NIH Scientists Shed Light on Mystery Surrounding Hepatitis B Virus Discovery Is Decades in the MakingScientists from the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), part of the National Institutes of Health, and the University of Oxford, U.K., have shed light on a long-standing enigma about the structure of a protein related to the Hepatitis B virus. Their findings, reported in Structure, could lead to new therapeutic strategies for chronic liver disease.
World-wide, some 350 million people are chronically infected with Hepatitis B Virus (HBV), of whom 620,000 die each year from HBV-related liver disease. Like any other pathogen, HBV expresses protein antigens that trigger the body’s immune system to defend itself. A relatively small and simple virus, HBV has three major clinical antigens that elicit an immune response: the surface antigen (which is also used safely and effectively to vaccinate individuals against HBV), the core antigen (HBcAg), and the e-antigen (HBeAg). Illustration of the hepatitis B virus e-antigen.
The HBV core antigen and the e-antigen are basically two versions of the same protein, but the core antigen is important for virus production, while the e-antigen is not. The e-antigen plays a role in establishing immune tolerance and chronic HBV infection. In addition, the core antigen assembles into the shell (capsid) that houses the genetic material of the virus, while the e-antigen is secreted into the bloodstream in an unassembled form. The relationship between the e-antigen and the core antigen has been a mystery for the past three decades.
In the new study, Alasdair Steven, Ph.D., Senior Investigator in the NIAMS Laboratory of Structural Biology Research, and Paul Wingfield, Ph.D., Chief of the NIAMS Protein Expression Laboratory developed a unique antibody that binds to and forms a stable complex with e-antigen. This complex was found to form well-diffracting crystals whose analysis allowed the structure of the complex to be determined. They discovered that the e-antigen subunit has essentially the same fold as the core antigen subunit, but that it pairs into dimers (two associated subunits) in an entirely different way, with a relative rotation of 140 degrees between the subunits. The rotation obviates the protein's ability to assemble and transforms its antigenic character. This switch represents a novel mechanism for regulating a protein's structure and function.
Understanding the e-antigen structure provides a framework upon which future studies can build to fully elucidate its role in HBV persistence and possibly a way to prevent the establishment of chronic liver infections. 作者: StephenW 时间: 2013-1-9 13:16
Dimattia, M.A., et al., Antigenic Switching of Hepatitis B Virus by Alternative Dimerization of the Capsid Protein. Structure, 2012. http://www.ncbi.nlm.nih.gov/pubmed/23219881
Wu, W., et al., Specificity of an anti-capsid antibody associated with Hepatitis B Virus-related acute liver failure. J Struct Biol, 2013. 181(1): p. 53-60. http://www.ncbi.nlm.nih.gov/pubmed/23079477作者: MP4 时间: 2013-1-11 15:36
谢谢,很有意思.
Antigenic Switching of Hepatitis B Virus by Alternative Dimerization of the Capsid Protein.Dimattia MA, Watts NR, Stahl SJ, Grimes JM, Steven AC, Stuart DI, Wingfield PT.
SourceDivision of Structural Biology, Henry Wellcome Building for Genomic Medicine, University of Oxford, Roosevelt Drive, Headington OX3 7BN, UK; Laboratory of Structural Biology Research , National Institute of Arthritis and Musculoskeletal and Skin Diseases; National Institutes of Health, Bethesda, MD 20892, USA.
AbstractChronic hepatitis B virus (HBV) infection afflicts millions worldwide with cirrhosis and liver cancer. HBV e-antigen (HBeAg), a clinical marker for disease severity, is a nonparticulate variant of the protein (core antigen, HBcAg) that forms the building-blocks of capsids. HBeAg is not required for virion production, but is implicated in establishing immune tolerance and chronic infection. Here, we report the crystal structure of HBeAg, which clarifies how the short N-terminal propeptide of HBeAg induces a radically altered mode of dimerization relative to HBcAg (∼140° rotation), locked into place through formation of intramolecular disulfide bridges. This structural switch precludes capsid assembly and engenders a distinct antigenic repertoire, explaining why the two antigens are cross-reactive at the T cell level (through sequence identity) but not at the B cell level (through conformation). The structure offers insight into how HBeAg may establish immune tolerance for HBcAg while evading its robust immunogenicity.
J Struct Biol. 2013 Jan;181(1):53-60. doi: 10.1016/j.jsb.2012.10.004. Epub 2012 Oct 16.
Specificity of an anti-capsid antibody associated with Hepatitis B Virus-related acute liver failure.
Wu W, Chen Z, Cheng N, Watts NR, Stahl SJ, Farci P, Purcell RH, Wingfield PT, Steven AC.
Source
Laboratory of Structural Biology, National Institute for Arthritis, Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, MD 20892, United States.
Abstract
Previously, the livers of patients suffering from acute liver failure (ALF), a potentially fatal syndrome arising from infection by Hepatitis B Virus (HBV), were found to contain massive amounts of an antibody specific for the core antigen (HBcAg) capsid. We have used cryo-electron microscopy and molecular modeling to define its epitope. HBV capsids are icosahedral shells with 25Å-long dimeric spikes, each a 4-helix bundle, protruding from the contiguous "floor". Of the anti-HBcAg antibodies previously characterized, most bind around the spike tip while one binds to the floor. The ALF-associated antibody binds tangentially to a novel site on the side of the spike. This epitope is conformational. The Fab binds with high affinity to its principal determinants but has lower affinities for quasi-equivalent variants. The highest occupancy site is on one side of a spike, with no detectable binding to the corresponding site on the other side. Binding of one Fab per dimer was also observed by analytical ultracentrifugation. The Fab did not bind to the e-antigen dimer, a non-assembling variant of capsid protein. These findings support the propositions that antibodies with particular specificities may correlate with different clinical expressions of HBV infection and that antibodies directed to particular HBcAg epitopes may be involved in ALF pathogenesis.
在此之前,急性肝功能衰竭(ALF),因感染乙型肝炎病毒(HBV),一个潜在的致命性综合征患者的肝脏被发现含有大量的特异性抗体为核心抗原(HBcAg)衣壳。我们使用低温电子显微镜和分子模型,以确定其抗原表位。 HBV衣壳二十面体壳长25A-二聚体尖峰,每一个四螺旋束,突出从连续的“地板”。的抗-HBcAg的抗体先前其特征在于,最绑定的尖钉尖端周围,而1绑定到地板上。 ALF相关抗体结合切向一种新型的网站上侧的尖峰。这是构象表位。的Fab结合高亲和力,其主要的决定因素,但具有较低的亲和力准等同变异体。的最高占用站点是一个尖峰的一侧上,与没有检测到结合到相应的站点的另一侧。也观察到了超高速离心一厂每二聚体的结合。的Fab没有绑定到的e-抗原的二聚体,非组装的核衣壳蛋白的变体。这些研究结果都支持的主张,尤其是特异性的抗体可能与HBV感染不同的临床表现,尤其是乙肝病毒核心抗原表位的抗体可能参与了ALF发病。 作者: LITTLEKING 时间: 2013-2-10 16:50