Division of Plasma Protein Therapeutics, Office of Tissues and Advanced Therapies, Center for Biologics Evaluation and Research, Food and Drug Administration, Silver Spring, MD 20993-0002.
Department of Transfusion Medicine, Warren Grant Magnuson Clinical Center, NIH, Bethesda, MD 20892
Proc Natl Acad Sci U S A. 2021 Jul 13;118(28). doi: 10.1073/pnas.2104242118.
Epitope III, a highly conserved amino acid motif of APTYSW on the hepatitis C virus (HCV) E2 glycoprotein, resides in the critical loop that binds to the host receptor CD81, thus making it one of the most important antibody targets for blocking HCV infections. Here, we have determined the X-ray crystal structure of epitope III at a 2.0-Å resolution when it was captured by a site-specific neutralizing antibody, monoclonal antibody 1H8 (mAb1H8). The snapshot of this complex revealed that epitope III has a relatively rigid structure when confined in the binding grooves of mAb1H8, which confers the residue specificity at both ends of the epitope. Such a high shape complementarity is reminiscent of the "lock and key" mode of action, which is reinforced by the incompatibility of an antibody binding with an epitope bearing specific mutations. By subtly positioning the side chains on the three residues of Tyr, Ser, and Trp while preserving the spatial rigidity of the rest, epitope III in this cocrystal complex adopts a unique conformation that is different from previously described E2 structures. With further analyses of molecular docking and phage display-based peptide interactions, we recognized that it is the arrangements of two separate sets of residues within epitope III that create these discrete conformations for the epitope to interact selectively with either mAb1H8 or CD81. These observations thus raise the possibility that local epitope III conformational dynamics, in conjunction with sequence variations, may act as a regulatory mechanism to coordinate "mAb1H8-like" antibody-mediated immune defenses with CD81-initiated HCV infections.
抗原表位 III 是丙型肝炎病毒 (HCV) E2 糖蛋白上高度保守的氨基酸模体 APTYSW，位于与宿主受体 CD81 结合的关键环中，因此成为阻断 HCV 感染的最重要的抗体靶标之一。在这里，我们通过特异性中和抗体单克隆抗体 1H8（mAb1H8）捕获时，确定了抗原表位 III 在 2.0-Å 分辨率下的 X 射线晶体结构。该复合物的快照显示，当抗原表位 III 被限制在 mAb1H8 的结合槽中时，它具有相对刚性的结构，从而赋予抗原表位两端的残基特异性。这种高形状互补性让人联想到“锁和钥匙”作用模式，而抗体与具有特定突变的抗原表位的不兼容性则加强了这种作用模式。通过巧妙地调整 Tyr、Ser 和 Trp 三个残基的侧链，同时保持其余部分的空间刚性，抗原表位 III 在这个共晶复合物中采用了一种独特的构象，与之前描述的 E2 结构不同。通过进一步分析分子对接和噬菌体展示肽相互作用，我们认识到，正是抗原表位 III 内两组独立的残基排列，为抗原表位与 mAb1H8 或 CD81 选择性相互作用创造了这些离散的构象。这些观察结果表明，局部抗原表位 III 构象动力学与序列变异一起，可能作为一种调节机制，协调“mAb1H8 样”抗体介导的免疫防御与 CD81 启动的 HCV 感染。
