Two of the three genetic variants of goat α_s1‐casein which are synthesized at a reduced level have an internal deletion possibly due to altered RNA splicing

Abstract

This paper describes the elucidation of the primary structure of the three genetic variants of goat asl-casein, asl-Cn D, E and F, which have been found to be associated with reduced amounts of asl-casein in milk. Variant E has the same electrophoretic mobility as variant B, but differs from the latter by the substitutions of Arg for Lys and of Thr for Ala at positions 100 and 195. A genetically controlled event which does not affect the amino acid sequence of this variant might be responsible for its lower rate of synthesis compared to that of asl-casein B. The deletion of 11 amino acids at positions 59 -69 and of 37 amino acids at positions 59 -95 in variant B leads to variants D and F. In both cases the deletions, which start at the same position of the polypeptide chain, include the major phosphorylation site of the protein. On the basis of sequence data for casein genes and cDNAs, it was concluded that the deletions occurring in the D and F variants are due to the exclusion of one and several exons, respectively. The observed deletions in the proteins could thus be the consequence of splice site mutations which would induce altered RNA processing and hence reduce the rate of synthesis of the casein.Early work on the casein family, which includes three phosphoproteins, asl-, as2-, and fl-casein, and one glycophosphoprotein, k--casein, was performed on proteins isolated from cow's milk. The primary structures of these four polypeptides, which contain 169 -209 amino acid residues, have already been established [l]. Whereas genetic polymorphism is widespread for asl-, fl-, and k--casein, it is more restricted and is breed-dependent for as2-casein [2, 31. Segregation studies led to the conclusion that the four proteins are controlled by a cluster of closely linked genes. In this cluster no recombination was observed in a total of 448 informative families [4]. The unit of inheritance is thus the haplotype which is a combination of individual alleles of each of the four clustered loci.Although the same classes of caseins have been identified in goat's milk [5], so far, in this species, only asl and as2-casein are known to exhibit genetic polymorphism [6]. Segregation studies have confirmed that asl-and as2-casein genes are closely linked suggesting that, as in the cow, casein expression in the goat is controlled by a cluster of genes.Electrophoretic patterns of goat asl-casein are unusually complex due to differences in the mobilities of different variants and to varying intensities of electrophoretic bands which reflect different amounts of protein [6]. It is now established [6, 71 that polymorphism of asl-casein is under the control of at least seven alleles denoted as: asl-CnA, asl-CnB, asl-Cnc, asl-CnD, asl-Cn", asl-CnF and asl-Cno. Alleles asl-CnA, asl-

The impact of genetic polymorphisms on the protein composition of ruminant milks

“…Studies performed on goat milk have reported extensive genetic polymorphism of a s1 -casein with at least 14 autosomal alleles at the goat a s1 -casein locus, distributed in seven different classes of protein variants associated with four levels of expression [34]. a s1 -Casein A, B, C and E variants differ from each other in amino acid substitutions, while a s1 -casein F and G variants, associated with a low level of protein synthesis, are internally deleted [14,60]. The establishment of the overall organisation of the goat a s1 -casein gene (Fig.…”

Influence du locus de la caséine αs1 sur les performances laitières et les paramètres génétiques des chèvres de race Alpine

“…Le logiciel utilisé pour estimer les effets des allèles est le PEST (Groeneveld et Kovac, 1990 (Leroux et al, 1990Leroux, 1992 (Remeuf, 1993;Grosclaude et al, 1994;Vassal et al, 1994). Cet (Brignon et al, 1990;Leroux et al, 1992;Martin 1993;Grosclaude et al, 1994). Le …”

Caractéristiques organoleptiques de fromages de chèvre fabriqués à partir de laits contenant des variants génétiques différents de la caséine αs1

“…Le polymorphisme des caséines de chèvre a été dé-crit par les généticiens: pour le gène de la caséine a s 1' il existe au moins 7 variants génétiques, amenant à des taux de caséine différents (Boulanger et al, 1984 ;Brignon et al, 1990). De plus, la teneur en caséine totale du lait est en relation directe avec la teneur en caséine a s 1 (Leroux et al,1990).…”