The Primary Structure of the Ovine β‐Caseins

Abstract

Ovine whole casein contains 2 multiphosphorylated β‐casein components designated as β1 and β2‐caseins. The complete sequence of β1‐casein and the partial sequence of β2‐casein have been determined from the intact proteins and from the peptides isolated from cyanogen bromide and tryptic digests. The ovine β1 and β2‐caseins have the same polypeptide chain and appear to differ only in that they contain 6 and 5 phosphates respectively. The amino acid composition of ovine β‐casein can be written as: Asp4, Asn4, Thr10, ThrP1, Ser9, SerP5, Glu19, Gln21, Pro34, Gly5, Ala4, Val21, Met6, IIe9, Leu22, Tyr3, Phe9, Trp1, Lys12, His5, Arg3. Compared to bovine β‐casein A2, which is made up of 209 residues, ovine β1‐casein has a deletion of 2 residues (either Pro‐179–Tyr‐180 or Tyr‐180–Pro‐181) and 20 largely conservative amino acid substitutions. Although 20% of the substitutions involve proline residues, the proline contents of ovine β1 and bovine β2‐caseins are very similar, around 16%. The average hydrophobicity, calculated according to Bigelow, is 5.51 kJ/residue, which is similar to that calculated for bovine β‐casein A2. The cluster of 4 phosphorylated serine residues and the highly charged nature of the amino terminal region observed for bovine β‐casein are conserved in the ovine β‐caseins. The substitution from IIe‐12 (bovine) to Thr‐12 (ovine) results in a new phosphorylation site, according to the phosphorylation code proposed for caseins. This site is only partially phosphorylated hence the occurrence of both β1 and β2‐caseins in ovine milk. Copyright © 1979, Wiley Blackwell. All rights reserved