Colloidal Calcium Phosphate in the Reconstituted Milk Micelle May Direct Wild-type Recombinant Human β-Casein to Fold Like the Native Protein

Native human β-casein (CN) at all phosphorylation levels exhibits reproducible behavior and appears to have a unique, stable folding pattern. In contrast, the recombinant non-phosphorylated form of human β-CN (β-CN-0P) with the exact amino acid sequence (wild-type), expressed and purified from Escherichia coli, differs greatly in its behavior from the native protein and the complexes formed are unstable to thermal cycling. However, when it was incorporated into reconstituted milk micelles, using bovine κ-CN at a κ/β molar ratio of 1/3 with added Ca2+ ions and inorganic phosphate (Pi) at levels that would ordinarily precipitate, its association behavior vs. temperature as monitored by turbidity (OD400 nm) approximated that of native β-CN-0P. This suggests that the milk micelle system, and particularly the colloidal calcium phosphate, may act as a ‘molecular chaperon’ to direct the folding of the molecule into the highly stable conformation found in the purified native human β-CN molecule.