Nucleotide Sequence Evolution at the κ-Casein Locus: Evidence for Positive Selection within the Family Bovidae

摘要

κ-Casein is a mammalian milk protein involved in a number of important physiological processes. In the gut, the ingested protein is split into an insoluble peptide (para κ-casein) and a soluble hydrophilic glycopeptide (caseinomacropeptide). Caseinomacropeptide is responsible for increased efficiency of digestion, prevention of neonate hypersensitivity to ingested proteins, and inhibition of gastric pathogens. Variation within this peptide has significant effects associated with important traits such as milk production. The nucleotide sequences for regions of κ-casein exon and intron four were determined for representatives of the artiodactyl family Bovidae. The pattern of nucleotide substitution in κ-casein sequences for distantly related bovid taxa demonstrates that positive selection has accelerated their divergence at the amino acid sequence level. This selection has differentially influenced the molecular evolution of the two κ-casein split peptides and is focused within a 34-codon region of caseinomacropeptide.