Akesson P, Schmidt K H, Cooney J, Björck L
Department of Medical and Physiological Chemistry, Lund University, Sweden.
Biochem J. 1994 Jun 15;300 ( Pt 3)(Pt 3):877-86. doi: 10.1042/bj3000877.
M1 protein and Protein H are surface proteins simultaneously present at the surface of certain strains of Streptococcus pyogenes, important pathogenic bacteria in humans. The present study concerns the structure, protein-binding properties and relationship between these two molecules. The gene encoding M1 protein (emm1) was found immediately upstream of the Protein H gene (sph). Both genes were preceded by a promoter region. Comparison of the sequences revealed a high degree of similarity in the signal peptides, the C repeats located in the central parts of the molecules and in the C-terminal cell-wall-attached regions, whereas the N-terminal sequences showed no significant similarity. Protein H has affinity for the Fc region of IgG antibodies. Also M1 protein, isolated from streptococcal culture supernatants or from Escherichia coli expressing emm1, was found to bind human IgGFc. When tested against polyclonal IgG from eight other mammalian species, M1 protein and Protein H both showed affinity for baboon, rabbit and pig IgG. M1 protein also reacted with guinea-pig IgG, whereas both streptococcal proteins were negative in binding experiments with rat, mouse, bovine and horse IgG. The two proteins were also tested against other members of the immunoglobulin super family: human IgM, IgA, IgD, IgE, beta 2-microglobulin, and major histocompatibility complex (MHC) class-I and class-II antigens. M1 protein showed no affinity for any of these molecules whereas Protein H reacted with MHC class-II antigens. M1 protein is known to bind albumin and fibrinogen also. The binding sites for these two plasma proteins and for IgGFc were mapped to different sites on M1 protein. Thus albumin bound to the C repeats and IgGFc to a region (S) immediately N-terminal of the C repeats. Finally, fibrinogen bound further towards the N-terminus but close to the IgGFc-binding site. On the fibrinogen molecule, fragment D was found to mediate binding to M1 protein. The IgGFc-binding region of M1 protein showed no similarity to that of Protein H. Still, competitive binding experiments demonstrated that the two streptococcal proteins bound to overlapping sites on IgGFc.
M1蛋白和H蛋白是化脓性链球菌某些菌株表面同时存在的表面蛋白，化脓性链球菌是人类重要的病原菌。本研究关注这两种分子的结构、蛋白结合特性以及它们之间的关系。编码M1蛋白的基因（emm1）位于H蛋白基因（sph）的紧邻上游。两个基因之前都有一个启动子区域。序列比较显示，信号肽、位于分子中部的C重复序列以及C末端细胞壁附着区域具有高度相似性，而N末端序列没有显著相似性。H蛋白对IgG抗体的Fc区域具有亲和力。同样，从链球菌培养上清液或表达emm1的大肠杆菌中分离出的M1蛋白也被发现能与人IgG Fc结合。当用来自其他八种哺乳动物的多克隆IgG进行测试时，M1蛋白和H蛋白对狒狒、兔和猪的IgG都表现出亲和力。M1蛋白也与豚鼠IgG发生反应，而在与大鼠、小鼠、牛和马的IgG结合实验中，两种链球菌蛋白均为阴性。这两种蛋白还针对免疫球蛋白超家族的其他成员进行了测试：人IgM、IgA、IgD、IgE、β2 -微球蛋白以及主要组织相容性复合体（MHC）I类和II类抗原。M1蛋白对这些分子均无亲和力，而H蛋白与MHC II类抗原发生反应。已知M1蛋白还能结合白蛋白和纤维蛋白原。这两种血浆蛋白以及IgG Fc的结合位点被定位到M1蛋白上的不同位点。因此，白蛋白结合到C重复序列，而IgG Fc结合到紧邻C重复序列N末端的一个区域（S）。最后，纤维蛋白原进一步结合到更靠近N末端但靠近IgG Fc结合位点的位置。在纤维蛋白原分子上，发现片段D介导与M1蛋白的结合。M1蛋白的IgG Fc结合区域与H蛋白的该区域没有相似性。然而，竞争性结合实验表明，这两种链球菌蛋白结合到IgG Fc上的重叠位点。
