Caractérisation des variants génétiques des caséines α sl et β bovines

The three major casein species of cow's milk are known to show a genetic polymorphism: four variants of α sl ‐casein (A, B, C, D), five of β‐casein (A 1 , A 2 , A 3 , 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 (α sl ‐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 α sl ‐, 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 α sl ‐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 α sl ‐ and β‐caseins. We have already, in recent publications, characterized the α sl C, α sl A and βA 3 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 α sl B and βA 2 caseins recently established by Mercier et al. [13] and Ribadeau‐Dumas et al. [14], deals with the four other variants, α sl D, βA 1 , βB, βC. The variant α sl D differs from α sl B in that the alanyl residue in position 53 of the polypeptide chain is replaced by a phosphothreonyl residue (53 Ala → Thr P ). There is no serine → proline substitution as previously proposed [15]. Compared to βA 2 , the three variants βA 1 , 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 α sl ‐ 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 α sl ‐Cn locus accounts for the rare occurrence of recombinant types between α sl ‐Cn c and β‐Cn A1 , β‐Cn B , β‐Cn C , since the distances between the corresponding mutation sites would be only 73, 128 and 43 codons, respectively.