PHOSPHORYLATION, GLYCOSYLATION AND AMINO-ACID-SEQUENCE OF COMPONENT PP3 FROM THE PROTEOSE PEPTONE FRACTION OF BOVINE-MILK

Component PP3 is a phosphorylated glycoprotein with an apparent molecu lar mass of 28 kDa isolated from the proteose peptone fraction of bovi ne milk. The function of the protein is not known. The primary structu re has been determined and shown to contain 135 amino acid residues (E MBL accession no. P80195). It was phosphorylated at Ser29, Ser34, Ser3 8, Ser40 and Ser46. Two O-linked carbohydrate groups were found at Thr 16 and Thr86, while one N-linked carbohydrate group was present at Asn 77. Thr16 was only approximately 50 % glycosylated. The amino sugar de tected by the amino acid analyser at Thr86 was mainly galactosamine bu t a small amount of glucosamine was also present. The amino sugars fou nd in the carbohydrate group linked to Asn77 were both glucosamine and galactosamine. A fragment of PP3 has been isolated from milk and show n to correspond to residues 54-135. This fragment was probably generat ed by plasmin hydrolysing the Arg53-Ser54 bond.