Glycosylation of κ-casein: Genetic and nongenetic variation and effects on rennet coagulation properties of milk

The aims of this study were to investigate genetic and nongenetic variation in the degree of glycosylation of kappa-casein (kappa-CN) and to estimate the effects of glycosylated (G-kappa CN) and unglycosylated (U-kappa CN) kappa-CN contents on milk coagulation properties of Simmental cows. Measures of contents of the main casein fractions, G-kappa CN, and U-kappa CN, and assessment of genotypes at CSN2, CSN3, and BLG were obtained by reversed-phase HPLC analysis of 2,015 individual milk samples. Content of total kappa-CN (kappa-CNtot, g/L) was the sum of G-kappa CN and U-kappa CN, and the glycosylation degree of kappa-CN (GD) was measured as the ratio of G-kappa CN to kappa-CNtot. Rennet coagulation time (RCT) and curd firmness were measured by using a computerized renneting meter. Measures of curd firmness were adjusted for RCT before statistical analysis. Variance components of kappa-CNtot, G-kappa CN, U-kappa CN, and GD were estimated by Bayesian procedures and univariate linear models that included the class effects of the herd-test-day, parity, days in milk, genotypes at milk protein genes, and animal. These class effects, those of G-kappa CN, U-kappa CN, and content of other caseins, and the linear effect of milk pH were accounted for by models investigating the influence of kappa-CN glycosylation on coagulation properties. The GD ranged from 22 to 76%, indicating that variation in G-kappa CN depends on the variation both in kappa-CNtot and in the efficiency of kappa-CN glycosylation. Genotype CSN3 BB exhibited high G-kappa CN and U-kappa CN relative to that of CSN3 AA. Heritability of G-kappa CN, U-kappa CN, and GD was high and ranged from 0.46 to 0.56. A large proportion of the additive genetic variation in G-kappa CN and U-kappa CN was attributable to influence of CSN3 and BLG, but these genes did not affect variation in GD, and across-genotypes differences in the trait were small or trivial. Average RCT of the milk class having the highest G-kappa CN was, on average, 2 min (standard deviation 0.5) shorter than that of the lowest class. Conversely, U-kappa CN and content of other caseins were not associated with any effect on RCT, except for a slight delay in coagulation when U-kappa CN was very high. Curd firmness increased when the contents of both kappa-CN fractions and other caseins increased. This study provides evidence that the positive association between RCT and kappa-CN content is exclusively attributable to the glycosylated fraction of the protein. Because exploitable additive genetic variation in G-kappa CN exists, improvement of kappa-CN composition through selective breeding might be an effective way to enhance milk coagulation properties.