Methionine oxidation induces amyloid fibril formation by full-length apolipoprotein A-I.
Apolipoprotein A-I (apoA-I) is the major protein component of HDL, where it plays an important role in cholesterol transport. The deposition of apoA-I derived amyloid is associated with various hereditary systemic amyloidoses and atherosclerosis; however, very little is known about the mechanism of apoA-I amyloid formation. Methionine residues in apoA-I are oxidized via several mechanisms in vivo to form methionine sulfoxide (MetO), and significant levels of methionine oxidized apoA-I (MetO-apoA-I) are present in normal human serum. We investigated the effect of methionine oxidation on the structure, stability, and aggregation of full-length, lipid-free apoA-I. Circular dichrosim spectroscopy showed that oxidation of all three methionine residues in apoA-I caused partial unfolding of the protein and decreased its thermal stability, reducing the melting temperature (T(m)) from 58.7 degrees C for native apoA-I to 48.2 degrees C for MetO-apoA-I. Analytical ultracentrifugation revealed that methionine oxidation inhibited the native self association of apoA-I to form dimers and tetramers. Incubation of MetO-apoA-I for extended periods resulted in aggregation of the protein, and these aggregates bound Thioflavin T and Congo Red. Inspection of the aggregates by electron microscopy revealed fibrillar structures with a ribbon-like morphology, widths of approximately 11 nm, and lengths of up to several microns. X-ray fibre diffraction studies of the fibrils revealed a diffraction pattern with orthogonal peaks at spacings of 4.64 A and 9.92 A, indicating a cross-beta amyloid structure. This systematic study of fibril formation by full-length apoA-I represents the first demonstration that methionine oxidation can induce amyloid fibril formation.
Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, University of Melbourne, Parkville, Victoria 3010, Australia.
Yuan Qi
YQ
Wong
Katrina J
KJ
Binger
Geoffrey J
GJ
Howlett
Michael D W
MD
Griffin
D000682:Amyloid / Q000096:biosynthesis* / Q000737:chemistry / Q000648:ultrastructure; D016632:Apolipoprotein A-I / Q000737:chemistry* / Q000378:metabolism* / Q000648:ultrastructure; D052160:Benzothiazoles; D018360:Crystallography, X-Ray; D006801:Humans; D066298:In Vitro Techniques; D008715:Methionine / Q000737:chemistry*; D046529:Microscopy, Electron, Transmission; D008958:Models, Molecular; D010084:Oxidation-Reduction; D017510:Protein Folding; D055503:Protein Multimerization; D055550:Protein Stability; D013816:Thermodynamics; D013844:Thiazoles / Q000378:metabolism
D000682:Amyloid; D016632:Apolipoprotein A-I; D052160:Benzothiazoles; D013844:Thiazoles; C009462:thioflavin T; D008715:Methionine