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J. Virol., Jan 1997, 291-298, Vol 71, No. 1
IR Muylaert, R Galler and CM Rice
The flavivirus NS1 protein is a highly conserved nonstructural glycoprotein
that is capable of eliciting protective immunity. NS1 homodimers are
secreted from virus-infected mammalian cells, but the protein is also
present at the plasma membrane and in the lumen of intracellular vesicles.
Based on these properties, it has been speculated that NS1 may function in
virus maturation or release. To gain further insight into NS1 function, we
used clustered charged-amino- acid-to-alanine mutagenesis to create 28
clustered substitutions in the NS1 protein of yellow fever virus. To screen
for conditional mutations, full-length RNAs containing each mutation were
assayed for plaque formation at 32 and 39 degrees C after RNA transfection.
We found that 9 mutations were lethal, 18 allowed plaque formation at both
temperatures, and 1, ts25, was strongly heat sensitive and was unable to
form plaques at 39 degrees C. Lethal mutations clustered in the
amino-terminal half of NS1, whereas those leading to impaired replication
relative to the parent were distributed throughout the protein.
High-multiplicity infections at 39 degrees C demonstrated that ts25 was
defective for RNA accumulation, leading to depressed viral protein
synthesis and delayed virus production. Although ts25 secreted less NS1
than did the parent, temperature shift experiments failed to demonstrate
any temperature-dependent differences in polyprotein processing, NS1
stability and secretion, or release of infectious virus. The ts lesion of
ts25 was shown to be due to a single alanine substitution for Arg-299, a
residue which is conserved among flaviviruses. These results argue that NS1
plays an essential but as yet undefined role in flavivirus RNA
amplification.
Copyright © 1997, American Society for Microbiology
Genetic analysis of the yellow fever virus NS1 protein: identification of a temperature-sensitive mutation which blocks RNA accumulation
Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri 63110-1093, USA.
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