Interaction properties of singly phosphorylated human beta-casein: similarities with other phosphorylation levels.

The singly and doubly phosphorylated forms of the human beta-caseins may function differently in casein micelle formation in human milk than the forms with higher phosphorylation (levels 3 to 5). This paper reports properties of the singly phosphorylated protein and compares them with the same previously measured properties for the forms with 0, 2, 3, and 5 phosphoryl groups, including the monomer characteristics, such as the extinction coefficient, partial specific volume, and molecular weight; the Ca2+ binding; the self-association and solubility patterns as monitored visually and with laser light scattering, sedimentation velocity, intrinsic viscosity, and ultraviolet spectroscopy; and the fluorescence polarization and intensity that are due to the binding of a fluorescent probe and to environmental changes in the vicinity of an intrinsic fluorophore. Changes could be attributed to relatively minor electrostatic differences between entities that vary in phosphoryl content. Major differences in interaction patterns between phosphorylation levels must therefore reside in the more complicated system in which inorganic o-phosphate is also present, thus permitting the proteins to interact with and be crosslinked by colloidal calcium phosphate.