Invited review: Caseins and the casein micelle: Their biological functions, structures, and behavior in foods

Abstract

A typical casein micelle contains thousands of casein molecules, most of which form thermodynamically stable complexes with nanoclusters of amorphous calcium phosphate. Like many other unfolded proteins, caseins have an actual or potential tendency to assemble into toxic amyloid fibrils, particularly at the high concentrations found in milk. Fibrils do not form in milk because an alternative aggregation pathway is followed that results in formation of the casein micelle. As a result of forming micelles, nutritious milk can be secreted and stored without causing either pathological calcification or amyloidosis of the mother's mammary tissue. The ability to sequester nanoclusters of amorphous calcium phosphate in a stable complex is not unique to caseins. It has been demonstrated using a number of noncasein secreted phosphoproteins and may be of general physiological importance in preventing calcification of other biofluids and soft tissues. Thus, competent noncasein phosphoproteins have similar patterns of phosphorylation and the same type of flexible, unfolded conformation as caseins. The ability to suppress amyloid fibril formation by forming an alternative amorphous aggregate is also not unique to caseins and underlies the action of molecular chaperones such as the small heat-shock proteins. The open structure of the protein matrix of casein micelles is fragile and easily perturbed by changes in its environment. Perturbations can cause the polypeptide chains to segregate into regions of greater and lesser density. As a result, the reliable determination of the native structure of casein micelles continues to be extremely challenging. The biological functions of caseins, such as their chaperone activity, are determined by their composition and flexible conformation and by how the casein polypeptide chains interact with each other. These same properties determine how caseins behave in the manufacture of many dairy products and how they can be used as functional ingredients in other foods. aBStraCtA typical casein micelle contains thousands of casein molecules, most of which form thermodynamically stable complexes with nanoclusters of amorphous calcium phosphate. Like many other unfolded proteins, caseins have an actual or potential tendency to assemble into toxic amyloid fibrils, particularly at the high concentrations found in milk. Fibrils do not form in milk because an alternative aggregation pathway is followed that results in formation of the casein micelle. As a result of forming micelles, nutritious milk can be secreted and stored without causing either pathological calcification or amyloidosis of the mother's mammary tissue. The ability to sequester nanoclusters of amorphous calcium phosphate in a stable complex is not unique to caseins. It has been demonstrated using a number of noncasein secreted phosphoproteins and may be of general physiological importance in preventing calcification of other biofluids and soft tissues. Thus, competent noncasein phosphoproteins have similar patterns of phosp...

The functional roles of the unstructured N- and C-terminal regions in αB-crystallin and other mammalian small heat-shock proteins

“…potentially amyloid fibrilforming α-synuclein, to return to its natively unfolded (intrinsically disordered) state (Cox et al 2014;Treweek et al 2015). From our other studies, it is apparent that the unrelated molecular chaperones, clusterin, caseins and 14-3-3ζ, all exhibit a very similar mechanism of ATP-independent sHsp-like chaperone action (Carver et al 2003;Holt et al 2013;Thorn et al 2015;Williams et al 2011). In E. coli, curli proteins (e.g.…”

“…One explanation for sHsp oligomerisation is that it may protect against fibril formation, for example, within the ACD, in addition to the protection provided by the unstructured flanking regions. Other protein oligomers associate for such a reason, as we have shown for the unstructured milk casein proteins in which micelle (oligomer) formation by the four unrelated caseins (either with themselves individually, or with other caseins, or all of them to form the casein micelle in milk) prevents amyloid fibril formation by κ-and α s2 -casein via mutual chaperone interaction and also (principally within the casein micelle) by the chaperone action of the β-and α s1 -caseins (Holt et al 2013;Holt and Carver 2012;Thorn et al 2015). Similarly, transthyretin fibril formation requires initial dissociation from a tetrameric species prior to a conformational change within the monomer which leads to an aggregation-prone intermediate (Colon and Kelly 1992).…”

Extracellular Protein Phosphorylation, the Neglected Side of the Modification

“…The formed nanoclusters with a core of ACP surrounded by casein molecules assemble to casein micelles. These structures ensure supersaturation of calcium and phosphate in milk without calcification (57,58). Similar sequestration of ACP as calcium phosphate nanoclusters was demonstrated for OPN peptides suggesting that such nanoclusters of other secreted phosphoproteins might be involved in the inhibition of calcification in soft and mineralized tissues, the extracellular matrix or biofluids (59).…”

Letter to the Editor: A response to Horne and Lucey (2017)

“…Our reviews showed how exciting new ways of thinking about caseins and casein micelles are possible. They have proved popular with many casein researchers, as was recognised in 2016 when the review article published in this journal (Holt et al, 2013) was given the ADSA Most-Cited Paper Award in the Dairy Foods section. In the process of developing our argument some long-established notions about the nature of caseins were re-evaluated, among which was the relative importance of the hydrophobic effect in casein-casein interactions.…”

“…In 2013, we wrote a review in the Journal of Dairy Science (Holt et al, 2013) and another in the International Dairy Journal in 2015 (Thorn et al, 2015), both dealing with the application of modern protein science concepts to the chemistry and physics of caseins. These were substantial documents with an important message.…”