DIFFERENTIAL SPLICING OF PREMESSENGER RNA PRODUCES MULTIPLE FORMS OF MATURE CAPRINE ALPHA(S1)-CASEIN

The identity of multiple forms of caprine alpha(s1)-casein in variants A, B, and C has been determined by structural characterisation using mass spectrometry, automated Edman degradation and peptide mapping. Ma ture goat alpha(s1)-casein exists as a mixture of at least four molecu lar species which differ in peptide chain length. The main component c orresponds to the 199-residues fern already described. The other three , in lesser amounts, were shelter forms of alpha(s1)-casein and differ ed for the deleted peptides 141-148, as shown previously for ovine alp ha(s1)-casein, peptide 110-117, or Gln78. Analysis of alpha(s1)-casein mRNA from milk somatic cells demonstrated that these forms originated from skipping events at the level of exon 13 (codifying for peptide 1 10-117) and 16 codifying for peptide 141-148) and from the presence of a cryptic splice site within exon 11 (whose first CAG triplet encodes Gln78) during primary transcript processing. The finding of these spl icing abnormalities in the three common variants A, B, and C suggests that this is a general feature of alpha(s1)-casein in goat. A further source of heterogeneity of caprine alpha(s1)-casein was identified in the discrete phosphorylation of seryl residues. Eight serine residues (at positions 44, 46, 64 to 68 and 75) are fully phosphorylated (excep t in variant A because of the replacement Glu77-->Gln which prevents p hosphorylation of Ser75). Conversely, Ser115 and Ser41 are phosphoryla ted only to about 50% and 20%, respectively. Ser12, although located i n a consensus triplet, is never phosphorylated, similarly to the ovine alpha(s1)-casein variants. These results confirm that there are stabi lised mechanisms of simultaneous synthesis of alpha(s1)-casein at diff erent length and of post-translational modification in both caprine an d ovine species.