Sample preparation for proteomic analysis was performed as described previously [13 (link)]. Peptides were resuspended in 5% formic acid and analyzed by liquid chromatography tandem mass spectrometry (MS/MS). Ten microliters was injected onto a C18 trap column (Thermo Scientific Acclaim), flushed into an analytical column (Agilent), and eluted via a mobile phase gradient of 5%–80% solvent B over 120 minutes. The eluted material was directly applied to the nanospray source of a QSTAR Elite instrument (Applied Biosystems), and data analysis was conducted with Analyst 2.0 (Applied Biosystems).