Protein Reduction, Alkylation, and Digestion

Aliquots with 100 mg of proteins from each sample were added to 100 ml of 200 mM triethyl ammonium bicarbonate TEAB (Sigma-Aldrich, St. Louis, MO). Reduction was performed by adding 5 ml of 200 mM tris (2-carboxyethyl) phosphine TCEP (Sigma-Aldrich, St. Louis, MO) to each replicate and incubating for 1 h at 55°C. Alkylation was carried out by adding 5 ml of 375 mM iodoacetamide (Bio-Rad Laboratories, Hercules, CA) to each sample and incubating for 30 min at room temperature. After alkylation, 1 ml of pre-chilled acetone was added and precipitation was allowed to proceed for 3 h at 20°C. Acetone-precipitated protein pellets were suspended in 100 ml of 200 mM TEAB and digested overnight at 37°C with 2.5 μg of sequencing grade modified trypsin (Promega Corp., Madison, WI) as previously described [13 (link),14 (link)].