatography. Tandem mass spectrometry is then used to identify the peptides.
Shotgun proteomics arose from the difficulties of using previous technologies to separate complex mixtures. In 1975, two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) was described by O’Farrell and Klose with the ability to resolve complex protein mixtures. The development of matrix-assisted laser desorption ionization (MALDI), electrospray ionization (ESI), and database searching continued to grow the field of proteomics. However these methods still had difficulty identifying and separating low-abundance proteins, aberrant proteins, and membrane proteins. Shotgun proteomics emerged as a method that could resolve even these proteins.
Shotgun proteomics allows global protein identification as well as the ability to systematically profile dynamic proteomes. It also avoids the modest separation efficiency and poor mass spectral sensitivity associated with intact protein analysis.
The dynamic exclusion filtering that is often used in shotgun proteomics maximizes the number of identified proteins at the expense of random sampling. This problem may be exacerbated by the undersampling inherent in shotgun proteomics.
Cells containing the protein complement desired are grown. Proteins are then extracted from the mixture and digested with a protease to produce a peptide mixture. The peptide mixture is then loaded directly onto a microcapillary column and the peptides are separated by hydrophobicity and charge. As the peptides elute from the column, they are ionized and separated by m/z in the first stage of tandem mass spectrometry. The selected ions undergo collision-induced dissociation or other process to induce fragmentation. The charged fragments are separated in the second stage of tandem mass spectrometry.
The "fingerprint" of each peptide's fragmentation mass spectrum is used to identify the protein from which they derive by searching against a sequence database with commercially available software (e.g. SEQUEST or MASCOT). Examples of sequence databases are the Genpept database or the PIR database. After the database search, each peptide-spectrum match (PSM) needs to be evaluated for validity. This analysis allows researchers to profile various biological systems.
Issues with peptide identification
Peptides that are degenerate (shared by two or more proteins in the database) may make it difficult to identify the protein to which they belong. Additionally, some proteome samples of vertebrates have a large number of paralogs. Finally, alternative splicing in higher eukaryotes can result in many identical protein subsequences.
With the human genome sequenced, the next step is the verification and functional annotation of all predicted genes and their protein products. Shotgun proteomics can be used for functional classification or comparative analysis of these protein products. It can be used in projects ranging from large-scale whole proteome to focusing on a single protein family. It can be done in research labs or commercially.
One example of this is a study by Washburn, Wolters, & Yates in which they used shotgun proteomics on the proteome of a Saccharomyces cerevisiae strain grown to mid-log phase. They were able to detect and identify 1,484 proteins as well as identify proteins rarely seen in proteome analysis, including low-abundance proteins like transcription factors and protein kinases. They were also able to identify 131 proteins with three or more predicted transmembrane domains.
Vaisar et al. uses shotgun proteomics to implicate protease inhibition and complement activation in the antiinflammatory properties of high-density lipoprotein. In a study by Lee et al., higher expression level of hnRNP A2/B1 and Hsp90 were observed in human hepatoma HepG2 cells than in wild type cells. This led to a search for reported functional roles mediated in concert by both these multifunctional cellular chaperones.
- Shotgun lipidomics
- Protein mass spectrometry
- Top-down proteomics
- Bottom-up proteomics
- Mass spectrometry software
- Alves, P; Arnold, RJ; Novotny, MV; Radivojac, P; Reilly, JP; Tang, H (2007). "Advancement in protein inference from shotgun proteomics using peptide detectability.". Pacific Symposium on Biocomputing. Pacific Symposium on Biocomputing: 409–20. PMID 17990506.
- Washburn MP, Wolters D, Yates JR (2001). "Large-scale analysis of the yeast proteome by multidimensional protein identification technology". Nat. Biotechnol. 19 (3): 242–247. doi:10.1038/85686. PMID 11231557.
- Wolters DA, Washburn MP, Yates JR (2001). "An automated multidimensional protein identification technology for shotgun proteomics". Anal. Chem. 73 (23): 5683–5690. doi:10.1021/ac010617e. PMID 11774908.
- Hu L, Ye M, Jiang X, Feng S, Zou H (2007). "Advances in hyphenated analytical techniques for shotgun proteome and peptidome analysis--a review". Anal. Chim. Acta 598 (2): 193–204. doi:10.1016/j.aca.2007.07.046. PMID 17719892.
- Fournier ML, Gilmore JM, Martin-Brown SA, Washburn MP (2007). "Multidimensional separations-based shotgun proteomics". Chem. Rev. 107 (8): 3654–86. doi:10.1021/cr068279a. PMID 17649983.
