Chapter 4 - Post-translational modifications of caseins

Abstract

Caseins are phosphoproteins and constitute about 80% of the protein in milk. Caseins exhibit a high degree of heterogeneity as a result of post-translational modifications (PTMs). Phosphorylation of the α- and β-caseins and glycosylation of κ-casein are the best-known modifications and are critical for the formation and stability of casein micelles. κ-Casein has long been known to exhibit a high degree of variability in glycosylation. This chapter summarizes current understanding on PTMs of the αs1-, αs2-, β-, and κ-caseins. Caseins are assembled in a colloidal complex with calcium phosphate and small amounts of other minerals. The casein micelle structure is critical in determining the physical properties of milk and for the provision of amino acids, calcium, and phosphorus for infant nutrition. A critical factor in micelle formation and stability is the presence PTMs such as phosphorylation, glycosylation, and perhaps disulfide bond formation that occur in the endoplasmic reticulum and/or Golgi complex after synthesis of the polypeptide chain. In recent years, the development of proteomic technologies has greatly enhanced our ability to analyze milk proteins, particularly with respect to PTMs. Two-dimensional electrophoresis (2-DE) provides a high-resolution methodology for displaying the heterogeneity of the major milk proteins. Advances in mass spectrometry (MS) have enhanced our ability to analyze the proteins arrayed on 2-D gels. Therefore, not only is it possible to resolve many proteins and their isoforms but it is also possible to characterize them, particularly with respect to the many PTMs that affect their electrophoretic mobility. Furthermore, the chapter includes an extended discussion on the functional significance of κ-casein heterogeneity.