The relationships among bovine α_S-casein phosphorylation isoforms suggest different phosphorylation pathways

<p>Casein (CN) phosphorylation is an important posttranslational modification and is one of the key factors responsible for constructing and stabilizing casein micelles. Variation in phosphorylation degree of α_S-CN is of great interest because it is suggested to affect milk technological properties. This study aimed to investigate the variation in phosphorylation degree of α_S-CN among milk of individual cows and to explore relationships among different phosphorylation isoforms of α_S-CN. For this purpose, we analyzed morning milk samples from 529 French Montbéliarde cows using liquid chromatography coupled with electrospray ionization mass spectrometry. We detected 3 new phosphorylation isoforms: α_S2-CN-9P, α_S2-CN-14P, and α_S2-CN-15P in bovine milk, in addition to the known isoforms α_S1-CN-8P, α_S1-CN-9P, α_S2-CN-10P, α_S2-CN-11P, α_S2-CN-12P, and α_S2-CN-13P. The relative concentrations of each α_S-CN phosphorylation isoform varied considerably among individual cows. Furthermore, the phenotypic correlations and hierarchical clustering suggest at least 2 regulatory systems for phosphorylation of α_S-CN: one responsible for isoforms with lower levels of phosphorylation (α_S1-CN-8P, α_S2-CN-10P, and α_S2-CN-11P), and another responsible for isoforms with higher levels of phosphorylation (α_S1-CN-9P, α_S2-CN-12P, α_S2-CN-13P, and α_S2-CN-14P). Identifying all phosphorylation sites of α_S2-CN and investigating the genetic background of different α_S2-CN phosphorylation isoforms may provide further insight into the phosphorylation mechanism of caseins.