Is the apocrine milk secretion process observed in the goat species rooted in the perturbation of the intracellular transport mechanism induced by defective alleles at the αs1-Cn locus?

Abstract

The structural and quantitative variability of caprine αs1-casein induced by the extensive polymorphism recorded at the corresponding locus strongly influences the composition (proteins as well as lipids) and the technological behaviour of milk. Immuno-histo-chemistry studies coupled with electron microscopy analysis have shown that a dysfunction exists in the intracellular transport of caseins when αs1-casein is lacking. Casein accumulation in the endoplasmic reticulum leads to a dilation of the cisternae that could disturb the whole secretion process (including lipids). Despite a long controversy, goat milk secretion is still considered to occur through an apocrine process contrary to the merocrine process described for cow's milk. We suggest that the apocrine pathway of secretion described in the goat could be the consequence of the dysfunction observed in the intracellular transport of caseins when αs1casein is lacking. To obtain further clues in the favour of such a hypothesis, we compared the protein and lipid fractions of milks from goats homozygous for different αs1-casein alleles.