A Selective Precipitation Purification Procedure for Multiple Phosphoseryl-Containing Peptides and Methods for Their Identification

Abstract

Multiple phosphoseryl-containing sequences of proteins stabilize amorphous calcium phosphate and have been implicated in the regulation of biomineralization, protein structure, and enzyme activity. To facilitate studies on the identification and characterization of multiple phosphoseryl-containing sequences of proteins we have developed a simple and efficient purification procedure involving precipitation of Ca2+/ethanol-induced aggregates of the multiple phosphoseryl-containing peptides from enzymic digests. The multiple phosphoseryl-containing peptides of a tryptic digest of casein were selectively precipitated using Ca2+ (20 mol/mol protein) and 50% (v/v) ethanol at pH 3.5, 4.6, and 8.0. The individual peptides of the precipitates were purified using anion-exchange fast-performance liquid chromatography and reversed-phase HPLC and then identified by solid-phase sequence analysis and amino acid composition analysis after vapor-phase hydrolysis. Prior to sequence analysis the phosphopeptides were covalently coupled to arylamine membranes and the phosphoseryl residues converted to S-ethylcysteinyl residues by calcium-ion-catalyzed β-elimination in the presence of ethanethiol. The modified peptides were sequenced using an Applied Biosystems Inc. automated protein sequencer fitted with a membrane cartridge. Only peptides containing the cluster sequence -Ser(P)-Ser(P)-Ser(P)- were precipitated by Ca2+/ethanol at pH 3.5. The pH 4.6 precipitate contained all the cluster peptides plus two diphosphorylated peptides containing -Ser(P)-Glu-Ser(P)- and -Ser(P)-Thr-Ser(P)-. At pH 8.0, a monophosphorylated peptide containing -Ser(P)-Glu-Glu- was also present in the precipitate with the diphosphorylated and cluster peptides. The recoveries of the peptides in the pH 8.0 selective precipitate ranged from 83 to 95% of that present in the hydrolysate. All of the phosphopeptides in the casein tryptic hydrolysate were selectively precipitated at pH 8.0 by Ca2+/ethanol, except the two monophosphorylated peptides containing -Ser(P)-Ala-Glu- and -Ser(P)-Thr-Glu-, suggesting that, for a monophosphorylated peptide, the minimal motif for Ca2+/ethanol-induced precipitation is -Ser(P)-Glu-Glu-.