Alternative nonallelic deletion is constitutive of ruminant alpha(s1)-casein

Multiple forms of alpha(s1)-casein were identified in the four major rumina nt species by structural characterization of the protein fraction. While al pha(s1)-casein phenotypes were constituted by a mixture of at least seven m olecular forms in ovine and caprine species, there were only two forms in b ovine and water buffalo species. In ovine and caprine forms the main compon ent corresponded to the 199-residue-long form, and the deleted proteins dif fered from the complete one by the absence of peptides 141-148, 110-117, or Gln78, or a combination of such deletions. The deleted segments correspond ed to the sequence regions encoded by exons 13 and 16, and by the first tri plet of exon 11 (CAG), suggesting that the occurrence of the short protein forms is due to alternative skipping, as previously demonstrated for some c aprine and ovine phenotypes. The alternative deletion of Gln78 in alpha(s1) -casein, the only form common to the milk of all the species examined and l ocated in a sequence region joining the polar phosphorylation cluster and t he hydrophobic C-terminal domain of the protein, may play a functional role in the stabilization of the milk micelle structure.