Ponnuraj Karthe, Xu Yuanyuan, Macon Kevin, Moore Dwight, Volanakis John E, Narayana Sthanam V L
Center for Biophysical Sciences and Engineering, School of Optometry, University of Alabama at Birmingham, Birmingham, AL 35294, USA.
Mol Cell. 2004 Apr 9;14(1):17-28. doi: 10.1016/s1097-2765(04)00160-1.
The C-terminal fragment, Bb, of factor B combines with C3b to form the pivotal C3-convertase, C3bBb, of alternative complement pathway. Bb consists of a von Willebrand factor type A (vWFA) domain that is structurally similar to the I domains of integrins and a serine protease (SP) domain that is in inactive conformation. The structure of the C3bBb complex would be important in deciphering the activation mechanism of the SP domain. However, C3bBb is labile and not amenable to X-ray diffraction studies. We engineered a disulfide bond in the vWFA domain of Bb homologous to that shown to lock I domains in active conformation. The crystal structures of Bb(C428-C435) and its inhibitor complexes reveal that the adoption of the "active" conformation by the vWFA domain is not sufficient to activate the C3-convertase catalytic apparatus and also provide insights into the possible mode of C3-convertase activation.
B因子的C末端片段Bb与C3b结合，形成替代补体途径的关键C3转化酶C3bBb。Bb由一个与整合素I结构域在结构上相似的血管性血友病因子A（vWFA）结构域和一个处于无活性构象的丝氨酸蛋白酶（SP）结构域组成。C3bBb复合物的结构对于解读SP结构域的激活机制至关重要。然而，C3bBb不稳定，不适合进行X射线衍射研究。我们在Bb的vWFA结构域中设计了一个二硫键，该二硫键与已显示能将I结构域锁定在活性构象的二硫键同源。Bb（C428 - C435）及其抑制剂复合物的晶体结构表明，vWFA结构域采用“活性”构象不足以激活C3转化酶催化装置，同时也为C3转化酶激活的可能模式提供了见解。
