Polymorphism in casein protein aggregation and amyloid fibril formation
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Caseins are a diverse group of proteins present in milk. They exhibit a strong tendency to associate with themselves and with each other, generating a variety of oligomeric species and structures. In mammary tissue, this tendency leads to the synthesis and secretion in milk of the casein micelle, the primary functional state of caseins. In the absence of their micellar counterparts, at least two of the four bovine caseins, κ- and αs2, preferentially form rope-like amyloid fibrils which themselves display a considerable degree of polymorphism. The heterogeneity of casein assemblies, along with the intrinsic flexibility and dynamism of their polypeptide chains, preclude detailed characterization by high-resolution techniques such as X-ray crystallography and NMR spectroscopy, while small-angle X-ray and neutron-scattering techniques provide limited information without recourse to modeling. In this chapter we describe the use of transmission electron microscopy and complementary techniques to examine and differentiate between casein assemblies formed under various conditions, with particular focus on the formation of κ- and αs2-casein amyloid fibrils. The ribbons, loops, spheres, rods and other structures observed highlight the polymorphism in casein fibril formation and aggregation in general, and the utility of bio-nanoimaging techniques, such as electron microscopy, for the characterization of aggregation-prone, misfolded proteins.