Phosphorylation of human plasma alpha2-Heremans-Schmid glycoprotein (human fetuin) in vivo.
A fraction of alpha2-Heremans-Schmid (alpha2-HS) glycoprotein (human fetuin) isolated from plasma was phosphorylated at serine-120 and serine-312 as shown by MS and peptide fragment sequencing after tryptic digestion. Serine-312-containing peptides were phosphorylated to 77% as determined from relative peak heights in the mass spectrum, which together with the phosphorylation of serine-120 implies a molar degree of phosphorylation of at least 1. Approximately 20% of the circulating fetuin plasma pool was phosphorylated to approx. 1 mol of phosphate/mol of protein. The remainder did not contain phosphate, resulting in an average phosphorylation degree for the protein in plasma of approx. 0.2 mol/mol. The isolated alpha2-HS glycoprotein was a heterodimer in which the entire C-terminal part of the connecting peptide including threonine-321 was present, but traces of C-terminally trimmed connecting peptide fragments were also found. The short B-chain was O-glycosylated to approx. 40%, whereas the N-glycosylation of asparagine-138 and asparagine-158 seemed to be 100%. This finding, for the first time, that circulating human plasma fetuin is partly phosphorylated, implies that the effects of phosphorylated alpha2-HS glycoprotein on insulin signal transduction seen in different cell systems could be relevant to its physiological function in vivo.
Haglund
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Department of Medical Biochemistry and Microbiology, Biomedical Centre, University of Uppsala, Box 582, SE-751 23 Uppsala, Sweden. asa..haglund@imbim.uu.se
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D000328:Adult; D000595:Amino Acid Sequence; D001798:Blood Proteins / Q000737:chemistry* / Q000302:isolation & purification; D002846:Chromatography, Affinity; D002852:Chromatography, Ion Exchange; D005260:Female; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D008875:Middle Aged; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry; D010748:Phosphopeptides / Q000737:chemistry / Q000302:isolation & purification; D010766:Phosphorylation; D010768:Phosphoserine / Q000032:analysis; D011247:Pregnancy; D011263:Pregnancy Trimester, Third; D011499:Protein Processing, Post-Translational; D060749:alpha-2-HS-Glycoprotein; D000509:alpha-Fetoproteins / Q000737:chemistry* / Q000302:isolation & purification
C556354:AHSG protein, human; D001798:Blood Proteins; D010446:Peptide Fragments; D010748:Phosphopeptides; D060749:alpha-2-HS-Glycoprotein; D000509:alpha-Fetoproteins; D010768:Phosphoserine