Characterization and genotyping of the caprine kappa-casein variants

Kappa-Casein (kappa-CN) is the milk protein that determines the size and specific function of milk micelles, and its cleavage by chymosin is responsible for milk coagulation. We have previously detected and characterized four variants of the goat kappa-CN in Spanish, French, and Italian breeds by screening the major part of the coding region in exon 4. Here we have sequenced and analyzed the full coding region of the kappa-CN gene which includes exons 3 and 4. No additional mutations were found, with exception of a single nucleotide substitution in exon 3, which had no amino acid change. However, the analysis of the association between the different mutations resulted in two new variants designated kappa-CN F and G. The novel variants are present in the Italian breeds Teramana, Girgentana, and Sarda (variant F). A protocol for rapid simultaneous genotyping of all known kappa-CN variants using the primer extension method was described, and a total of 210 animals from nine European breeds were genotyped. Alleles A and B are the most frequent variants occurring in the majority of breeds with highest prevalence of the B variant, except for the Canaria breed where the A allele is more frequent. Sequence data suggest that the F variant is the original type of caprine kappa-CN, other alleles being derived from this type following two different trunks by successive mutations.