The primary structure of a low‐ M r multiphosphorylated variant of β ‐casein in equine milk

• Published in: Proteomics (Weinheim) 2007-04, Vol.7 (8), p.1327-1335
• Main Authors: Miclo, Laurent, Girardet, Jean‐Michel, Egito, Antonio S., Mollé, Daniel, Martin, Patrice, Gaillard, Jean‐Luc
• Format: Article
• Language: English
• ISSN: 1615-9853; 1615-9861
• DOI: 10.1002/pmic.200600683

Highly phosphorylated casein with a low molecular mass was isolated from Haflinger mare's milk by RP‐HPLC. It accounts for 4.0% of the casein content. Its mass was determined by LC‐ESI‐MS before and after treatment by alkaline phosphatase. The molecular mass found for the apo‐form (10 591 ± 2 Da) is in agreement with its primary structure, which was established by ESI‐MS/MS from tryptic peptides. It appeared that this short protein (94 amino acid residues) is an internally truncated form of the full‐length equine β‐casein (226 residues). This low‐ M r variant of equine β‐casein displays a large deletion (residues 50–181), due to a cryptic splice site usage occurring within exon 7 during the course of primary transcripts processing. The phosphorylation pattern of this equine β‐casein variant was investigated by LC‐ESI‐MS and 2‐DE. Seven phosphorylation forms were identified with one to seven phosphate groups with p I s ranging between 4.67 and 4.01. The major isoforms carry five and six phosphate groups.