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J Virol, January 1998, p. 624-632, Vol. 72, No. 1
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Mutagenesis of the NS3 Protease of Dengue Virus
Type 2
Rosaura P. C.
Valle and
Barry
Falgout*
Laboratory of Vector-Borne Viral Diseases,
Center for Biologics Evaluation and Research, Food and Drug
Administration, Bethesda, Maryland 20852-1448
Received 7 May 1997/Accepted 2 October 1997
The flavivirus protease is composed of two viral proteins, NS2B and
NS3. The amino-terminal portion of NS3 contains sequence and structural
motifs characteristic of bacterial and cellular trypsin-like proteases.
We have undertaken a mutational analysis of the region of NS3 which
contains the catalytic serine, five putative substrate binding
residues, and several residues that are highly conserved among
flavivirus proteases and among all serine proteases. In all, 46 single-amino-acid substitutions were created in a cloned NS2B-NS3 cDNA
fragment of dengue virus type 2, and the effect of each mutation on the
extent of self-cleavage of the NS2B-NS3 precursor at the NS2B-NS3
junction was assayed in vivo. Twelve mutations almost completely or
completely inhibited protease activity, 9 significantly reduced it, 14 decreased cleavage, and 11 yielded wild-type levels of activity.
Substitution of alanine at ultraconserved residues abolished NS3
protease activity. Cleavage was also inhibited by substituting some
residues that are conserved among flavivirus NS3 proteins. Two (Y150
and G153) of the five putative substrate binding residues could not be
replaced by alanine, and only Y150 and N152 could be replaced by a
conservative change. The two other putative substrate binding residues,
D129 and F130, were more freely substitutable. By analogy with the
trypsin model, it was proposed that D129 is located at the bottom of
the substrate binding pocket so as to directly interact with the basic
amino acid at the substrate cleavage site. Interestingly, we found that significant cleavage activity was displayed by mutants in which D129
was replaced by E, S, or A and that low but detectable protease activity was exhibited by mutants in which D129 was replaced by K, R,
or L. Contrary to the proposed model, these results indicate that D129
is not a major determinant of substrate binding and that its
interaction with the substrate, if it occurs at all, is not essential.
This mutagenesis study provided us with an array of mutations that
alter the cleavage efficiency of the dengue virus protease. Mutations
that decrease protease activity without abolishing it are candidates
for introduction into the dengue virus infectious full-length
cDNA clone with the aim of creating potentially attenuated virus
stocks.
*
Corresponding author. Mailing address: FDA/CBER, 1401 Rockville Pike (HFM-451), Rockville, MD 20852-1448. Phone: (301)
827-1890. Fax: (301) 496-1810. E-mail:
falgout{at}A1.cber.fda.gov.
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