Phylogenetic variations in the calcium-dependent electrophoretic shift of alpha-lactalbumin

Characterization and partial purification of bovine alpha-lactalbumin and beta-casein produced in milk of transgenic mice

Bovine alpha-lactalbumin (alpha-LA) and bovine beta-casein (beta-CN), from milk from transgenic mice were characterized and partially purified using electrophoretic, immunoblotting, and chromatographic methods. The transgenically expressed bovine milk proteins were identified using PAGE or by a combination of preparative isoelectrofocusing followed by Western immunoblotting. The heterologous bovine alpha-IA and bovine beta-CN had molecular masses that were identical to those of those of the native proteins. The estimated expression of the proteins was 1.0 mg/ml of milk for alpha-LA and 3.0 mg/ml for beta-CN. The calcium binding of bovine alpha-LA suggested that the protein produced in murine milk has the same electrophoretic shift as native bovine alpha-LA after the removal of calcium. Nitrogen-linked glycosylation of native and murine synthesized bovine alpha-LA was identified by peptide-N-glycosidase F treatment, and the N-terminal amino acid sequence of HPLC-purified bovine alpha-LA from mouse milk was confirmed to be identical to native bovine alpha-LA. In addition, the phosphorylation of the bovine beta-CN expressed in the milk of transgenic mice was the same as that of native bovine beta-CN, as determined by phosphatase digestion.

Functional implications resulting from disruption of the calcium-binding loop in bovine alpha-lactalbumin

The strong calcium-binding site of alpha-lactalbumin comprises the carboxylate side chains of aspartic acid 82, 87, and 88 and the carbonyl oxygens of residues 79 and 84. A single methionine residue was selectively modified by controlled CNBr cleavage to yield homoserine at position 90. The CNBr-cleaved alpha-lactalbumin lost the ability to bind calcium strongly as monitored by intrinsic fluorescence, electrophoretic mobility, atomic absorption, and x-ray fluorescence. Remarkably, the modified protein was still competent in lactose biosynthesis, although activity was reduced to 1/40th that of the native form of the protein. Although the strong calcium-binding site was destroyed as a result of the cleavage of the calcium-binding loop, a secondary calcium site was retained that directly affects a rate enhancement of lactose biosynthesis when saturated, resulting in approximately a two- to threefold increase in rate at 1 mM CaCl2 with an activation equilibrium constant of 350 +/- 40 microM.