The two-faced nature of milk casein proteins: Amyloid fibril formation and chaperone-like activity

Molecular chaperones are a diverse group of proteins that stabilise partially folded target proteins to prevent their misfolding, aggregation and potential precipitation under conditions of cellular stress, e.g. elevated temperature. Protein aggregation, particularly the formation of highly ordered protein aggregates termed amyloid fibrils, is of considerable research interest because of its intimate association with a wide range of debilitating diseases, including Alzheimer's, Parkinson's and Huntington's diseases and type II diabetes. In this review, we discuss the ability of the milk casein proteins to act in a chaperone-like manner. This property is of biological importance since at least two of the casein proteins, α_S2- and κ-casein, have a propensity to assemble into amyloid fibrils under physiological conditions. The fibril-forming propensity of α_s2- and κ-casein, the possibility of its occurrence in mammary tissue, and the ability of the other casein proteins, α_s1- and β-casein, to inhibit the aggregation of α_s2- and κ-casein and other proteins, are discussed. The results have application in the use of casein proteins in a systematic manner to stabilise other proteins at high temperature and under shear conditions, as occurs in the industrial treatment of milk and milk-based products.