Alix regulates egress of hepatitis B virus naked capsid particles in an ESCRT-independent manner

Abstract

SummaryHepatitis 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 releasemediating 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.

Hepatocytes Traffic and Export Hepatitis B Virus Basolaterally by Polarity-Dependent Mechanisms

“…It is known that viral envelope proteins are assembled and modified in the endoplasmic reticulum (ER) and associate with the genome-containing core on passage through the ER-Golgi complex before export (13). Additionally, HBV export pathways that involve regulated endosomal structures and even some that are independent of the endosomal machinery have been described, albeit in nonpolarized hepatocyte models (2,12). The trafficking and export of hepatitis C virus and its core protein have been shown to depend on endosomal structures in HuH7 hepatocyte-derived cells (11).…”

A Novel Pyridazinone Derivative Inhibits Hepatitis B Virus Replication by Inducing Genome-Free Capsid Formation

“…HBV secretes three major types of viral particles in cell culture medium, including enveloped virion and subviral particles and nonenveloped "naked" capsid (36,45,46). According to a recent study (36), the comigration of the capsid and envelope proteins on the agarose gel and their cofractionation on the density gradient indicated the capsids detected at the virion position were enveloped.…”

Protein-Protein Interactions Suggest Novel Activities of Human Cytomegalovirus Tegument Protein pUL103

“…Proteins that bind to ALIX typically interact via ALIX-specific late domains that are important for virion production in many RNA and DNA viruses (33)(34)(35)(36)(37). Late domains in retrovirus gag proteins can bind the V domain of ALIX to facilitate viral budding at the cell membrane (reviewed in references 19, 36, and 37).…”