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Serum amyloid A forms stable oligomers that disrupt vesicles at lysosomal pH and contribute to the pathogenesis of reactive amyloidosis

Significance Although acidic conditions favor the misfolding of various proteins in amyloid diseases, the molecular underpinnings of this process are unclear. We used an array of spectroscopic, biochemical, and electron microscopic methods to unravel the effects of pH on serum amyloid A, a protein precursor of reactive amyloidosis, the major complication of chronic inflammation and one of the major human systemic amyloidoses worldwide. We found that at lysosomal pH this protein forms unusually stable proteolysis-resistant soluble oligomers that have solvent-accessible apolar surfaces, disrupt lipid vesicles, and undergo an α-helix to β-sheet transition in the presence of lipids. Such oligomers are likely to escape lysosomal degradation, accumulate in the lysosomes, and disrupt cellular membranes, thereby contributing to the development of amyloid A amyloidosis. Serum amyloid A (SAA) is an acute-phase plasma protein that functions in innate immunity and lipid homeostasis. SAA is a protein precursor of reactive AA amyloidosis, the major complication of chronic inflammation and one of the most common human systemic amyloid diseases worldwide. Most circulating SAA is protected from proteolysis and misfolding by binding to plasma high-density lipoproteins. However, unbound soluble SAA is intrinsically disordered and is either rapidly degraded or forms amyloid in a lysosome-initiated process. Although acidic pH promotes amyloid fibril formation by this and many other proteins, the molecular underpinnings are unclear. We used an array of spectroscopic, biochemical, and structural methods to uncover that at pH 3.5–4.5, murine SAA1 forms stable soluble oligomers that are maximally folded at pH 4.3 with ∼35% α-helix and are unusually resistant to proteolysis. In solution, these oligomers neither readily convert into mature fibrils nor bind lipid surfaces via their amphipathic α-helices in a manner typical of apolipoproteins. Rather, these oligomers undergo an α-helix to β-sheet conversion catalyzed by lipid vesicles and disrupt these vesicles, suggesting a membranolytic potential. Our results provide an explanation for the lysosomal origin of AA amyloidosis. They suggest that high structural stability and resistance to proteolysis of SAA oligomers at pH 3.5–4.5 help them escape lysosomal degradation, promote SAA accumulation in lysosomes, and ultimately damage cellular membranes and liberate intracellular amyloid. We posit that these soluble prefibrillar oligomers provide a missing link in our understanding of the development of AA amyloidosis.