Mass spectrometry of peptides:. MALDI Mass Spectrometry: Each sample was spotted in triplicate using 1 µL aliquots on the MALDI target wells, and, before drying mixed with an equal volume of matrix consisting of 5 g/L CHCA in 1:1 ACN:0.1% TFA, 10 mM (NH4)2SO3. Ten, 1000 shot spectra were acquired from each sample well and merged using the methodology described previously 22 using an Applied Biosystems 4800 Proteomics Analyzer mass spectrometer (Framingham, MA) operated in linear negative mode. The spectra were calibrated internally using the tyrosinated CNBr cleaved peptide of the chicken erythrocyte a-isotype and a synthetic unmodified tubulin peptide. MSMS fragmentation of peptides was accomplished in negative ion mode by isolating specific m/z peptides observed in the linear spectra by using the timed ion selector of the instrument and collecting reflector mode fragment ion spectra. External calibration of the MSMS spectra was accomplished using glu-fibrinopeptide peptide y-series fragments in positive ion mode. The accurate mass of the unidentified peak was determined using a SimulTOF MALDI-TOF instrument (SimulTOF, Sudbury, MA) operated at 8000 resolution In negative ion, reflector mode. This instrument enabled the determination of monoisotopic rather than average values for the m/z. Electrospray Mass Spectrometry: Peptide samples were injected into an Agilent 6550 iFunnel Q-TOF mass spectrometer operated in the negative ion mode equipped with a JetStream ESI or nanoESI source. For separation, reversed phase chromatography (C18-Eclipse, 2.1 mm x 50 mm, 1.8 µm) was performed with a gradient of 5-45% B (acetonitrile, 0.1% formic acid) in 10 minutes at a flow rate of 150 µL/min. Eluted peptides were analyzed via the mass spectrometer operated at m/z 300-1700 in high sensitivity mode. Tandem MS spectra were collected via collision induced dissociation at 40V. Spectra were manually inspected for the ions of interest corresponding to predicted m/z values. -GEGEEEGEEY