Variation in the Glycosylation Pattern of Bovine K-Casein with Micelle Size and Its Relationship to a Micelle Model

Bovine casein micelles were separated into seven size classes and the supernatant by differential centrifugation. The K-casein was isolated from each class, and s020.w, the weight-average molecular weight (Mw), the hexose content, and the proportion glycosylated were determined for each. Glycosylation appears to follow complete micelle formation so that surface or near-surface K-casein is modified while that incorporated into the interior is unchanged. The largest micelles seem to have little interior K-casein but instead act as a coat-core system with K-casein all on the surface. The single class of largest micelles, fraction 1, contained K-casein with the highest Mw, the largest amount of associated hexose and a proportion glycosylated approaching that of supernatant K-casein. The proportion of glycosylated K-casein in the remaining micelle size classes appeared to be inversely related to micelle size with the exception of fraction 2 which probably contained some fraction 1-type micelles. The hexose to glycosylated protein ratio generally followed the MW and s020,W values. Further fractionation of the K-caseins according to polymer size by gel filtration revealed that, except for the K-casein from the largest micelles, hexose content and the hexose to glycosylated protein ratio were greatest for the larger sized polymers and decreased slightly as polymer size decreased. A higher hexose content for K-casein polymers of high molecular weight than for those of lower molecular weight would favor a previously proposed model [Slattery, C. W., & Evard, R. (1973) Biochim. Biophys. Acta 317. 529].