Structure Primaire de la caséine β bovine Séquence complète.

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]. [ABSTRACT FROM AUTHOR]