Wang Limin, Colón Wilfredo
Rensselaer Polytechnic Institute, Department of Chemistry and Chemical Biology, Troy, NY 12180, USA.
Protein Sci. 2005 Jul;14(7):1811-7. doi: 10.1110/ps.051387005. Epub 2005 Jun 3.
Serum Amyloid A (SAA) is an acute phase reactant protein that is predominantly found bound to high-density lipoprotein in plasma. Upon inflammation, the plasma concentration of SAA can increase dramatically, occasionally leading to the development of amyloid A (AA) amyloidosis, which involves the deposition of SAA amyloid fibrils in major organs. We previously found that the murine isoform SAA2.2 exists in aqueous solution as a hexamer containing a central channel. Here we show using various biophysical and biochemical techniques that the SAA2.2 hexamer can be totally dissociated into monomer by approximately 2 M urea, with the concerted loss of its alpha-helical structure. However, limited trypsin proteolysis experiments in urea showed a conserved digestion profile, suggesting the preservation of major backbone topological features in the urea-denatured state of SAA2.2. The marginal stability of hexameric SAA2.2 and the presence of residual structure in the denatured monomeric protein suggest that both forms may interconvert in vivo to exert different functions to meet the various needs during normal physiological conditions and in response to inflammatory stimuli.
血清淀粉样蛋白A（SAA）是一种急性期反应蛋白，主要以与血浆中高密度脂蛋白结合的形式存在。在炎症反应时，血浆中SAA的浓度会急剧升高，偶尔会导致淀粉样蛋白A（AA）淀粉样变性的发生，这涉及SAA淀粉样原纤维在主要器官中的沉积。我们之前发现，小鼠同种型SAA2.2在水溶液中以含有中央通道的六聚体形式存在。在此，我们使用各种生物物理和生化技术表明，SAA2.2六聚体可被约2 M尿素完全解离为单体，同时其α-螺旋结构协同丧失。然而，在尿素中进行的有限胰蛋白酶消化实验显示出保守的消化图谱，这表明在SAA2.2的尿素变性状态下，主要的主链拓扑特征得以保留。六聚体SAA2.2的边缘稳定性以及变性单体蛋白中残余结构的存在表明，这两种形式可能在体内相互转化，以发挥不同功能，满足正常生理条件下以及应对炎症刺激时的各种需求。
