Models for Casein Micelle Formation

Abstract

The synthesis and secretion of milk involves the formation of a distinctive disaccharide, lactose, and two quite unique biological structures, the fat globule and the casein protein-complex (the casein micelle). The mechanism of biosynthesis of lactose and the origin of the fat globule are beginning to be understood, but the precise mechanism of bioassembly of the casein micelle has not been fully elucidated. The physical and chemical properties of the major micelle components αs1-, β-, and κ-casein are briefly reviewed, as well as the formation of synthetic micelles in the presence of calcium ions by native and dephosphorylated caseins. Several conflicting models for the casein micelle have been proposed based upon these properties. In order to determine which of these models might be applicable to casein micelle formation, correlations between the chemical and physical properties of the caseins and the actual events which occur in vivo in casein bio-assembly have been attempted. It is proposed that phosphorylation of caseins occurs in the Golgi apparatus of lactating mammary cells and that, subsequently, casein micelle formationThe synthesis and secretion of milk involves the formation of a distinctive disaccharide, lactose, and two quite unique biological structures, the fat globule and the casein protein-complex (the casein micelle). The mechanism of biosynthesis of lactose and the origin of the fat globule are beginning to be understood, but the precise mechanism of bio-assembly of the casein micelle has not been fully elucidated. The physical and chemical properties of the major micelle components αs1-, β-, and κ-casein are briefly reviewed, as well as the formation of synthetic micelles in the presence of calcium ions by native and dephosphoryl-ated caseins. Several conflicting models for the casein micelle have been proposed based upon these properties. In order to determine which of these models might be applicable to casein micelle formation, correlations between the chemical and physical properties of the caseins and the actual events which occur in vivo in casein bio-assembly have been attempted. It is proposed that phosphorylation of caseins occurs in the Golgi apparatus of lactating mammary cells and that, subsequently, casein micelle formation occurs in Golgi vacuoles through the condensation of preformed subunits. These observations lend support to a model for the formation of the casein micelle from protein subunits of about 100 angstroms in diameter.