Alix regulates egress of hepatitis B virus naked capsid particles in an ESCRT-independent manner.
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.
Department of Medical Microbiology and Hygiene,University Medical Center of the Johannes Gutenberg University, Mainz, Germany.
Andreas
A
Bardens
Tatjana
T
Döring
Jens
J
Stieler
Reinhild
R
Prange
D001692:Biological Transport / Q000502:physiology; D002135:Calcium-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002213:Capsid / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D056827:Endosomal Sorting Complexes Required for Transport / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006515:Hepatitis B virus / Q000235:genetics / Q000378:metabolism* / Q000502:physiology / Q000648:ultrastructure; D006801:Humans; D017434:Protein Structure, Tertiary; D034741:RNA, Small Interfering / Q000378:metabolism; D014758:Viral Core Proteins / Q000378:metabolism; D014771:Virion / Q000378:metabolism*; D057074:Virus Release; D014779:Virus Replication
D002135:Calcium-Binding Proteins; D018797:Cell Cycle Proteins; D056827:Endosomal Sorting Complexes Required for Transport; C117020:PDCD6IP protein, human; D034741:RNA, Small Interfering; D014758:Viral Core Proteins