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Journal of Virology, July 1999, p. 5364-5372, Vol. 73, No. 7
Institutes of Molecular and Cellular
Virology1 and Applied
Virology,2 Friedrich-Loeffler-Institutes,
Federal Research Centre for Virus Diseases of Animals, D-17498 Insel
Riems, Germany
Received 2 February 1999/Accepted 29 March 1999
Glycoprotein M (gM), the product of the UL10 gene of pseudorabies
virus (PrV), is one of the few nonessential glycoproteins conserved
throughout the Herpesviridae. In contrast to wild-type PrV
strains, the UL10 gene product of the attenuated PrV vaccine strain
Bartha (PrV-Ba) is not modified by N-glycans due to a
mutation in the DNA sequence encoding the consensus N-glycosylation
motif. To assay function of the UL10 protein in PrV-Ba, a UL10-deletion mutant (PrV-Ba-UL10
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Inhibition of Virion Maturation by Simultaneous
Deletion of Glycoproteins E, I, and M of Pseudorabies Virus
) was isolated. Surprisingly, in
contrast to gM-deleted wild-type PrV, PrV-Ba-UL10
was
severely impaired in plaque formation, inducing only foci of very few
infected RK13, Vero, and PSEK cells and tiny plaques on MDBK cells.
Since this effect was significantly more dramatic than in wild-type
PrV, additional mutations known to be present in PrV-Ba were analyzed
for their contribution to this phenotype. trans-complementation of the mutated PrV-Ba UL21 or gC
protein by the wild-type version had no influence on the observed
phenotype. In contrast, complementation of the gE/gI deletion rescued
the phenotype. The synergistic effect of deletions in gE/gI and gM on
plaque size was verified by construction of a gE/I/M triple mutant
derived from wild-type PrV which exhibited the same phenotype. The
dramatic effect of deletion of gM on plaque size in a
gE/I
virus background was mainly attributable to a
function of gM, and not of the gM/gN complex, as shown by analysis of a
gE/I/N triple mutant. Interestingly, despite the strong effect on
plaque size, penetration was not significantly impaired. In
noncomplementing cells infected with the gE/I/M triple mutant, electron
microscopy showed absence of secondary envelopment in the cytoplasm but
occurrence of intracytoplasmic accumulations of nucleocapsids in
association with electron dense material, presumably tegument proteins.
These structures were not observed after infection of cells expressing either gE/I or gM. We suggest that gE/I and gM are required for late
stages in virion morphogenesis prior to final envelopment in the cytoplasm.
*
Corresponding author. Mailing address: Institute of
Molecular and Cellular Virology, Friedrich-Loeffler-Institutes, Federal Research Centre for Virus Diseases of Animals, D-17498 Insel Riems, Germany. Phone: 49-38351-7102. Fax: 49-38351-7151. E-mail:
mettenleiter{at}rie.bfav.de
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