Phosvitin, a Phosphoglycoprotein

The purified preparations of phosvitin, the phosphoprotein of hens' egg yolk, contain 6.5% carbohydrate. Analysis showed 6 residues of hexose, 5 of glucosamine (assumed to occur as N-acetyl derivatives), and 2 of sialic acid per molecular weight of 40,000. In order to determine the mode of attachment of the polysaccharide moiety, phosvitin has been digested with pronase. A sequence of fractionation steps that included gel filtration, chromatography on DEAE-cellulose and DEAE-Sephadex afforded isolation of a glycopeptide containing all of the carbohydrate attached to an aspartic acid residue. Edman degradation on this peptide and on a smaller peptide isolated from the tryptic digest of a partially dephosphorylated preparation of the glycopeptide yielded the sequence [see PDF for sequence] In addition to the demonstration of the occurrence of a sequence of 8 phosphoserine residues, at least 3 of the 5 nonphosphorylated serines of phosvitin were located near the single carbohydrate moiety of this protein.