The structural mechanism of Serum Amyloid A -mediated Secondary amyloid formation (135.34)

Abstract

Abstract Secondary amyloidosis is a severe complication of many chronic inflammatory diseases, such as rheumatoid arthritis and atherosclerosis, when the C-terminally cleaved fragments of acute phase protein Serum Amyloid A (SAA) are deposited in tissues as amyloid fibrils. The pathologic process of secondary amyloidosis involves a conversion of the structure of native SAA into a predominantly antiparallel β-sheet secondary structure. We have now determined the X-ray structure of SAA in a monomeric and a hexameric form at 2.2Å and 2.7Å resolution, respectively. The hexameric SAA structure revealed a dimeric architecture arranged as two trimers. Strong interactions between the C-terminal loop region and four-helix-bundle core of the SAA monomers create a stable structure to prevent secondary structure refolding during amyloidogenesis. The presence of multimeric forms in human serum was confirmed by size-exclusion experiments. Recombinant monomeric, trimeric and hexameric SAA protein were purified from Escherichia coli and characterized. Binding studies show that the monomeric and trimeric but not the hexameric SAA bind to Toll-like receptor 2 and 4. In line with these experiments, SAA may undergo a transition between hexamers and monomers, which would activate Toll-like receptors to further complicate the chronic inflammation in disease situation.