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J. Virol., Mar 1997, 2072-2082, Vol 71, No. 3
H Granzow, F Weiland, A Jons, BG Klupp, A Karger and TC Mettenleiter
We reinvestigated major steps in the replicative cycle of pseudorabies
virus (PrV) by electron microscopy of infected cultured cells. Virions
attached to the cell surface were found in two distinct stages, with a
distance of 12 to 14 nm or 6 to 8 nm between virion envelope and cell
surface, respectively. After fusion of virion envelope and cell membrane,
immunogold labeling using a monoclonal antibody against the envelope
glycoprotein gE demonstrated a rapid drift of gE from the fusion site,
indicating significant lateral movement of viral glycoproteins during or
immediately after the fusion event. Naked nucleocapsids in the cytoplasm
frequently appeared close to microtubules prior to transport to nuclear
pores. At the nuclear pore, nucleocapsids invariably were oriented with one
vertex pointing to the central granulum at a distance of about 40 nm and
viral DNA appeared to be released via the vertex region into the
nucleoplasm. Intranuclear maturation followed the typical herpesvirus
nucleocapsid morphogenesis pathway. Regarding egress, our observations
indicate that primary envelopment of nucleocapsids occurred at the inner
leaflet of the nuclear membrane by budding into the perinuclear cisterna.
This nuclear membrane-derived envelope exhibited a smooth surface which
contrasts the envelope obtained by putative reenvelopment at tubular
vesicles in the Golgi area which is characterized by distinct surface
projections. Loss of the primary envelope and release of the nucleocapsid
into the cytoplasm appeared to occur by fusion of envelope and outer
leaflet of the nuclear membrane. Nucleocapsids were also found engulfed by
both lamella of the nuclear membrane. This vesiculation process released
nucleocapsids surrounded by two membranes into the cytoplasm. Our data also
indicate that fusion between the two membranes then leads to release of
naked nucleocapsids in the Golgi area. Egress of virions appeared to occur
via transport vesicles containing one or more virus particles by fusion of
vesicle and cell membrane. Our data thus support biochemical data and
mutant virus studies of (i) two steps of attachment, (ii) the involvement
of microtubules in the transport of nucleocapsids to the nuclear pore, and
(iii) secondary envelopment in the trans-Golgi area in PrV infection.
Copyright © 1997, American Society for Microbiology
Ultrastructural analysis of the replication cycle of pseudorabies virus in cell culture: a reassessment
Institutes of Diagnostic Virology, Friedrich-Loeffler-Institutes, Federal Research Centre for Virus Diseases of Animals, Insel Riems, Germany. Harald.Granzow@rie.bfav.de
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