Exploratory Research Center on Life and Living Systems (ExCELLS), Institute for Molecular Science (IMS), National Institutes of Natural Sciences, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan.
Faculty and Graduate School of Pharmaceutical Sciences, Nagoya City University, 3-1 Tanabe-dori, Mizuho-ku, Nagoya 467-8603, Japan.
Int J Mol Sci. 2022 Feb 14;23(4):2090. doi: 10.3390/ijms23042090.
Immunoglobulin G (IgG) adopts a modular multidomain structure that mediates antigen recognition and effector functions, such as complement-dependent cytotoxicity. IgG molecules are self-assembled into a hexameric ring on antigen-containing membranes, recruiting the complement component C1q. In order to provide deeper insights into the initial step of the complement pathway, we report a high-speed atomic force microscopy study for the quantitative visualization of the interaction between mouse IgG and the C1 complex composed of C1q, C1r, and C1s. The results showed that the C1q in the C1 complex is restricted regarding internal motion, and that it has a stronger binding affinity for on-membrane IgG2b assemblages than C1q alone, presumably because of the lower conformational entropy loss upon binding. Furthermore, we visualized a 1:1 stoichiometric interaction between C1/C1q and an IgG2a variant that lacks the entire C1 domain in the absence of an antigen. In addition to the canonical C1q-binding site on Fc, their interactions are mediated through a secondary site on the C domain that is cryptic in the presence of the C1 domain. Our findings offer clues for novel-modality therapeutic antibodies.
免疫球蛋白 G（IgG）采用模块化多结构域结构，介导抗原识别和效应功能，如补体依赖性细胞毒性。IgG 分子在含有抗原的膜上自组装成六聚体环，募集补体成分 C1q。为了更深入地了解补体途径的初始步骤，我们报告了一项高速原子力显微镜研究，用于定量可视化小鼠 IgG 与由 C1q、C1r 和 C1s 组成的 C1 复合物之间的相互作用。结果表明，C1 复合物中的 C1q 内部运动受到限制，并且它与膜上 IgG2b 组装体的结合亲和力强于单独的 C1q，这可能是由于结合时构象熵损失较低。此外，我们在没有抗原的情况下可视化了 C1/C1q 与缺乏整个 C1 结构域的 IgG2a 变体之间的 1:1 化学计量相互作用。除了 Fc 上的典型 C1q 结合位点外，它们的相互作用还通过 C 结构域上的次要位点介导，该位点在存在 C1 结构域时是隐匿的。我们的发现为新型治疗性抗体提供了线索。
