Caractérisation des variants génétiques des caséines αsl et β bovines

Abstract

The three major casein species of cow's milk are known to show a genetic polymorphism: four variants of αsl‐casein (A, B, C, D), five of β‐casein (A1, A2, A3, B, C) and two of χ‐casein (A, B) have been discovered until the present time in the western breeds of cattle (Bos taurus) (see review by Aschaffenburg [1]). These variants, whose relative mobilities in gel electrophoresis at pH 8.6 and at pH 3.0 are shown in Fig.1, represent markers of the structural loci synthesizing the three casein species (αsl‐Cn, β‐Cn and χ‐Cn). The salient feature of the genetics of cattle caseins lies in the very close linkage of these three loci, which are likely to be clustered on the chromosome [2–5]. As a consequence of this close linkage, associations of αsl‐, β‐and χ‐casein variants are transmitted en bloc, as “gene complexes”. Our interpretation of available data on the frequencies of the gene complexes in cattle populations had led us to propose a phylogeny of the genetic variants of αsl‐and β‐caseins, and also to suggest that the mutation sites corresponding to the more common ones are particularly close together on the chromosome [6]. These hypotheses prompted us to identify and to position the alterations specific for each of the variants of αsl‐ and β‐caseins. We have already, in recent publications, characterized the αslC, αslA and βA3 variants [7–9]. The present work, which in the case of β‐casein was guided by indications on amino‐acid substitutions given by Pion et al. [10], Peterson et al. [11], Groves and Gordon [12], and which refers to the sequences of αsl B and βA2 caseins recently established by Mercier et al. [13] and Ribadeau‐Dumas et al. [14], deals with the four other variants, αslD, βA1, βB, βC. The variant αsl D differs from αsl B in that the alanyl residue in position 53 of the polypeptide chain is replaced by a phosphothreonyl residue (53 Ala → ThrP). There is no serine → proline substitution as previously proposed [15]. Compared to βA2, the three variants βA1, βB and βC all differ by a 67 Pro → His substitution. In βB, in addition to the 67 Pro → His substitution, the seryl residue in position 122 is replaced by an arginyl residue, as previously mentioned [16]. In βC, in addition to the 67 Pro → His substitution, the glutamyl residue in position 37 is replaced by a lysyl residue, and the seryl residue in position 35 is no more phosphorylated. Table 3 summarizes our results on all the genetic variants of αsl‐ and β‐caseins. These results make it possible to deduce phylogenic relationships (Fig. 6) that support and expand our former hypotheses [6]. The concept that the initiator end of β‐Cn locus is contiguous to the terminal end of αsl‐Cn locus accounts for the rare occurrence of recombinant types between αsl‐Cnc and β‐CnA1, β‐CnB, β‐CnC, since the distances between the corresponding mutation sites would be only 73, 128 and 43 codons, respectively. Copyright © 1972, Wiley Blackwell. All rights reserved