Primary structure of bovine beta casein. Complete sequence

Abstract

This article is the sixth and last of a series devoted to the primary structure of bovine β‐casein [1–5]. In the preceding paper, we presented a partial sequence of this protein in which 156 out of the 209 amino‐acid residues were exactly positioned [5]. Only a few data were given on the tryptic peptides T2 and T3 [1] which corresponded to the unknown part of the molecule. In the present communication, we establish the amino‐acid sequence of these 2 peptides, and discuss the salient features of the β‐casein molecule.Bovine β‐casein A2 is a phosphoprotein constituted of a single polypeptide chain containing 209 amino‐acid residues. Among them are five phosphoseryl residues. From sequence data, it can be written: Asp4, Asn5, Thr9, Ser11, SerP5, Glu17, Gln22, Pro35, Gly5, Ala5, Val19, Met6, Ile10, Leu22, Tyr4, Phe9, Trp, Lys11, His5, Arg4. Its exact molecular weight is 23982, a little higher than that of αs1‐casein, which is 23616 [6]. The total number of 27 amide groups (Asn + Gln) agrees well with previous data obtained by Pion et al. [7] and Peterson et al. [8]. The average hydrophobicity, calculated according to Bigelow [9], is 1330 and it places β‐casein among the most hydrophobic proteins.The sequence obtained emphasizes the uneven charge distribution along the peptide chain. A high negative net charge is located in the first 50 residues. The high proline residue content (16.7%) and the rather uniform repartition of this residue exclude the possibility of a high α‐helix content. Furthermore, the complete absence of segments containing more than 2 adjacent prolyl residues seems to preclude the possibility of poly‐L‐proline structure that was expected from optical‐rotatory‐dispersion studies [10].An interesting extension of the present work is the discovery by Gordon et al [11] that γ‐, TS‐, R‐ and S‐caseins are fragments of β‐casein likely originating from tryptic‐like cleavages of the peptide chain next to the carbonyl groups of some lysyl residues. The γ‐, TS · A2‐ and S‐, TS · B‐ and R‐caseins respectively represent residues 29 to 209, 106 to 209 and 108 to 209. These results explain the occurrence of only one phosphorus atom in the γ‐caseins, the absence of phosphorus in the TS‐, R‐ and S‐caseins [12]. Localization of the substitutions which differentiate the known variants of bovine β‐casein also explains the relationship between β‐, γ‐, R‐, S‐ and TS‐caseins [12,13].The NH2‐terminal tryptic phosphopeptide, previously analyzed by Peterson et al. [14], has been sequenced independently by Manson and Annan [15]. Their results are identical to those reported in a preceding paper [4].