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A Protein Epitope Signature Tag (PrEST) library allows SILAC-based absolute quantification and multiplexed determination of protein copy numbers in cell lines.

Mass spectrometry-based proteomics increasingly relies on relative or absolute quantification. In relative quantification, stable isotope based methods often allow mixing at early stages of sample preparation, whereas for absolute quantification this has generally required recombinant expression of full length, labeled protein standards. Here we make use of a very large library of Protein Epitope Signature Tags (PrESTs) that has been developed in the course of the Human Protein Atlas Project. These PrESTs are expressed recombinantly in E. coli and they consist of a short and unique region of the protein of interest as well as purification and solubility tags. We first quantify a highly purified, stable isotope labeling of amino acids in cell culture (SILAC)-labeled version of the solubility tag and use it determine the precise amount of each PrEST by its SILAC ratios. The PrESTs are then spiked into cell lysates and the SILAC ratios of PrEST peptides to peptides from endogenous target proteins yield their cellular quantities. The procedure can readily be multiplexed, as we demonstrate by simultaneously determining the copy number of 40 proteins in HeLa cells. Among the proteins analyzed, the cytoskeletal protein vimentin was found to be most abundant with 20 million copies per cell, while the transcription factor and oncogene FOS only had 6000 copies. Direct quantification of the absolute amount of single proteins is possible via a SILAC experiment in which labeled cell lysate is mixed both with the heavy labeled solubility tag and with the corresponding PrEST. The SILAC-PrEST combination allows accurate and streamlined quantification of the absolute or relative amount of proteins of interest in a wide variety of applications.

Pubmed ID: 21964433 RIS Download

Mesh terms: Chromatography, Liquid | Enzyme-Linked Immunosorbent Assay | Epitopes | HeLa Cells | Humans | Isotope Labeling | Mass Spectrometry | Peptide Fragments | Proteins | Proteomics | Recombinant Proteins

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Ensembl

A collection of genome databases for vertebrates and other eukaryotic species with DNA and protein sequence search capabilities. The goal of Ensembl is to automatically annotate the genome, integrate this annotation with other available biological data and make the data publicly available via the web. The range of available data has also expanded to include comparative genomics, variation and regulatory data. Ensembl allows users to: upload and analyze data and save it to an Ensembl account; search for a DNA or protein sequence using BLAST or BLAT; fetch desired data from the public database, using the Perl API; download the databases via FTP in FASTA, MySQL and other formats; and mine Ensembl with BioMart and export sequences or tables in text, HTML, or Excel format. The DNA sequences and assemblies used in the Ensembl genebuild are provided by various projects around the world. Ensembl has entered into an agreement with UCSC and NCBI with regard to sequence identifiers in order to improve consistency between the data provided by different genome browsers. The site also links to the Ensembl blog with updates on new species and sequences as they are added to the database.

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