Three new bovine αs-CN phosphorylation isoforms reveal different phosphorylation pathways

Abstract Text:

Casein (CN) phosphorylation is an important post-translational modification, and it 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. Our objective was 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 (LC/ESI-MS). We detected three new phosphorylation isoforms: α_s2-CN-9P, -14P, and -15P in bovine milk, in addition to the known isoforms α_s1-CN-8P and -9P, and α_s2-CN-10P, -11P, -12P, and -13P. The relative concentrations of each α_s-CN phosphorylation isoform varied considerably among milk of individual cows (coefficient of variation ≥8). Furthermore, the phenotypic correlations and hierarchical clustering suggest two regulatory systems for the phosphorylation of α_s-CN: one responsible for isoforms with lower levels of phosphorylation (α_s1-CN-8P, α_s2-CN-10P and -11P), and the other responsible for isoforms with higher levels of phosphorylation (α_s1-CN-9P, α_s2-CN-12P, -13P and -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.

Keywords: phosphorylation, casein, milk protein composition, LC/ESI-MS