Phosphorylation of αS1-casein is regulated by different genes

Abstract

Casein phosphorylation is a posttranslational modification catalyzed by kinase enzymes that attach phosphate groups to specific AA in the protein sequence. This modification is one of the key factors responsible for the stabilization of calcium phosphate nanoclusters in casein micelles and for the internal structure of the casein micelles. αS1-Casein (αs1-CN) is of special interest because it constitutes up to 40% of the total casein fraction in milk, and it has 2 common phosphorylation states, with 8 (αS1-CN-8P) and 9 (αS1-CN-9P) phosphorylated serine residues. Factors affecting this variation in the degree of phosphorylation are not currently known. The objective of this research was to determine the genetic background of αS1-CN-8P and αS1-CN-9P. The genetic and phenotypic correlation between αS1-CN-8P and αS1-CN-9P was low (0.18 and 0.19, respectively). This low genetic correlation suggests a different genetic background. These differences were further investigated by means of a genome-wide association study, which showed that both αS1-CN-8P and αS1-CN-9P were affected by a region on Bos taurus autosome (BTA) 6, but only αS1-CN-8P was affected by a region on BTA11 that contains the gene that encodes for β-lactoglobulin (β-LG), and only αS1-CN-9P was affected by a region on BTA14 that contains the diacylglycerol acyltransferase 1 (DGAT1) gene. Estimated effects of β-LG protein genotypes showed that only αS1-CN-8P was associated with the β-LG A/B polymorphism (g.1772G>A and g.3054C>T); the AA genotype of β-LG was associated with a lower concentration of αS1-CN-8P (−0.32% wt/wt) than the BB genotype (+0.41% wt/wt). Estimated effects of DGAT1 K232A genotypes showed that only αS1-CN-9P was associated with the DGAT1 gene polymorphism; DGAT1 AA genotype was associated with a higher αS1-CN-9P concentration (+0.53% wt/wt) than the DGAT1 KK genotype (−0.44% wt/wt). The results give insight in phosphorylation of αS1-CN-8P and αS1-CN-9P, which seem to be regulated by a different set of genes.