title: Improved enrichment strategies for phosphorylated peptides on titanium dioxide using methyl esterification and pH gradient elution.
abstract: Improvements to phosphopeptide enrichment protocols employing titanium dioxide (TiO2) are described and applied to identification of phosphorylation sites on recombinant human cyclin-dependent kinase 2 (CDK2). Titanium dioxide binds phosphopeptides under acidic conditions, and they can be eluted under basic conditions. However, some nonphosphorylated peptides, particularly acidic peptides, bind and elute under these conditions as well. These nonphosphorylated peptides contribute significantly to ion suppression of phosphopeptides and also increase sample complexity. We show here that the conversion of peptide carboxylates to their corresponding methyl esters sharply reduces nonspecific binding, improving the selectivity for phosphopeptides, just as has been reported for immobilized metal affinity chromatography (IMAC) columns. We also present evidence that monophosphorylated peptides can be effectively fractionated from multiply phosphorylated peptides, as well as acidic peptides, via stepwise elution from TiO2 using pH step gradients from pH 8.5 to pH 11.5. These approaches were applied to human CDK2 phosphorylated in vitro by yeast CAK1p in the absence of cyclin. We confirmed phosphorylation at T160, a site previously documented and shown to be necessary for CDK2 activity. However, we also discovered several novel sites of partial phosphorylation at S46, T47, T165, and Y168 when ion-suppressing nonphosphorylated peptides were eliminated using the new protocols.
authors.affiliation: National Resource for Proteomics and Pathways, University of Michigan, Ann Arbor, MI 48109, USA. essimon@umich.edu
authors.forename: Eric S
authors.initials: ES
authors.lastname: Simon
authors.forename: Matthew
authors.initials: M
authors.lastname: Young
authors.forename: Antonia
authors.initials: A
authors.lastname: Chan
authors.forename: Zhao-Qin
authors.initials: ZQ
authors.lastname: Bao
authors.forename: Philip C
authors.initials: PC
authors.lastname: Andrews
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002264:Carboxylic Acids / Q000737:chemistry / Q000378:metabolism; D005591:Chemical Fractionation; D051357:Cyclin-Dependent Kinase 2 / Q000737:chemistry / Q000378:metabolism; D004951:Esterification; D006801:Humans; D010748:Phosphopeptides / Q000737:chemistry* / Q000378:metabolism*; D010766:Phosphorylation; D018892:Proton-Motive Force; D013379:Substrate Specificity; D053719:Tandem Mass Spectrometry; D014025:Titanium / Q000737:chemistry*
chemical_list: D002264:Carboxylic Acids; D010748:Phosphopeptides; C009495:titanium dioxide; D014025:Titanium; D051357:Cyclin-Dependent Kinase 2