- Nesvizhskii AI (2007). "Protein identification by tandem mass spectrometry and sequence database searching". Methods Mol. Biol. 367: 87–119. doi:10.1385/1-59745-275-0:87. ISBN 1-59745-275-0. PMID 17185772.
- O'Farrell, PH (May 25, 1975). "High resolution two-dimensional electrophoresis of proteins". The Journal of Biological Chemistry 250 (10): 4007–21. PMC 2874754. PMID 236308.
- Klose, J (1975). "Protein mapping by combined isoelectric focusing and electrophoresis of mouse tissues. A novel approach to testing for induced point mutations in mammals". Humangenetik 26 (3): 231–43. PMID 1093965.
- Wu CC, MacCoss MJ (2002). "Shotgun proteomics: tools for the analysis of complex biological systems". Current Opinion in Molecular Therapeutics 4 (3): 242–50. PMID 12139310.
- Zhang B, VerBerkmoes NC, Langston MA, Uberbacher E, Hettich RL, Samatova NF (2006). "Detecting Differential and Correlated Protein Expression in Label-Free Shotgun Proteomics". J Proteome Res. 5 (11): 2909–2918. doi:10.1021/pr0600273. PMID 17081042.
- Tolmachev AV, Monroe ME, Purvine SO, Moore RJ, Jaitly N, Adkins JN, Anderson GA, Smith RD (2008). "Characterization of strategies for obtaining confident identifications in bottom-up proteomics measurements using hybrid FT MS instruments". Anal Chem. 80 (22): 8514–8525. doi:10.1021/ac801376g. PMC 2692492. PMID 18855412.
- Eng, Jimmy K.; McCormack, Ashley L.; Yates, John R. (1 November 1994). "An approach to correlate tandem mass spectral data of peptides with amino acid sequences in a protein database". Journal of the American Society for Mass Spectrometry 5 (11): 976–989. doi:10.1016/1044-0305(94)80016-2. PMID 24226387.
- Cerqueira, Fabio R; Ferreira, Ricardo S; Oliveira, Alcione P; Gomes, Andreia P; Ramos, Humberto JO; Graber, Armin; Baumgartner, Christian (1 January 2012). "MUMAL: Multivariate analysis in shotgun proteomics using machine learning techniques". BMC Genomics 13: S4. doi:10.1186/1471-2164-13-S5-S4.
- Vaisar, Tomas; Pennathur, Subramaniam; Green, Pattie S.; Gharib, Sina A.; Hoofnagle, Andrew N.; Cheung, Marian C.; Byun, Jaeman; Vuletic, Simona; Kassim, Sean; Singh, Pragya; Chea, Helen; Knopp, Robert H.; Brunzell, John; Geary, Randolph; Chait, Alan; Zhao, Xue-Qiao; Elkon, Keith; Marcovina, Santica; Ridker, Paul; Oram, John F.; Heinecke, Jay W. (1 March 2007). "Shotgun proteomics implicates protease inhibition and complement activation in the antiinflammatory properties of HDL". Journal of Clinical Investigation 117 (3): 746–756. doi:10.1172/JCI26206. PMC 1804352. PMID 17332893.
- Lee, Chih-Lei; Hsiao, He-Hsuan; Lin, Chia-Wei; Wu, Szu-Pei; Huang, Shiuan-Yi; Wu, Chi-Yue; Wang, Andrew H.-J.; Khoo, Kay-Hooi (1 December 2003). "Strategic shotgun proteomics approach for efficient construction of an expression map of targeted protein families in hepatoma cell lines". Proteomics 3 (12): 2472–2486. doi:10.1002/pmic.200300586. PMID 14673797.
- Marcotte EM (2007). "How do shotgun proteomics algorithms identify proteins?". Nat. Biotechnol. 25 (7): 755–7. doi:10.1038/nbt0707-755. PMID 17621303.
- Veenstra, Timothy D.; Yates, John R. (2006). Proteomics for Biological Discovery. New York: Wiley-Liss. ISBN 0-471-16005-9.
- Yates JR, Ruse CI, Nakorchevsky A (2009). "Proteomics by Mass Spectrometry: Approaches, Advances, and Applications" (PDF). Annu. Rev. Biomed. Eng. 11: 49–79. doi:10.1146/annurev-bioeng-061008-124934. PMID 19400705.
- Liao, L; McClatchy, DB; Yates, JR (Jul 16, 2009). "Shotgun proteomics in neuroscience". Neuron 63 (1): 12–26. doi:10.1016/j.neuron.2009.06.011. PMC 2746002. PMID 19607789.