Structural studies of a serum amyloid A octamer that is primed to scaffold lipid nanodiscs

Abstract

Serum amyloid A (SAA) is a highly conserved acute-phase protein that acts on multiple pro-inflammatory pathways during the inflammatory response and is used as a biomarker of inflammation. It has also been linked to beneficial roles in tissue repair through improved clearance of lipids and cholesterol. In patients with chronic inflammatory diseases, elevated SAA may contribute to increased severity of the underlying condition. The majority of circulating SAA is bound to high-density lipoprotein (HDL), stabilizing SAA and altering its functional properties, likely through altered accessibility of protein-protein interaction sites on SAA. While high-resolution structures for lipid-free forms of SAA have been reported, their relationship with the lipid or HDL-bound forms of the protein, has not been established. We used multiple biophysical techniques, including SAXS, TEM, SEC-MALS, native gel electrophoresis, glutaraldehyde crosslinking, and trypsin digestion to characterize the lipid-free and lipid-bound forms of SAA. SAXS and TEM data show the presence of soluble octamers of SAA with structural similarity to the ring-like structures reported for lipid-free ApoA-I. These SAA octamers represent a previously uncharacterized structure for lipid-free SAA and are capable of scaffolding lipid nanodiscs with similar morphology to those formed by ApoA-I. SAA-lipid nanodiscs contain four SAA molecules and have similar exterior dimensions as the lipid-free SAA octamer, suggesting that relatively few conformational rearrangements are required for lipid binding. This study suggests a new model for SAA-lipid interactions and provides new insight into the ability of SAA to stabilize protein-lipid nanodiscs or even replace ApoA-I on HDL particles during inflammation.