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

David C Thorn; Elmira Bahraminejad; Aidan B Grosas; Tomas Koudelka; Peter Hoffmann; Jitendra P Mata; Glyn L Devlin; Margaret Sunde; Heath Ecroyd; Carl Holt; John A Carver

doi:10.1016/j.bpc.2020.106530

Native disulphide-linked dimers facilitate amyloid fibril formation by bovine milk α_S2-casein

Biophys Chem. 2021 Mar:270:106530. doi: 10.1016/j.bpc.2020.106530. Epub 2020 Dec 15.

Authors

Affiliations

¹ Research School of Chemistry, The Australian National University, Acton, ACT 2601, Australia.
² Institute of Experimental Medicine, University of Kiel, Kiel 24105, Germany.
³ Future Industries Institute, University of South Australia, Mawson Lakes, SA 5095, Australia.
⁴ Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia.
⁵ Victorian Health and Human Services Building Authority, Melbourne, Victoria 3000, Australia.
⁶ Discipline of Pharmacology, School of Medical Sciences, Faculty of Medicine and Health and Sydney Nano, University of Sydney, Sydney, NSW 2006, Australia.
⁷ Molecular Horizons and School of Chemistry and Molecular Bioscience, University of Wollongong and Illawarra Health and Medical Research Institute, Wollongong, NSW 2522, Australia.
⁸ Institute of Molecular, Cell & Systems Biology, University of Glasgow, Glasgow G12 8QQ, Scotland, United Kingdom.
⁹ Research School of Chemistry, The Australian National University, Acton, ACT 2601, Australia. Electronic address: john.carver@anu.edu.au.

PMID: 33545456
DOI: 10.1016/j.bpc.2020.106530

Abstract

Bovine milk α_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 °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 α_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 α_S2-casein is a functional, yet amyloidogenic, structure.

Keywords: Amyloid fibril; Dimer; Intermolecular disulphide; Intramolecular disulphide; Milk casein protein; Monomer.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Amyloid / chemistry*
Amyloid / ultrastructure
Animals
Caseins / chemistry*
Caseins / ultrastructure
Cattle
Cysteine / analysis
Disulfides / analysis
Milk / chemistry
Protein Multimerization*

Substances

Amyloid
Caseins
Disulfides
Cysteine