Quantitative Proteomic Analysis of A549 Cells Infected with Human Respiratory Syncytial Virus

Munday, Diane C.; Emmott, Edward; Surtees, Rebecca; Lardeau, Charles-Hugues; Wu, Weining; Duprex, W. Paul; Dove, Brian K.; Barr, John N.; Hiscox, Julian A.

The generation of the peak list, SILAC-and extracted ion currentbased quantitation, calculated posterior error probability and false discovery rate based on search engine results, peptide to protein group assembly, and data filtration and presentation were carried out using MaxQuant. The derived peak list was searched with the Mascot search engine (version 2.1.04; Matrix Science, London, UK) against a concatenated database combining 80,412 proteins from the Interna-tional Protein Index human protein database version 3.6 (forward database) and the reversed sequences of all proteins (reverse database). Alternatively, database searches were done using Mascot (Matrix Science) as the database search engine, and the results were saved as a peptide summary before quantification using MSQuant (http://msquant.sourceforge.net/). Parameters allowed included up to three missed cleavages and two labeled amino acids (arginine and lysine). Initial mass deviation of the precursor and fragment ions were up to 7 ppm and 0.5 Da, respectively. The minimum required peptide length was set to six amino acids. To pass statistical evaluation, posterior error probability (PEP) for peptide identification (MS/MS spectra) should be below or equal to 0.1. The required false positive rate was set to 5% at the peptide level. False positive rates or PEPs for peptides were calculated by recording the Mascot score and peptide sequence length-dependent histograms of forward and reverse hits separately and then using Bayes' theorem in deriving the probability of a false identification for a given top scoring peptide. At the protein level, the false discovery rate was calculated as the product of the PEP of the peptides of a protein where only peptides with distinct sequences were taken into account. If a group of identified peptide sequences belongs to multiple proteins and these proteins cannot be distinguished with no unique peptide reported, these proteins are reported as a protein group in MaxQuant. Proteins were quantified if at least one MaxQuant-quantifiable SILAC pair was present. Identification was set to a false discovery rate of 1% with a minimum of two quantifiable peptides. The set value for false positive rate/PEP at the peptide level ensures that the worst identified peptide has a probability of 0.05 of being false, and proteins are sorted by the product of the false positive rates of their peptides where only peptides with distinct sequences are recognized. During the search, proteins are successively included starting with the best identified proteins until a false discovery rate of 1% is reached, an estimation based on the fraction of reverse protein hits. Enzyme specificity was set to trypsin allowing for cleavage N-terminal to proline and between aspartic acid and proline. Carbamidomethylation of cysteine was searched as a fixed modification; N-acetyl protein and oxidation of methionine were searched as variable modifications.