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J. Virol., 02 1997, 1598-1607, Vol 71, No. 2
XH Li, P Valdez, RE Olvera and JC Carrington
The NIb protein of tobacco etch potyvirus (TEV) possesses several
functions, including RNA-dependent RNA polymerase and nuclear translocation
activities. Using a reporter protein fusion strategy, NIb was shown to
contain two independent nuclear localization signals (NLS I and NLS II).
NLS I was mapped to a sequence within amino acid residues 1 to 17, and NLS
II was identified between residues 292 and 316. Clustered point mutations
resulting in substitutions of basic residues within the NLSs were shown
previously to disrupt nuclear translocation activity. These mutations also
abolished TEV RNA amplification when introduced into the viral genome. The
amplification defects caused by each NLS mutation were complemented in
trans within transgenic cells expressing functional NIb, although the level
of complementation detected for each mutant differed significantly.
Combined with previous results (X. H. Li and J. C. Carrington, Proc. Natl.
Acad. Sci. USA 92:457-461, 1995), these data suggest that the NLSs overlap
with essential regions necessary for NIb trans-active function(s). The fact
that NIb functions in trans implies that it must interact with one or more
other components of the genome replication apparatus. A yeast two-hybrid
system was used to investigate physical interactions between NIb and
several other TEV replication proteins, including the multifunctional
VPg/proteinase NIa and the RNA helicase CI. A specific interaction was
detected between NIa and NIb. Deletion of any of five regions spanning the
NIb sequence resulted in NIb variants that were unable to interact with
NIa. Clustered point mutations affecting the conserved GDD motif or NLS II
within the central region of NIb, but not mutations affecting NLS I near
the N terminus, reduced or eliminated the interaction. The C-terminal
proteinase (Pro) domain of NIa, but not the N-terminal VPg domain,
interacted with NIb. The effects of NIb mutations within NLS I, NLS II, and
the GDD motif on the interaction between the Pro domain and NIb were
identical to the effects of these mutations on the interaction between
full-length NIa and NIb. These data are compatible with a model in which
NIb is directed to replication complexes through an interaction with the
Pro domain of NIa.
Copyright © 1997, American Society for Microbiology
Functions of the tobacco etch virus RNA polymerase (NIb): subcellular transport and protein-protein interaction with VPg/proteinase (NIa)
Department of Biology, Texas A&M University, College Station 77843, USA.
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