Agrawal A, Shrive A K, Greenhough T J, Volanakis J E
Division of Clinical Immunology and Rheumatology, Department of Medicine, University of Alabama, Birmingham, AL 35294, USA.
J Immunol. 2001 Mar 15;166(6):3998-4004. doi: 10.4049/jimmunol.166.6.3998.
The host defense functions of human C-reactive protein (CRP) depend to a great extent on its ability to activate the classical complement pathway. The aim of this study was to define the topology and structure of the CRP site that binds C1q, the recognition protein of the classical pathway. We have previously reported that residue Asp(112) of CRP plays a major role in the formation of the C1q-binding site, while the neighboring Lys(114) hinders C1q binding. The three-dimensional structure of CRP shows the presence of a deep, extended cleft in each protomer on the face of the pentamer opposite that containing the phosphocholine-binding sites. Asp(112) is part of this marked cleft that is deep at its origin but becomes wider and shallower close to the inner edge of the protomer and the central pore of the pentamer. The shallow end of the pocket is bounded by the 112-114 loop, residues 86-92 (the inner loop), the C terminus of the protomer, and the C terminus of the pentraxin alpha-helix 169-176, particularly Tyr(175). Mutational analysis of residues participating in the formation of this pocket demonstrates that Asp(112) and Tyr(175) are important contact residues for C1q binding, that Glu(88) influences the conformational change in C1q necessary for complement activation, and that Asn(158) and His(38) probably contribute to the correct geometry of the binding site. Thus, it appears that the pocket at the open end of the cleft is the C1q-binding site of CRP.
人C反应蛋白（CRP）的宿主防御功能在很大程度上取决于其激活经典补体途径的能力。本研究的目的是确定CRP与经典途径识别蛋白C1q结合位点的拓扑结构和结构。我们之前报道过，CRP的天冬氨酸残基（Asp112）在C1q结合位点的形成中起主要作用，而相邻的赖氨酸残基（Lys114）则阻碍C1q结合。CRP的三维结构显示，在五聚体中与含磷酸胆碱结合位点相对的一面上，每个原体都存在一个深而延伸的裂隙。Asp112是这个明显裂隙的一部分，裂隙在起始处较深，但在靠近原体内边缘和五聚体中心孔处变宽变浅。裂隙浅端由112 - 114环、86 - 92残基（内环）、原体的C末端以及五聚素α - 螺旋169 - 176的C末端界定，尤其是酪氨酸残基（Tyr175）。对参与形成该裂隙的残基进行突变分析表明，Asp112和Tyr175是C1q结合的重要接触残基，谷氨酸残基（Glu88）影响补体激活所需的C1q构象变化，天冬酰胺残基（Asn158）和组氨酸残基（His38）可能有助于结合位点的正确几何形状。因此，似乎裂隙开口端的口袋状结构就是CRP的C1q结合位点。
