Factors influencing degree of glycosylation and phosphorylation of caseins in individual cow milk samples

The aim of this study was to examine variations in posttranslational modifications (PTM) of caseins (CN) in milk from individual cows and determine how these differ between breeds, across lactation, and between variants. Furthermore, we examined the variation of casein PTM in relation to rennet coagulation properties of milk. In total, detailed protein composition of milk from 892 Danish Holstein and Jersey cows was determined by liquid chromatography/electrospray ionization-mass spectrometry. The method measured relative contents of the main milk proteins as well as several variants and PTM. The results showed that the 2 breeds had distinct milk protein composition. Milk from Danish Holstein cows was mainly characterized by higher relative contents of β-CN, α-lactalbumin (α-LA), and β-lactoglobulin, and a higher fraction of glycosylated κ-CN (G κ-CN), whereas milk from Danish Jersey cows was characterized by higher relative contents of κ-CN, αS2-CN, and the less phosphorylated forms of αS1-CN and αS2-CN. Univariate linear models including days in milk and parity as class effects showed variation in the detailed protein profile across and between lactations; in particular, changes in the degree of glycosylation of κ-CN were pronounced, but changes in αS1-CN 8P to total αS1-CN and αS2-CN 11P to αS2-CN were also observed over lactation for both breeds. The phosphorylated forms of αS1-CN and αS2-CN were, to some extent, correlated. Further, the κ-CN BB genotype was associated with higher relative contents of both unglycosylated κ-CN (UG κ-CN) and G κ-CN compared with κ-CN AA; κ-CN AB showed intermediate results in both breeds. The influence of protein composition on rennet coagulation properties was explored based on 4 classes for curd firming rate: noncoagulation, and poor, average, and good coagulation. The results revealed breed differences: Holstein milk, higher relative content of κ-CN to total protein, and higher content of G κ-CN were associated with improved milk coagulation. In contrast, relative content of α-LA was the main component associated with milk coagulation properties in Danish Jerseys and it was shown to affect milk coagulation properties negatively. In addition, variation in phosphorylation degrees of αS1-CN also played a role. This study demonstrates that although the genetic influence of glycosylation seems to be the same in both breeds, nongenetic variation differs, which is further reflected in different associations with milk coagula