The primary structure of water buffalo αs1- and β-casein:: Identification of phosphorylation sites and characterization of a novel β-casein variant

The primary structure of water buffalo alpha(s1)-casein and of beta-casein A and B variants has been determined using a combination of mass spectrometry and Edman degradation procedures. The phosphorylated residues were localized on the tryptic phosphopeptides after performing a beta-elimination/thiol derivatization. Water buffalo alpha(s1)-casein, resolved in three discrete bands by isoelectric focusing, was found to consist of a single protein containing eight, seven, or six phosphate groups. Compared to bovine alpha(s1)-casein C variant, the water buffalo alpha(s1)-casein presented ten amino acid substitutions, seven of which involved charged amino acid residues. With respect to bovine beta A(2)-casein variant, the two water buffalo beta-casein variants A and B presented four and five amino acid substitutions, respectively. In addition to the phosphoserines, a phosphothreonine residue was identified in variant A. From the phylogenetic point of view, both water buffalo beta-casein variants seem to be homologous to bovine beta A(2)-casein.