Aggregation and fibrillation of bovine serum albumin

The all-α helix multi-domain protein bovine serum albumin (BSA) aggregates at elevated temperatures. Here we show that these thermal aggregates have amyloid properties. They bind the fibril-specific dyes Thioflavin T and Congo Red, show elongated although somewhat worm-like morphology and characteristic amyloid X-ray fiber diffraction peaks. Fibrillation occurs over minutes to hours without a lag phase, is independent of seeding and shows only moderate concentration dependence, suggesting intramolecular aggregation nuclei. Nevertheless, multi-exponential increases in dye-binding signal and changes in morphology suggest the existence of different aggregate species. Although β-sheet content increases from 0 to ca. 40% upon aggregation, the aggregates retain significant amounts of α-helix structure, and lack a protease-resistant core. Thus BSA is able to form well-ordered β-sheet rich aggregates which nevertheless do not possess the same structural rigidity as classical fibrils. The aggregates do not permeabilize synthetic membranes and are not cytotoxic. The ease with which a multidomain all-α helix protein can form higher-order β-sheet structure, while retaining significant amounts of α-helix, highlights the universality of the fibrillation mechanism. However, the presence of non-β-sheet structure may influence the final fibrillar structure and could be a key component in aggregated BSA's lack of cytotoxicity.