The Human Complement Regulator Factor H Binds Pneumococcal Surface Protein PspC via Short Consensus Repeats 13 to 15

The innate ability of Streptococcus pneumoniae to resist complement activation and complement-mediated phagocytosis may be a direct consequence of the ability of the bacteria to bind components of the complement regulatory system. One such component, factor H (fH), is a crucial fluid-phase negative regulator of the alternative pathway of complement and is utilized by a number of pathogenic organisms to resist complement attack. The pneumococcal surface protein C (PspC [also known as CbpA] and SpsA) has been shown to bind fH, although the exact binding site within one or more of the 20 short consensus repeats (SCRs) of the molecule is not known. The purpose of the current study was to map specific SCRs on fH responsible for this binding. Initial experiments utilizing type 2 pneumococcal strain D39 and its isogenic PspC-negative derivative (D39/pspC mutant) showed that fH binding was PspC dependent. A purified recombinant protein derivative of PspC that lacked the proline-rich region (PspCΔPro) had a reduced binding efficiency for fH, thereby directly showing the importance of this region for the fH interaction. We have specifically shown by inhibition experiments that SCRs responsible for heparin and C3b binding of fH are not involved in binding PspC and the interaction between fH and PspC is largely hydrophobic, since no inhibition was observed in the presence of high concentrations of NaCl. Construction of SCR proteins encompassing the whole fH molecule showed that SCRs 8 to 15 (SCR 8-15) mediated binding to PspC. Further localization experiments revealed that SCR 13 and SCR 15 were required for full binding, although partial binding was retained when either SCR was removed.

肺炎链球菌（Streptococcus pneumoniae）抵御补体激活及补体介导吞噬作用的内在能力，可能源于该细菌结合补体调节系统组分的特性。其中一类关键组分为补体因子H（factor H, fH），它是补体旁路途径的重要液相负调控因子，众多致病微生物可利用其躲避补体攻击。已有研究证实，肺炎球菌表面蛋白C（pneumococcal surface protein C，PspC，又称CbpA、SpsA）能够结合fH，但目前尚不明确该结合位点位于fH的20个短同源重复序列（short consensus repeats, SCRs）中的哪一个或哪几个区域。本研究旨在定位fH上负责与PspC结合的特定SCR区域。
初步实验以2型肺炎球菌菌株D39及其同基因PspC缺陷株（D39/pspC突变株）为研究对象，结果显示fH的结合依赖于PspC。缺失富脯氨酸区域的PspC重组蛋白衍生物（PspCΔPro）与fH的结合效率显著降低，直接证实了该区域在fH相互作用中的重要性。
本研究通过抑制实验明确证实，fH中负责结合肝素与补体C3b（C3b）的SCR区域并未参与PspC结合；且fH与PspC的相互作用主要以疏水方式介导，因为高浓度氯化钠（NaCl）并未对该结合产生抑制效果。针对覆盖fH全分子的SCR重组蛋白的构建实验显示，SCR 8至15（SCR 8-15）可介导与PspC的结合。进一步的定位实验表明，SCR 13与SCR 15是实现完全结合所必需的，但若移除其中任一SCR，仍可保留部分结合活性。