Bovine beta-casein expressed in transgenic mouse milk is phosphorylated and incorporated into micelles

Transgenic mice carrying the bovine beta-casein gene linked to the promoter sequence of the sheep beta-lactoglobulin milk protein gene were produced. Four of seven lines expressed the transgene protein with mice from the two highest expressing lines producing 4 to 5 mg ml(-1) of this protein in their milk. The foreign protein was associated with the casein micelles and did not segregate into the whey fraction on centrifugation of milk samples. Following purification, the protein was characterized by amino acid analysis, gel electrophoresis, capillary zone electrophoresis, and trypsinolysis. The results showed that the transgene protein was authentic, phosphorylated bovine beta-casein A(1).