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The casein micelle system of bovine milk is unique in that protein aggregates of similar spherical shape but extreme variability of size are formed by the self-assembly of three major nonidentical subunits. The monomeric subunits appear to be approximately the same size and shape with similar amphiphilic natures, the chief difference in properties being in the carbohydrate-containing kappa-casein which acts to stabilize the system against precipitation by calcium ion. Micelle models with kappa-casein exclusively in the interior lack a stabilization mechanism and can be eliminated. Statistical considerations of a chain polymer model also lead to its rejection. Electron microscopy reveals spherical submicellar aggregates which at present can be accounted for by only three models. Of these three, the experimental data are predicted only by one in which, alphas 1-, beta-, and kappa-casein subunits are associated into spherical soap micelle-like particles with the kappa-casein segregated into one portion, giving these submicelles an amphiphilic nature. The alphas 1- and beta-caseins are hydrophobic while the kappa-casein portion of the submicelle surface is hydrophilic. Of particular interest is the ability of this micelle model to explain the formation of a minimum micelle which is larger than a submicellar particle.

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Human beta-casein was separated according to the extent of phosphorylation and the fully phosphorylated moiety was characterized. Fully phosphorylated human beta-casein makes up to 13-15% of the beta-casein fraction. It has a partial specific volume, v, of 0.754 +/- 0.008 and an absorbancy, E1(1%)cm,280 nm of 6.4 +/- 0.2. Sedimentation and viscosity data yield a solvation of 2.9 g H2O/g protein and an axial ratio of about 5 for the monomer. This would be consistent with a prolate ellipsoid of 10 nm length and 2 nm width. There is one strong binding site for Ca2+ for each organic phosphate ester in the molecule. The protein will precipitate at room temperature upon the addition of either 10 mM Ca2+ or greater than 1 M NaCl. Increasing the temperature from 4 to 37 degrees C causes an apparent conformational change and an increase in protein aggregation which is further increased by the addition of NaCl at this temperature until a limiting size is reached at about 0.25 M NaCl. This limiting size polymer contains 95-105 monomers and is nearly spherical with a radius of about 15 nm and a solvation of 3 g H2O/g protein. If this polymer were the submicelle of human casein, it could account for the abnormally high solvation of human casein micelles but their small average size would be more difficult to reconcile without additional information concerning K-casein association. The addition of Ca2+ to the system introduces association patterns which are more complex and not easily assessed.