PDF(Pubmed)
Abstract：
Serum amyloid A (SAA) is a highly conserved acute-phase protein that plays roles in activating multiple pro-inflammatory pathways during the acute inflammatory response and is commonly used as a biomarker of inflammation. It has been linked to beneficial roles in tissue repair through improved clearance of lipids and cholesterol from sites of damage. In patients with chronic inflammatory diseases, elevated levels of SAA may contribute to increased severity of the underlying condition. The majority of circulating SAA is bound to lipoproteins, primarily high-density lipoprotein (HDL). Interaction with HDL not only stabilizes SAA but also alters its functional properties, likely through altered accessibility of protein-protein interaction sites on SAA. While high-resolution structures for lipid-free, or apo-, forms of SAA have been reported, their relationship with the HDL-bound form of the protein, and with other possible mechanisms of SAA binding to lipids, has not been established. Here, we have 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. The 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. The SAA-lipid nanodiscs contain four SAA molecules and have similar exterior dimensions as the lipid-free SAA octamer, suggesting that relatively few conformational rearrangements may be required to allow SAA interactions with lipid-containing particles such as HDL. This study suggests a new model for SAA-lipid interactions and provides new insight into how SAA might stabilize protein-lipid nanodiscs or even replace ApoA-I as a scaffold for HDL particles during inflammation.
摘要：
血清淀粉样蛋白A（SAA）是一种高度保守的急性期蛋白，在急性炎症反应期间在激活多种促炎途径中起作用，通常用作炎症的生物标志物。通过改善脂质和胆固醇从损伤部位的清除，它与组织修复中的有益作用有关。在慢性炎症性疾病患者中，SAA水平升高可能导致基础疾病的严重程度增加.大多数循环SAA与脂蛋白结合，主要是高密度脂蛋白（HDL）。与HDL的相互作用不仅稳定了SAA，而且改变了其功能特性，可能是通过改变SAA上蛋白质-蛋白质相互作用位点的可及性。虽然高分辨率结构的无脂，或apo-，已经报道了SAA的形式，它们与HDL结合形式的蛋白质的关系，以及SAA与脂质结合的其他可能机制，尚未建立。这里,我们使用了多种生物物理技术，包括SAXS,TEM,SEC-MALS,天然凝胶电泳，戊二醛交联，和胰蛋白酶消化以表征SAA的无脂质和脂质结合形式。SAXS和TEM数据显示存在SAA的可溶性八聚体，其结构与无脂ApoA-I报道的环状结构相似。这些SAA八聚体代表无脂质SAA的先前未表征的结构，并且能够支架具有与ApoA-I形成的相似形态的脂质纳米盘。SAA-脂质纳米盘包含四个SAA分子，并且具有与无脂质SAA八聚体相似的外部尺寸，这表明，可能需要相对较少的构象重排来允许SAA与含有脂质的颗粒如HDL相互作用。这项研究提出了SAA-脂质相互作用的新模型，并提供了关于SAA如何稳定蛋白质-脂质纳米盘或甚至取代ApoA-I作为炎症期间HDL颗粒的支架的新见解。
