Perkins Stephen J, Fung Ka Wai, Khan Sanaullah
Department of Structural and Molecular Biology, University College London , London , UK.
Front Immunol. 2014 Mar 31;5:126. doi: 10.3389/fimmu.2014.00126. eCollection 2014.
Complement factor H (CFH) is the major regulator of the central complement protein C3b in the alternative pathway of complement activation. A molecular view of the CFH interaction with native heparan sulfate (HS) is central for understanding the mechanism of how surface-bound CFH interacts with C3b bound to host cell surfaces. HS is composed of sulfated heparin-like S-regions that alternate with desulfated NA-regions. Solution structural studies of heparin (equivalent to the S-regions) and desulfated HS (the NA-regions) by scattering and ultracentrifugation showed that each structure was mostly extended and partially bent, but with greater bending and flexibility in the NA-regions compared to the S-regions. Their solution structures have been deposited in the Protein Data Bank. The largest HS oligosaccharides showed more bent and flexible structures than those for heparin. A folded-back domain structure for the solution structure of the 20 domains in CFH was determined likewise. CFH binds to the S-regions but less so to the NA-regions of HS. The bivalent interaction of CFH-heparin was observed by ultracentrifugation, and binding studies of CFH fragments with heparin-coated sensor chips. In common with other CFH interactions with its physiological and pathophysiological ligands, the CFH-heparin and CFH-C3b interactions have moderate micromolar dissociation constants K D, meaning that these complexes do not fully form in vivo. The combination of the solution structures and binding studies indicated a two-site interaction model of CFH with heparin at cell surfaces. By this, the bivalent binding of CFH to a cell surface is co-operative. Defective interactions at either of the two independent CFH-heparin sites reduce the CFH interaction with surface-bound C3b and lead to immune disorders.
补体因子H（CFH）是补体激活替代途径中中央补体蛋白C3b的主要调节因子。了解结合在表面的CFH如何与结合在宿主细胞表面的C3b相互作用的机制，关键在于对CFH与天然硫酸乙酰肝素（HS）相互作用的分子层面认识。HS由硫酸化的肝素样S区域和去硫酸化的NA区域交替组成。通过散射和超速离心对肝素（等同于S区域）和去硫酸化的HS（NA区域）进行的溶液结构研究表明，每种结构大多呈伸展状且部分弯曲，但与S区域相比，NA区域的弯曲度和柔韧性更大。它们的溶液结构已存入蛋白质数据库。最大的HS寡糖比肝素的结构更弯曲、更具柔韧性。同样确定了CFH中20个结构域的溶液结构的折返结构域结构。CFH与HS的S区域结合，但与NA区域的结合较少。通过超速离心以及CFH片段与肝素包被的传感器芯片的结合研究，观察到了CFH-肝素的二价相互作用。与CFH与其生理和病理生理配体的其他相互作用一样，CFH-肝素和CFH-C3b相互作用具有中等微摩尔解离常数KD，这意味着这些复合物在体内不会完全形成。溶液结构和结合研究的结合表明，CFH在细胞表面与肝素存在双位点相互作用模型。据此，CFH与细胞表面的二价结合具有协同性。两个独立的CFH-肝素位点中任何一个的相互作用缺陷都会降低CFH与结合在表面的C3b的相互作用，并导致免疫紊乱。
