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On studying protein phosphorylation patterns using bottom-up LC-MS/MS: the case of human alpha-casein
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Pharmaceutical Biosciences, MMS, Medical Mass Spectrometry.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Pharmaceutical Biosciences, MMS, Medical Mass Spectrometry.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Pharmaceutical Biosciences, MMS, Medical Mass Spectrometry.
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2007 (English)In: The Analyst, ISSN 0003-2654, E-ISSN 1364-5528, Vol. 132, no 8, 768-776 p.Article in journal (Refereed) Published
Abstract [en]

Most proteomics studies involving mapping post-translational modifications, such as the phosphorylation of serine and threonine, are performed today using the 'bottom-up' approach. This approach involves enzymatic cleavage of proteins, most often by trypsin, with subsequent nano-LC-MS/MS. The occupancy rates of phosphosites in proteins may differ by orders of magnitude, and thus the occupancy rate must be reported for each occupied phosphosite. To highlight potential pitfalls in quantifying the occupancy rates, αs1- casein from human milk was selected as a model molecule representing moderately phosphorylated proteins. For this purpose, human milk from one Caucasian woman in the eighth month of lactation was used. The phosphorylation level of caseins is believed to have major implications for the formation of micelles that are involved in delivering valuable calcium phosphate and other minerals to the new-born. Human αs1-casein has been reported to be much less phosphorylated than ruminant caseins, which may indicate a different function of caseins in humans. Revealing the phosphorylation pattern in human casein can thus shed light on its function. The current study found that the sequence region between the residues Ser70 and Ser76 in human αs1-casein is in fact phosphorylated, contrary to previous knowledge. The site of the most abundant phosphorylation is Ser75, in agreement with the known action of the mammary gland casein kinase. There is evidence for the second phosphorylation in that region, possibly at Ser73. Earlier reported positions of phosphorylations at Ser18 and Ser26 are also confirmed, but not the dominance of Ser18 phosphorylation. The occupancy rates at Ser18, Ser26 and Ser75 are estimated to be (7 ± 2), (20 ± 6) and (27 ± 9)%, respectively. Owing to differences in the ionization efficiency between phosphorylated and unphosphorylated peptides a 30% error margin is added to the occupancy rates. The highlighted pitfalls of the bottom-up strategy include the sensitivity of enzymes to proximal acidic and phosphorylated residues and the presence of multiple isoforms, including unexpected ones, of the tryptic peptides. The utility of the earlier introduced PhosTS_hunter and ModifiComb approaches for evading the latter pitfall is demonstrated.

Place, publisher, year, edition, pages
2007. Vol. 132, no 8, 768-776 p.
National Category
Medical and Health Sciences
URN: urn:nbn:se:uu:diva-95355DOI: 10.1039/b701902eISI: 000248229700017PubMedID: 17646876OAI: oai:DiVA.org:uu-95355DiVA: diva2:169532
Available from: 2007-01-17 Created: 2007-01-17 Last updated: 2011-01-28Bibliographically approved
In thesis
1. New Proteomics Methods and Fundamental Aspects of Peptide Fragmentation
Open this publication in new window or tab >>New Proteomics Methods and Fundamental Aspects of Peptide Fragmentation
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Nya Proteomik Metoder och Fundamentala Aspekter av Peptid Fragmentering
Abstract [en]

The combination of collision-activated dissociation, (CAD) and electron capture dissociation, (ECD) yielded a 125% increase in protein identification. The S-score was developed for measuring the information content in MS/MS spectra. This measure made it possible to single out good quality spectra that were not identified by a search engine. Poor quality MS/MS data was filtered out, streamlining the identification process.

A proteomics grade de novo sequencing approach was developed enabling to almost completely sequence 19% of all MS/MS data with 95% reliability in a typical proteomics experiment.

A new tool, Modificomb, for identifying all types of modifications in a fast, reliable way was developed. New types of modifications have been discovered and the extent of modifications in gel based proteomics turned out to be greater than expected.

PhosTShunter was developed for sensitive identification of all phosphorylated peptides in an MS/MS dataset.

Application of these programs to human milk samples led to identification of a previously unreported and potentially biologically important phosphorylation site.

Peptide fragmentation has been studied. It was shown emphatically on a dataset of 15.000 MS/MS spectra that CAD and ECD have different cleavage preferences with respect to the amino acid context.

Hydrogen rearrangement involving z• species has been investigated. Clear trends have been unveiled. This information elucidated the mechanism of hydrogen transfer.

Partial side-chain losses in ECD have been studied. The potential of these ions for reliably distinguishing Leu/Iso residues was shown. Partial sidechain losses occurring far away from the cleavage site have been detected.

A strong correlation was found between the propensities of amino acids towards peptide bond cleavage employing CAD and the propensity of amino acids to accept in solution backbone-backbone H-bonds and form stable motifs. This indicated that the same parameter governs formation of secondary structures in solution and directs fragmentation in peptide ions by CAD.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2007. 56 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 264
Bioinformatics, Proteomics, Peptide fragmentation, Bioinformatik
urn:nbn:se:uu:diva-7438 (URN)978-91-554-6775-X (ISBN)
Public defence
2007-02-08, B21, BMC, Husargatan 3, Uppsala, 14:15
Available from: 2007-01-17 Created: 2007-01-17 Last updated: 2013-09-04Bibliographically approved

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