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Journal of Virology, December 2000, p. 11472-11478, Vol. 74, No. 24
Zentrum für Molekulare Biologie
Heidelberg, 69120 Heidelberg, Germany
Received 15 May 2000/Accepted 19 September 2000
Hepadnaviruses are DNA viruses but, as pararetroviruses, their
morphogenesis initiates with the encapsidation of an RNA pregenome, and
these viruses have therefore evolved mechanisms to exclude nucleocapsids that contain incompletely matured genomes from
participating in budding and secretion. We provide here evidence that
binding of hepadnavirus core particles from the cytosol to their target membranes is a distinct step in morphogenesis, discriminating among
different populations of intracellular capsids. Using the duck
hepatitis B virus (DHBV) and a flotation assay, we found about half of
the intracellular capsids to be membrane associated due to an intrinsic
membrane-binding affinity. In contrast to free cytosolic capsids, this
subpopulation contained largely mature, double-stranded DNA genomes and
lacked core protein hyperphosphorylation, both features characteristic
for secreted virions. Against expectation, however, the selective
membrane attachment observed did not require the presence of the large
DHBV envelope protein, which has been considered to be crucial for
nucleocapsid-membrane interaction. Furthermore, removal of
surface-exposed phosphate residues from nonfloating capsids by itself
did not suffice to confer membrane affinity and, finally,
hyperphosphorylation was absent from nonenveloped nucleocapsids that
were released from DHBV-transfected cells. Collectively, these
observations argue for a model in which nucleocapsid maturation,
involving the viral genome, capsid structure, and capsid
dephosphorylation, leads to the exposure of a membrane-binding signal as a step crucial for selecting the matured nucleocapsid to be
incorporated into the capsid-independent budding of virus particles.
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Intracellular Hepadnavirus Nucleocapsids Are
Selected for Secretion by Envelope Protein-Independent Membrane
Binding
and
*
Corresponding author. Mailing address: Zentrum
für Molekulare Biologie Heidelberg, Im Neuenheimer Feld 282, D-69120 Heidelberg, Germany. Phone: 49-6221-54-68-85. Fax:
49-6221-54-58-93. E-mail: hshd{at}zmbh.uni-heidelberg.de
Present address: Zoologisches Institut, 8057 Zürich, Switzerland.
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