title: Selective isolation at the femtomole level of phosphopeptides from proteolytic digests using 2D-NanoLC-ESI-MS/MS and titanium oxide precolumns.
abstract: Selective detection of phosphopeptides from proteolytic digests is a challenging and highly relevant task in many proteomics applications. Often phosphopeptides are present in small amounts and need selective isolation or enrichment before identification. Here we report a novel automated method for the enrichment of phosphopeptides from complex mixtures. The method employs a two-dimensional column setup, with titanium oxide-based solid-phase material (Titansphere) as the first dimension and reversed-phase material as the second dimension. Phosphopeptides are separated from nonphosphorylated peptides by trapping them under acidic conditions on a TiO(2) precolumn. Nonphosphorylated peptides break through and are trapped on a reversed-phase precolumn after which they are analyzed by nanoflow LC-ESI-MS/MS. Subsequently, phosphopeptides are desorbed from the TiO(2) column under alkaline conditions, reconcentrated onto the reversed-phase precolumn, and analyzed by nanoflow LC-ESI-MS/MS. The selectivity and practicality of using TiO(2) precolumns for trapping phosphopeptides are demonstrated via the analysis of a model peptide RKISASEF, in a 1:1 mixture of a non- and a monophosphorylated form. A sample of 125 fmol of the phosphorylated peptide could easily be isolated from the nonphosphorylated peptide with a recovery above 90%. In addition, proteolytic digests of three different autophosphorylation forms of the 153-kDa homodimeric cGMP-dependent protein kinase are analyzed. From proteolytic digests of the fully autophosphorylated protein at least eight phosphorylation sites are identified, including two previously uncharacterized sites, namely, Ser-26 and Ser-44. Ser-26 is characterized as a minor phosphorylation site in purified PKG samples, while Ser-44 is identified as a novel in vitro autophosphorylation target. These results clearly show that TiO(2) has strong affinity for phosphorylated peptides, and thus, we conclude that this material has a high potential in the field of phosphoproteomics.
authors.affiliation: Department of Biomolecular Mass Spectrometry, Bijvoet Center for Biomolecular Research and Utrecht Institute for Pharmaceutical Sciences, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands. m.w.h.pinkse@pharm.uu.nl
authors.forename: Martijn W H
authors.initials: MW
authors.lastname: Pinkse
authors.forename: Pauliina M
authors.initials: PM
authors.lastname: Uitto
authors.forename: Martijn J
authors.initials: MJ
authors.lastname: Hilhorst
authors.forename: Bert
authors.initials: B
authors.lastname: Ooms
authors.forename: Albert J R
authors.initials: AJ
authors.lastname: Heck
mesh_terms: D001331:Automation; D002853:Chromatography, Liquid / Q000379:methods*; D036103:Nanotechnology; D010446:Peptide Fragments / Q000737:chemistry; D010748:Phosphopeptides / Q000032:analysis* / Q000302:isolation & purification; D011506:Proteins / Q000378:metabolism; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D014025:Titanium / Q000737:chemistry*
chemical_list: D010446:Peptide Fragments; D010748:Phosphopeptides; D011506:Proteins; C009495:titanium dioxide; D014025:Titanium