Department of Medical Microbiology and Hygiene,University Medical Center of the Johannes Gutenberg University, Mainz, Germany.
Cell Microbiol. 2011 Apr;13(4):602-19. doi: 10.1111/j.1462-5822.2010.01557.x. Epub 2010 Dec 28.
Hepatitis B virus (HBV) is an enveloped DNA virus that exploits the endosomal sorting complexes required for transport (ESCRT) pathway for budding. In addition to infectious particles, HBV-replicating cells release non-enveloped (nucleo)capsids, but their functional implication and pathways of release are unclear. Here, we focused on the molecular mechanisms and found that the sole expression of the HBV core protein is sufficient for capsid release. Unexpectedly, released capsids are devoid of a detectable membrane bilayer, implicating a non-vesicular exocytosis process. Unlike virions, naked capsid budding does not require the ESCRT machinery. Rather, we identified Alix, a multifunctional protein with key roles in membrane biology, as a regulator of capsid budding. Ectopic overexpression of Alix enhanced capsid egress, while its depletion inhibited capsid release. Notably, the loss of Alix did not impair HBV production, furthermore indicating that virions and capsids use diverse export routes. By mapping of Alix domains responsible for its capsid release-mediating activity, its Bro1 domain was found to be required and sufficient. Alix binds to core via its Bro1 domain and retained its activity even if its ESCRT-III binding site is disrupted. Together, the boomerang-shaped Bro1 domain of Alix appears to escort capsids without ESCRT.
乙型肝炎病毒 (HBV) 是一种包膜 DNA 病毒,利用内体分选复合物必需运输 (ESCRT) 途径进行出芽。除了传染性颗粒外,HBV 复制细胞还释放非包膜 (核)衣壳,但它们的功能意义和释放途径尚不清楚。在这里,我们专注于分子机制的研究,并发现仅表达 HBV 核心蛋白就足以促进衣壳的释放。出乎意料的是,释放的衣壳缺乏可检测到的膜双层,暗示了一种非囊泡胞吐过程。与病毒颗粒不同,裸露的衣壳出芽不需要 ESCRT 机制。相反,我们鉴定出 Alix,一种在膜生物学中具有关键作用的多功能蛋白,是衣壳出芽的调节剂。Alix 的异位过表达增强了衣壳的出芽,而其耗尽则抑制了衣壳的释放。值得注意的是,Alix 的缺失并不影响 HBV 的产生,进一步表明病毒颗粒和衣壳使用不同的输出途径。通过对 Alix 负责其衣壳释放介导活性的结构域进行映射,发现其 Bro1 结构域是必需和充分的。Alix 通过其 Bro1 结构域与核心结合,即使其 ESCRT-III 结合位点被破坏,其活性也得以保留。总之,Alix 的回旋镖状 Bro1 结构域似乎在没有 ESCRT 的情况下护送衣壳。