Variation in Phosphorylation of αS1-casein and αS2-casein

Phosphorylation of the caseins in milk is a posttranslational modification that occurs in the Golgi membranes of the mammary cell. During this process, catalyzed by two kinases, a phosphate groups is attached to certain serine or sometimes threonine residues of the caseins. Due to their strong calcium binding properties, these phosphate groups are not only important for the nutritional value of milk but also for the physical chemical properties of the casein  micelle. It is known that in milk multiple phosphorylation states of the different caseins exist. However, the relative occurrence of these different phosphorylation states and the variation between individual cows in the degree of phosphorylation of the caseins in their milk is unknown. We developed a capillary zone electrophoresis (CZE) method to determine the relative concentration of different phosphorylation states of αS1-casein (αS1-CN) and αS2-casein (αS2-CN) and used this method  to determine the variation in the degree of phosphorylation of αS1-CN and αS2-CN in milk samples of 2000 Dutch Holstein Friesian cows.

We show that the CZE method is a suitable method to quantify all known phosphorylation states of αS1-CN and αS2-CN except from αS1-CN-7P and αS2-CN-13P. On average 74% of αS1-CN contains 8 phosphate groups and 26% contains 9 phosphate groups. For αS2-CN the values are, 15% having 10, 52% having 11, and 33% having 12 phosphate groups. Between cows, large variation exists in the relative occurrence of these different phosphorylation states. Phenotypic correlations between the concentrations of the different phosphorylation states, based on measurements in milk samples of 2000 cows, show that the degree of phosphorylation of αS1-CN and αS2-CN is important for the protein composition of milk.