The effects of molecular crowding on the amyloid fibril formation of alpha-lactalbumin and the chaperone action of alpha-casein.

Amyloid fibrils arise from the slow aggregation of intermediately folded protein states. In this study the kinetics of the protein fibril formation of alpha-lactalbumin and its prevention by alphaS-casein in the presence and absence of the crowding agent, dextran (68 kDa), have been compared using a thioflavin T binding assay. It was found that alphaS-casein, a molecular chaperone found in bovine milk, is a potent in vitro inhibitor of alpha-lactalbumin fibrillization. The effect of alphaS-casein in preventing fibril formation was significant, although less than it is in the absence of the crowding agent, dextran. The interaction between the chaperone and the alpha-lactalbumin and structural change in the target protein are also shown using intrinsic fluorescence intensity, an ANS binding assay, CD spectroscopy and size-exclusion HPLC. In summary, alpha-casein interacts with alpha-lactalbumin and prevents amyloid formation but not as well as it does when the crowding agent, dextran, not present.