2.11. Mass spectrometry
Protein samples of interest were digested with trypsin either in
solution or in gel for tandem mass spectrometry (MS/MS) analysis
(Shevchenko et al., 1996). Digested peptide mixtures were separated
by on-line nanoLC and analyzed by electronspray MS/MS. The experiments
were performed on an Agilent 1200 nanoflow system connected
to an LTQ Orbitrap mass spectrometer (Thermo Electron, Bremen,
Germany) equipped with a nanoelectrospray ion source (Proxeon
Biosystems, Odense, Denmark). Data analysis was performed with the
MaxQuant software as described (Gruhler et al., 2005) supported by
Mascot or Andromeda as database search engines for peptide identifications.
Peaks in MS scans were determined as three-dimensional hills in
the mass-retention time plane. MS/MS peak lists were filtered to contain
at most six peaks per 100 Da interval and searched against a concatenated
forward and reversed version of the T. acidophilum and E. coli
databases (extracted from NCBInr) and Mouse database (International
Protein Index database: ftp://ftp.ebi.ac.uk/pub/databases/IPI/old/MOUSE)
and frequently observed contaminants like proteases and human keratins.
The initial mass tolerance in MS mode was set to 7 ppm and MS/MS mass
tolerance was 0.5 Da. Cysteine carbamidomethylation was searched as a
fixed modification, whereas N-acetyl protein and oxidized methionine
were searched as variable modifications