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

Bovine milk alpha(S2)-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 degrees C. Upon incubation at 37 degrees 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 Ala(81) to Lys(113) is incorporated into the fibril core, implying that this region, which is predicted by several algorithms to be amyloidogenic, initiates fibril formation of alpha(S2)-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 alpha(S2)-casein is a functional, yet amyloidogenic, structure.

Automated summary

Bovine milk alpha(S2)-casein, an intrinsically disordered protein, readily forms amyloid fibrils in vitro. The disulfide-linked dimer of this protein has a greater propensity to form amyloid fibrils than its monomeric counterpart when incubated at 37 degrees Celsius. Experimental results suggest that the dimer structure of this protein exhibits amyloid-like characteristics.