Differential splicing of pre-messenger RNA produces multiple forms of mature caprine α(s1)-casein

Abstract

The identity of multiple forms of caprine α(s1)-casein in variants A, B, and C has been determined by structural characterisation using mass spectrometry, automated Edman degradation and peptide mapping. Mature goat α(s1)-casein exists as a mixture of at least four molecular species which differ in peptide chain length. The main component corresponds to the 199-residues form already described. The other three, in lesser amounts, were shorter forms of α(s1)-casein and differed for the deleted peptides 141-148, as shown previously for ovine α(s1)-casein, peptide 110-117, or Gln78. Analysis of α(s1)-casein mRNA from milk somatic cells demonstrated that these forms originated from skipping events at the level of exon 13 (codifying for peptide 110-117) 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 splicing abnormalities in the three common variants A, B, and C suggests that this is a general feature of α(s1)-casein in goat. A further source of heterogeneity of caprine α(s1)-casein was identified in the discrete phosphorylation of seryl residues. Eight serine residues (at positions 34, 36, 64 to 68 and 75) are fully phosphorylated (except in variant α because of the replacement Glu77→Gln which prevents phosphorylation of Ser75). Conversely, Ser115 and Ser41 are phosphorylated only to about 50% and 20%, respectively. Ser12, although located in a consensus triplet, is never phosphorylated, similarly to the ovine α(s1)-casein variants. These results confirm that there are stabilised mechanisms of simultaneous synthesis of α(s1)-casein at different length and of post-translational modification in both caprine and ovine species.