ジャーナル: Proc Natl Acad Sci U S A / 年: 2019 タイトル: Insights into IgM-mediated complement activation based on in situ structures of IgM-C1-C4b. 著者: Thomas H Sharp / Aimee L Boyle / Christoph A Diebolder / Alexander Kros / Abraham J Koster / Piet Gros / 要旨: Antigen binding by serum Ig-M (IgM) protects against microbial infections and helps to prevent autoimmunity, but causes life-threatening diseases when mistargeted. How antigen-bound IgM activates ...Antigen binding by serum Ig-M (IgM) protects against microbial infections and helps to prevent autoimmunity, but causes life-threatening diseases when mistargeted. How antigen-bound IgM activates complement-immune responses remains unclear. We present cryoelectron tomography structures of IgM, C1, and C4b complexes formed on antigen-bearing lipid membranes by normal human serum at 4 °C. The IgM-C1-C4b complexes revealed C4b product release as the temperature-limiting step in complement activation. Both IgM hexamers and pentamers adopted hexagonal, dome-shaped structures with Fab pairs, dimerized by hinge domains, bound to surface antigens that support a platform of Fc regions. C1 binds IgM through widely spread C1q-collagen helices, with C1r proteases pointing outward and C1s bending downward and interacting with surface-attached C4b, which further interacts with the adjacent IgM-Fab and globular C1q-recognition unit. Based on these data, we present mechanistic models for antibody-mediated, C1q-transmitted activation of C1 and for C4b deposition, while further conformational rearrangements are required to form C3 convertases.
凍結剤: ETHANE / チャンバー内温度: 277.15 K / 装置: LEICA EM GP
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電子顕微鏡法
顕微鏡
FEI TITAN KRIOS
特殊光学系
位相板: VOLTA PHASE PLATE / エネルギーフィルター - 名称: GIF Quantum LS / エネルギーフィルター - スリット幅: 20 eV
撮影
フィルム・検出器のモデル: GATAN K2 SUMMIT (4k x 4k) 検出モード: COUNTING / 平均露光時間: 2.4 sec. / 平均電子線量: 1.48 e/Å2 詳細: Exposures of 2.4 sec were dose-fractionated into 6 movie frames per tilt angle. The total dose for each tilt series was 80 e-/A^2. Focusing to -300 nm was performed before each image ...詳細: Exposures of 2.4 sec were dose-fractionated into 6 movie frames per tilt angle. The total dose for each tilt series was 80 e-/A^2. Focusing to -300 nm was performed before each image acquisition using a low-dose routine