Native disulphide-linked dimers facilitate amyloid fibril formation by bovine milk αS2-casein

Bovine milk αS2-casein, an intrinsically disordered protein, readily forms amyloid fibrils in vitro and is implicated in the formation of amyloid fibril deposits in mammary tissue. Its two cysteine residues participate in the formation of either intra- or intermolecular disulphide bonds, generating monomer and dimer species. X-ray solution scattering measurements indicated that both forms of the protein adopt large, spherical oligomers at 20 °C. Upon incubation at 37 °C, the disulphide-linked dimer showed a significantly greater propensity to form amyloid fibrils than its monomeric counterpart. Thioflavin T fluorescence, circular dichroism and infrared spectra were consistent with one or both of the dimer isomers (in a parallel or antiparallel arrangement) being predisposed toward an ordered, amyloid-like structure. Limited proteolysis experiments indicated that the region from Ala81 to Lys113 is incorporated into the fibril core, implying that this region, which is predicted by several algorithms to be amyloidogenic, initiates fibril formation of αS2-casein. The partial conservation of the cysteine motif and the frequent occurrence of disulphide-linked dimers in mammalian milks despite the associated risk of mammary amyloidosis, suggest that the dimeric conformation of αS2-casein is a functional, yet amyloidogenic, structure. © 2020 Elsevier B.V

Description

Dedicated to the memory of Prof. Sir Chris Dobson FRS, who encouraged, supported, inspired, and mentored many of this manuscript's authors and provided significant intellectual and critical input into our investigations of protein aggregation, including of caseins.

Keywords

Milk, Casein, Proteins, Disulfides, Monomers, Dimers, Cows, X-ray diffraction

Citation

Thorn, D. C., Bahraminejad, E., Grosas, A. B., Koudelka, T., Hoffmann, P., Mata, J. P., .Devlin, G. L., Sunde, M., Ecroyd, H., Holt, C., & Carver, J. A. (2021). Native disulphide-linked dimers facilitate amyloid fibril formation by bovine milk αS2-casein. Biophysical Chemistry, 270, 106530. doi:10.1016/j.bpc.2020.106530