Caseins are highly phosphorylated milk proteins interacting with calcium phosphate to form large colloidal structures called casein micelles. Phosphorylation of casein is an important post-translational modification occurring in the mammary gland under the action of kinases. However, the molecular mechanism behind is not fully understood yet. These kinases recognize the consensus motif as a glutamic acid, phosphor-serine residue, or aspartate at the position n+2 relative to the target serine or threonine residue (Ser/Thr-x-Glu/SerP/Asp; Mercier, 1981). This modification is essential for the construction and the stabilization of the micelle structure which is based on interactions between phospho-serine or phospho-threonine residues of casein and calcium phosphate. αs2-casein (αs2-CN) is of particular interest since it is the most phosphorylated casein and exhibits various phosphorylation states. In addition, it has been the least investigated since its phosphorylated isoforms are difficult to purify and quantify. The objective of this study was to investigate the variation in the phosphorylation degree of αs2-CN among individual cows. Milk samples from 500 French Montbéliarde cows were collected as part of a R&D project (FROM’MIR)*, and were analyzed by liquid chromatography coupled with electrospray ionization mass spectrometry (LC/ESI-MS) to determine milk protein composition. Relative protein concentration of each αs2-CN phosphorylated isoform was quantified by mass signal intensity (Miranda et al., 2013). Method reproducibility was assessed by calculating the coefficient of variation (CV) of relative protein concentration for all αs2-CN phosphorylated isoforms from a reference milk sample that was analyzed after every 10 milk samples. In total, 50 reference milk samples were analyzed. αs2-CN was characterized with multiple phosphorylation states containing 10 to 14 phosphate groups (10P-14P). This is the first study that shows the presence of αs2-CN-14P, and the first method to determine the relative concentration for each αs2-CN phosphorylated isoform. CV values for reproducibility of the relative protein concentration were below 6% for all the αs2-CN phosphorylated isoforms. Furthermore, CV values for relative protein concentration regarding all the αs2-CN phosphorylated isoforms (> 10%) were larger than CV values for reproducibility, which implies a significant variation in phosphorylation degree of αs2-CN among individuals. Regarding to the correlation between the relative concentration of the different phosphorylation states of αs1-CN and αs2-CN, αs1-CN-9P had negative correlation with αs2-CN-10P and αs2-CN-11P (-0.74 and -0.72, respectively), but positive correlation with αs2-CN-13P and αs2-CN-14P (0.69 and 0.67, respectively). This observation supports the previous finding on the relationship between αs1-CN-8P and αs1-CN-9P, which suggests that two different sets of genes are involved in regulation of phosphorylation isoforms (Bijl et al., 2014).
|Publication status||Published - Oct 2015|
|Event||12th International Milk Genomics & Human Health Symposium - Sydney, Australia|
Duration: 26 Oct 2015 → 28 Oct 2015
|Conference||12th International Milk Genomics & Human Health Symposium|
|Period||26/10/15 → 28/10/15|