Phosphorylation, glycosylation and amino acid sequence of component PP3 from the proteose peptone fraction of bovine milk

Abstract

SummaryComponent PP3 is a phosphorylated glycoprotein with an apparent molecular mass of 28 kDa isolated from the proteose peptone fraction of bovine milk. The function of the protein is not known. The primary structure has been determined and shown to contain 135 amino acid residues (EMBL accession no. P80195). It was phosphorylated at Ser29, Ser34, Ser38, Ser40and Ser46. Two O-linked carbohydrate groups were found at Thr16and Thr86, while one N-linked carbohydrate group was present at Asn77. Thr16was only ∼ 50% glycosylated. The amino sugar detected by the amino acid analyser at Thr86was mainly galactosamine but a small amount of glucosamine was also present. The amino sugars found in the carbohydrate group linked to Asn77were both glucosamine and galactosamine. A fragment of PP3 has been isolated from milk and shown to correspond to residues 54–135. This fragment was probably generated by plasmin hydrolysing the Arg53–Ser54bond.