Center for Vaccines and Immunity, The Research Institute at Nationwide Children's Hospital, Columbus, OH 43205.
State Key Laboratory of Molecular Vaccinology and Molecular Diagnostics, National Institute of Diagnostics and Vaccine Development in Infectious Diseases, School of Public Health, Xiamen University, 361102 Xiamen, People's Republic of China.
Proc Natl Acad Sci U S A. 2018 May 1;115(18):4773-4778. doi: 10.1073/pnas.1721345115. Epub 2018 Apr 18.
The enterically transmitted hepatitis E virus (HEV) adopts a unique strategy to exit cells by cloaking its capsid (encoded by the viral ORF2 gene) and circulating in the blood as "quasi-enveloped" particles. However, recent evidence suggests that the majority of the ORF2 protein present in the patient serum and supernatants of HEV-infected cell culture exists in a free form and is not associated with virus particles. The origin and biological functions of this secreted form of ORF2 (ORF2) are unknown. Here we show that production of ORF2 results from translation initiated at the previously presumed AUG start codon for the capsid protein, whereas translation of the actual capsid protein (ORF2) is initiated at a previously unrecognized internal AUG codon (15 codons downstream of the first AUG). The addition of 15 amino acids to the N terminus of the capsid protein creates a signal sequence that drives ORF2 secretion via the secretory pathway. Unlike ORF2, ORF2 is glycosylated and exists as a dimer. Nonetheless, ORF2 exhibits substantial antigenic overlap with the capsid, but the epitopes predicted to bind the putative cell receptor are lost. Consistent with this, ORF2 does not block HEV cell entry but inhibits antibody-mediated neutralization. These results reveal a previously unrecognized aspect in HEV biology and shed new light on the immune evasion mechanisms and pathogenesis of this virus.
肠传播戊型肝炎病毒（HEV）通过将其衣壳（由病毒 ORF2 基因编码）包裹并以“准包膜”颗粒的形式在血液中循环，采用独特的策略从细胞中释放。然而，最近的证据表明，大多数存在于患者血清和 HEV 感染细胞培养上清液中的 ORF2 蛋白以游离形式存在，与病毒颗粒无关。这种分泌形式的 ORF2（ORF2）的起源和生物学功能尚不清楚。在这里，我们表明 ORF2 的产生是由先前假定的衣壳蛋白起始 AUG 起始密码子起始翻译引起的，而实际衣壳蛋白（ORF2）的翻译则起始于先前未被识别的内部 AUG 密码子（第一个 AUG 下游 15 个密码子）。在衣壳蛋白的 N 端添加 15 个氨基酸会产生一个信号序列，通过分泌途径驱动 ORF2 的分泌。与 ORF2 不同，ORF2 被糖基化并以二聚体形式存在。尽管如此，ORF2 与衣壳具有显著的抗原重叠，但预测与假定的细胞受体结合的表位丢失。与此一致，ORF2 不会阻止 HEV 细胞进入，但会抑制抗体介导的中和。这些结果揭示了 HEV 生物学中一个以前未被认识的方面，并为该病毒的免疫逃逸机制和发病机制提供了新的见解。
