Primary structures of caseins from 20 species, including two monotremes and two marsupials, have been compared. Sequences of the mature proteins are very divergent, whereas variation in amino acid composition is mostly restricted to a range of disorder-promoting residues. The number and size of clusters of phosphorylation sites in the caseins is variable, blurring the boundaries between them. Casein polar tract sequences were found in all caseins, though of variable lengths, and are chiefly responsible for weak and dynamic interactions among the tangled web of peptide chains in the matrix of casein micelles. The interactions take the predominant form of backbone-to-backbone contacts rather than the sequence-specific side chain interactions of the hydrophobic effect. It is suggested that the dynamic casein micelle matrix be represented by an ensemble of interchanging structures with different types and degrees of inhomogeneity, influenced by solvent quality and other environmental factors
