Functional and dysfunctional folding, association and aggregation of caseins

Abstract

Caseins are a group of closely related intrinsically disordered proteins (IDPs), best known for their occurrence in milk as stable, polydisperse, roughly spherical, amorphous particles, typically containing thousands of protein chains and hundreds of nanoclusters of calcium phosphate. The particles are called casein micelles though their structure bears no resemblance to detergent micelles. Caseins have an open and flexible conformation with a preponderance of poly-l-proline II secondary structure and hence cannot be described as hydrophobic proteins. Individually, and in combination, they associate through polar and non-polar interactions to form polydisperse fuzzy complexes (including the native casein micelle) while retaining their hydrated and flexible conformation to a large degree. Like many other IDPs, caseins are prone to form cytotoxic amyloid fibrils. However, they are also highly effective molecular chaperones so that a mixture of different caseins can form fuzzy complexes that are often self-limiting in size and, within which, amyloid fibril formation is suppressed. The remarkable ability of caseins to sequester nanoclusters of calcium phosphate in stable complexes is due to their flexible conformation and multiply-phosphorylated short sequences. These features combine to form a dense peptide shell around the calcium phosphate making the core-shell complex thermodynamically stable, even at high calcium and phosphate concentrations. Thus, the casein micelle provides a readily digested, high calcium food for the neonate. It also preserves the functional properties of caseins as IDPs and protects the mammary gland against amyloid formation and pathological calcification, dysfunctional processes that would reduce the future reproductive success of the mother.

Keywords: Amyloid; Calcium phosphate; Casein; Intrinsic disorder; Molecular chaperone.