Analysis of phosphoprotein p 19 by liquid chromatography/mass spectrometry: Identification of two proline-directed serine phosphorylation sites and a blocked amino terminus

James E. Labdon, Edward Nieves, Ulrich K. Schubart

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Abstract

p19 is a highly conserved 19-kDa cytosolic protein that undergoes phosphorylation in mammalian cells upon activation of several distinct signal transduction pathways. Its expression is widespread but developmentally regulated. To determine the in vivo phosphorylation site(s) of p19, the protein was purified from bovine brain and resolved into the unphosphorylated form (p19) and a mixture of the two predominant phospho-forms (pp19). Proteolytic fragments of p19 and pp19 were examined by liquid chromatography/ mass spectrometry (LC/MS). We detected ion masses corresponding to fragments spanning the entire amino acid sequence as deduced from the cDNA except for those predicted to contain an unmodified amino terminus. Instead, the digests revealed ions corresponding to peptides lacking the initiator methionine and containing an N-acetylated alanine at the amino terminus. The analysis of pp19, but not that of p19, revealed two sets of ions representing peptides whose m/z values differed by 80 atomic mass units, the incremental mass of a phosphate residue. These putative phosphate-bearing peptides were sensitive to alkaline phosphatase treatment. Using combined trypsin and V8 protease digestions, the phosphorylation sites were mapped to Ser-25 and Ser-38, in the peptides Leu-Ile-Leu-Ser*-Pro-Arg and Phe-Pro-Leu-Ser*-Pro-Pro-Lys, respectively. Interestingly, both phosphoserines are in a very similar sequence context, suggesting that a single proline-directed serine protein kinase, possibly p34cdc2, is responsible for phosphorylation of both sites in vivo.

Original languageEnglish (US)
Pages (from-to)3506-3513
Number of pages8
JournalJournal of Biological Chemistry
Volume267
Issue number5
StatePublished - Feb 15 1992

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Phosphorylation
Phosphoproteins
Liquid chromatography
Proline
Liquid Chromatography
Serine
Mass spectrometry
Mass Spectrometry
Ions
Peptides
prolylarginine
Proline-Directed Protein Kinases
Bearings (structural)
Phosphates
Phosphoserine
Signal transduction
Protein-Serine-Threonine Kinases
Alanine
Methionine
Trypsin

ASJC Scopus subject areas

  • Biochemistry

Cite this

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title = "Analysis of phosphoprotein p 19 by liquid chromatography/mass spectrometry: Identification of two proline-directed serine phosphorylation sites and a blocked amino terminus",
abstract = "p19 is a highly conserved 19-kDa cytosolic protein that undergoes phosphorylation in mammalian cells upon activation of several distinct signal transduction pathways. Its expression is widespread but developmentally regulated. To determine the in vivo phosphorylation site(s) of p19, the protein was purified from bovine brain and resolved into the unphosphorylated form (p19) and a mixture of the two predominant phospho-forms (pp19). Proteolytic fragments of p19 and pp19 were examined by liquid chromatography/ mass spectrometry (LC/MS). We detected ion masses corresponding to fragments spanning the entire amino acid sequence as deduced from the cDNA except for those predicted to contain an unmodified amino terminus. Instead, the digests revealed ions corresponding to peptides lacking the initiator methionine and containing an N-acetylated alanine at the amino terminus. The analysis of pp19, but not that of p19, revealed two sets of ions representing peptides whose m/z values differed by 80 atomic mass units, the incremental mass of a phosphate residue. These putative phosphate-bearing peptides were sensitive to alkaline phosphatase treatment. Using combined trypsin and V8 protease digestions, the phosphorylation sites were mapped to Ser-25 and Ser-38, in the peptides Leu-Ile-Leu-Ser*-Pro-Arg and Phe-Pro-Leu-Ser*-Pro-Pro-Lys, respectively. Interestingly, both phosphoserines are in a very similar sequence context, suggesting that a single proline-directed serine protein kinase, possibly p34cdc2, is responsible for phosphorylation of both sites in vivo.",
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TY - JOUR

T1 - Analysis of phosphoprotein p 19 by liquid chromatography/mass spectrometry

T2 - Identification of two proline-directed serine phosphorylation sites and a blocked amino terminus

AU - Labdon, James E.

AU - Nieves, Edward

AU - Schubart, Ulrich K.

PY - 1992/2/15

Y1 - 1992/2/15

N2 - p19 is a highly conserved 19-kDa cytosolic protein that undergoes phosphorylation in mammalian cells upon activation of several distinct signal transduction pathways. Its expression is widespread but developmentally regulated. To determine the in vivo phosphorylation site(s) of p19, the protein was purified from bovine brain and resolved into the unphosphorylated form (p19) and a mixture of the two predominant phospho-forms (pp19). Proteolytic fragments of p19 and pp19 were examined by liquid chromatography/ mass spectrometry (LC/MS). We detected ion masses corresponding to fragments spanning the entire amino acid sequence as deduced from the cDNA except for those predicted to contain an unmodified amino terminus. Instead, the digests revealed ions corresponding to peptides lacking the initiator methionine and containing an N-acetylated alanine at the amino terminus. The analysis of pp19, but not that of p19, revealed two sets of ions representing peptides whose m/z values differed by 80 atomic mass units, the incremental mass of a phosphate residue. These putative phosphate-bearing peptides were sensitive to alkaline phosphatase treatment. Using combined trypsin and V8 protease digestions, the phosphorylation sites were mapped to Ser-25 and Ser-38, in the peptides Leu-Ile-Leu-Ser*-Pro-Arg and Phe-Pro-Leu-Ser*-Pro-Pro-Lys, respectively. Interestingly, both phosphoserines are in a very similar sequence context, suggesting that a single proline-directed serine protein kinase, possibly p34cdc2, is responsible for phosphorylation of both sites in vivo.

AB - p19 is a highly conserved 19-kDa cytosolic protein that undergoes phosphorylation in mammalian cells upon activation of several distinct signal transduction pathways. Its expression is widespread but developmentally regulated. To determine the in vivo phosphorylation site(s) of p19, the protein was purified from bovine brain and resolved into the unphosphorylated form (p19) and a mixture of the two predominant phospho-forms (pp19). Proteolytic fragments of p19 and pp19 were examined by liquid chromatography/ mass spectrometry (LC/MS). We detected ion masses corresponding to fragments spanning the entire amino acid sequence as deduced from the cDNA except for those predicted to contain an unmodified amino terminus. Instead, the digests revealed ions corresponding to peptides lacking the initiator methionine and containing an N-acetylated alanine at the amino terminus. The analysis of pp19, but not that of p19, revealed two sets of ions representing peptides whose m/z values differed by 80 atomic mass units, the incremental mass of a phosphate residue. These putative phosphate-bearing peptides were sensitive to alkaline phosphatase treatment. Using combined trypsin and V8 protease digestions, the phosphorylation sites were mapped to Ser-25 and Ser-38, in the peptides Leu-Ile-Leu-Ser*-Pro-Arg and Phe-Pro-Leu-Ser*-Pro-Pro-Lys, respectively. Interestingly, both phosphoserines are in a very similar sequence context, suggesting that a single proline-directed serine protein kinase, possibly p34cdc2, is responsible for phosphorylation of both sites in vivo.

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