Pangburn M K, Pangburn K L, Koistinen V, Meri S, Sharma A K
Department of Biochemistry, University of Texas Health Science Center, Tyler, TX 75708, USA.
J Immunol. 2000 May 1;164(9):4742-51. doi: 10.4049/jimmunol.164.9.4742.
In the alternative pathway of complement (APC) factor H is the primary control factor involved in discrimination between potential pathogens. The APC deposits C3b on possible Ags, and the interaction with factor H determines whether the initial C3b activates the APC. Factor H is composed of a linear array of 20 homologous short consensus repeats (SCR) domains with many functional sites. Three of these sites are involved in binding C3b and regulating complement activation; others bind to sialic acid and/or heparin and are responsible for host recognition. Using site-directed mutations we have examined the contributions of each of these sites to target discrimination and to functional activities of factor H. Decay acceleration by SCR1-4 of C3/C5 convertases bound to nonactivators was strongly dependent on SCR domains 11-15 and 16-20. Loss of these regions caused a 97% loss of activity, with SCR16-20 being the most critical (>90% loss). On APC activators the pattern of site usage was different and unique on each. On yeast, deletion of the 10 C-terminal domains (SCR11-20) had no effect on specific activity. On rabbit erythrocytes, this deletion caused loss of 75% of the specific activity. An examination of binding affinity to C3b on the four cell types demonstrated that factor H exhibits a unique pattern of SCR involvement on each cell. The results reveal a complex molecular mechanism of discrimination between microbes and host in this ancient innate defense system and help explain the different rates and intensities of APC activation on different biological particles.
在补体替代途径(APC)中,因子H是参与区分潜在病原体的主要控制因子。APC在可能的抗原上沉积C3b,与因子H的相互作用决定了初始C3b是否激活APC。因子H由20个具有许多功能位点的同源短共有重复序列(SCR)结构域的线性阵列组成。其中三个位点参与结合C3b并调节补体激活;其他位点与唾液酸和/或肝素结合,负责宿主识别。我们使用定点突变研究了这些位点对因子H的靶标识别和功能活性的贡献。与非激活剂结合的C3/C5转化酶的SCR1-4介导的衰变加速强烈依赖于SCR结构域11-15和16-20。这些区域的缺失导致活性丧失97%,其中SCR16-20最为关键(活性丧失>90%)。在APC激活剂上,位点使用模式在每种激活剂上都不同且独特。在酵母上,删除10个C末端结构域(SCR11-20)对比活性没有影响。在兔红细胞上,这种删除导致比活性丧失75%。对四种细胞类型上与C3b的结合亲和力的研究表明,因子H在每种细胞上表现出独特的SCR参与模式。结果揭示了这个古老的先天防御系统中微生物与宿主之间识别的复杂分子机制,并有助于解释不同生物颗粒上APC激活的不同速率和强度。