Differences in the defective alleles E and F for the locus CSN1S1 in goats affects the profile of milk caseins

• Morphology of epithelial cells in mammary tissue and fractions of caseins in milk studied simultaneously.
• We give one more argument in favor of overexpression of the β- and κ-casein gene.
• The organization of the micelles may depend on the phosphorylation sites that are absent in the protein encoded by allele F.
• Epistatic interactions may be occurring between the genes encoding caseins in goats.

The objective in the current study was to associate different genotypes for the gene that codifies the αS1-casein with the profile of milk caseins and the morphology of the mammary gland epithelial cells in goats. The genotypes HH, HF, EE and EF for the αS1-casein gene (H = any high allele) were assessed based on the differences in the size of fragments amplified using polymerase chain reaction (PCR) and PCR-restriction fragment length polymorphism (PCR-RFLP). The milk casein fractions were measured by reversed phase HPLC. Mammary gland biopsies of animals with each of these genotypes were submitted to histological and ultrastructural microscopy analysis for the evaluation of secretory epithelial cells. The amount of total casein in milk from goats with genotype HH was only higher than in milk from those animals with the genotype EF, while the αS1-casein (αS1-Cn) fraction was different among all genotypes. The largest αS1-Cn fraction was found in milk from the animals with the genotype HH, and it progressively decreased in milk from animals with the genotypes HF, EE and EF. Other milk casein fractions (β-casein, αS2-casein and κ-casein) had a characteristic composition for each genotype. The κ-casein (κ-Cn) fraction was greater whenever the F allele was found, and the β-casein fraction was greater in EE homozygous animals. The ultrastructural analysis showed that mammary gland secretory cells from goats with the genotype EE and EF have highly dilated rough endoplasmic reticulum (RER) cisternae, whereas the dilation pattern in the animals with the genotype HF was intermediate between those with the genotype HH, and EE/EF. Although the amount of αS1-casein is critical for an efficient transport of other caseins by RER, the defective F allele affects the other milk casein fractions differently from the defective E allele, probably due to the lack of phosphorylation sites on the protein encoded by the F allele, which would affect, in turn, the way caseins interact during milk micelle formation, reflecting on casein fractions in milk.