Copresence of Deleted Protein Species Generates Structural Heterogeneity of Ovine αs1-Casein

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Abstract
Multiple forms of mature alpha(s1)-casein have been characterized in ovine variants A and D using a combination of mass spectrometry and automated Edman degradation. Mature ovine alpha(s1)-casein was found to be a heterogeneous mixture of at least seven molecular species. The main component, representing about 50% total alpha(s1)-casein, corresponded to the full-length (199 residues long) protein. The other components were alpha(s1)-casein of different lengths: 198 (less Gln78), 191 (less peptide 110-117), 191 residues (less peptide 140-148), 190 (less peptide 110-117 and Gln78), 190 (less peptide 140-148 and Gln78), and 183 (less peptides 110-117 and 140-148) residues long alpha(s1)-casein. Each of the alpha(s1)-casein multiple forms occurred at three different phosphorylation levels, due to the partial phosphorylation of both Ser115 (at about 50%) and Ser41 (at about 20%). In the case of deleted peptide 110-117, the protein heterogeneity linked to the partially phosphorylated Ser115 was abolished, and only two levels of phosphorylation were observed. These multiple forms differing in molecular weight and degree of phosphorylation may have been developed from an exon skipping during mRNA splicing in ovine alpha(s1)-casein, similar to that recently described in the case of its caprine counterpart.