Wikoff W R, Wang G, Parrish C R, Cheng R H, Strassheim M L, Baker T S, Rossmann M G
Department of Biological Sciences, Purdue University, West Lafayette, IN 47907-1392.
Structure. 1994 Jul 15;2(7):595-607. doi: 10.1016/s0969-2126(00)00062-9.
Members of the Parvovirus genus cause a variety of diseases in mammals, including humans. One of the major defences against viral infection is the presence of neutralizing antibodies that prevent virus particles from infecting target cells. The mechanism of neutralization is not well understood. We therefore studied the structure of canine parvovirus (CPV) complexed with the Fab fragment of a neutralizing antibody, A3B10, using image reconstruction of electron micrographs of vitrified samples, together with the already known structure of CPV from X-ray crystallographic data.
The structure of the complex of CPV with Fab A3B10 has been determined to 23 A resolution. The known CPV atomic structure was subtracted from the electron density of the complex, and the difference map was used to fit the atomic coordinates of a known Fab fragment, HyHEL-5. The long axis of each Fab molecule is oriented in a near radial direction, inclined away from the two-fold axes. The viral epitope consists of 14 amino acid residues found in loops 1, 2 and 3 on the capsid surface, which include previously identified escape mutations.
The mode of Fab binding suggests that the A3B10 neutralizing antibody cannot bind bivalently to the capsid across the two-fold axes, consistent with the observation that whole A3B10 antibody readily precipitates CPV. Since Fab A3B10 can also neutralize the virus, mechanisms of neutralization such as interference with cell attachment, cell entry, or uncoating, must be operative.
细小病毒属的成员可在包括人类在内的哺乳动物中引发多种疾病。对抗病毒感染的主要防御机制之一是存在中和抗体，其可阻止病毒颗粒感染靶细胞。中和机制尚不清楚。因此，我们利用玻璃化样品电子显微镜图像重建技术，结合已通过X射线晶体学数据获得的犬细小病毒（CPV）已知结构，研究了CPV与中和抗体A3B10的Fab片段形成的复合物的结构。
已确定CPV与Fab A3B10复合物的结构分辨率为23埃。从复合物的电子密度中减去已知的CPV原子结构，并利用差值图拟合已知Fab片段HyHEL-5的原子坐标。每个Fab分子的长轴沿近径向方向排列，偏离二重轴。病毒表位由衣壳表面环1、环2和环3中发现的14个氨基酸残基组成，其中包括先前鉴定出的逃逸突变。
Fab的结合模式表明，A3B10中和抗体不能通过二重轴与衣壳二价结合，这与完整的A3B10抗体能轻易沉淀CPV的观察结果一致。由于Fab A3B10也能中和病毒，因此诸如干扰细胞附着、细胞进入或脱壳等中和机制必然起作用。
