Title

Amyloid fibril formation by bovine milk alpha(s2)-casein occurs under physiological conditions yet is prevented by its natural counterpart, alpha(s1)-casein

RIS ID

25939

Publication Details

Thorn, D., Ecroyd, H., Sunde, M., Poon, S. & Carver, J. A. (2008). Amyloid fibril formation by bovine milk alpha(s2)-casein occurs under physiological conditions yet is prevented by its natural counterpart, alpha(s1)-casein. Biochemistry, 47 (12), 3926-3636.

Abstract

The calcified proteinaceous deposits, or corpora amylacea, of bovine mammary tissue often comprise a network of amyloid fibrils, the origins of which have not been fully elucidated. Here, we demonstrate by transmission electron microscopy, dye binding assays, and X-ray fiber diffraction that bovine milk alpha(s2)-casein, a protein synthesized and secreted by mammary epithelial cells, readily forms fibrils in vitro. As a component of whole alpha(s)-casein, alpha(s2)-casein was separated from alpha(s1)-casein under nonreducing conditions via cation-exchange chromatography. Upon incubation at neutral pH and 37 degrees C, the spherical particles typical of alpha(s2)-casein rapidly converted to twisted, ribbon-like fibrils similar to 12 nm in diameter, which occasionally formed loop structures. Despite. their irregular morphology, these fibrils possessed a P-sheet core structure and the ability to bind amyloidophilic dyes such as thioflavin T. Fibril formation was optimal at pH 6.5-6.7 and was promoted by higher incubation temperatures. Interestingly, the protein appeared to be less prone to fibril formation upon disulfide bond reduction with dithiothreitol. Thus, (alpha(s2)-casein is particularly susceptible to fibril formation under physiological conditions. However, our findings indicate that alpha(s2)-casein fibril formation is potently inhibited by its natural counterparts alpha(s1)-casein, while is Only partially inhibited by beta-casein. These findings highlight the inherent propensity of casein proteins to form amyloid fibrils and the importance of casein-casein interactions in preventing such fibril formation in vivo.

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Link to publisher version (DOI)

http://dx.doi.org/10.1021/bi701278c