Effects of protein genetic variants on their phosphorylation levels, milk composition, milk proteome, and milk coagulation ability in Chinese Holstein bovine milk

Milk samples were collected from 3625 Chinese Holstein cows to assess the effects of kappa-casein (kappa-CN) and beta-lactoglobulin (beta-LG) genetic variants on its milk coagulation properties. The results show that Chinese Holstein cows have a higher frequency of the kappa-CN AA and AB variants, and beta-LG of the AB and AA variants. Of these, kappa-CN B variants, the beta-LG AA and BB variants were more frequent in milk showing good coagulation. The effects of the genetic variants on milk composition, milk proteome, and protein phosphorylation sites were studied. The results showed that higher concentrations of protein and dry matter were found in kappa-CN BE variant. Moreover, large variations in milk proteome among different kappa-CN and beta-LG variants were observed. Highly phosphorylated for kappa-CN, especially Ser97, was observed in cows with the kappa-CN BE variant, but no effect of beta-LG variants on phosphorylation site was found. Of the various factors examined, variation of kappa-CN phosphorylation sites Ser97 may be the most important in affecting casein structure and milk coagulation ability. Some milk protein contents were found to be negative factors for milk coagulation. In summary, this study showed that kappa-CN genetic variants contained different milk compositions and phosphorylation site Ser97 influenced milk coagulation.