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J. Virol., Dec 1996, 8477-8484, Vol 70, No. 12
CL Tai, WK Chi, DS Chen and LH Hwang
To assess the RNA helicase activity of hepatitis C virus (HCV)
nonstructural protein 3 (NS3), a polypeptide encompassing amino acids 1175
to 1657, which cover only the putative helicase domain, was expressed in
Escherichia coli by a pET expression vector. The protein was purified to
near homogeneity and assayed for RNA helicase activity in vitro with
double-stranded RNA substrates prepared from a multiple cloning sequence
and an HCV 5' nontranslated region (5'-NTR) or 3'-NTR. The enzyme acted
successfully on substrates containing both 5' and 3' single-stranded
regions (standard) or on substrates containing only the 3' single-stranded
regions (3'/3') but failed to act on substrates containing only the 5'
single-stranded regions (5'/5') or on substrates lacking the
single-stranded regions (blunt). These results thus suggest 3' to 5'
directionality for HCV RNA helicase activity. However, a 5'/5' substrate
derived from the HCV 5'-NTR was also partially unwound by the enzyme,
possibly because of unique properties inherent in the 5' single- stranded
regions. Gel mobility shift analyses demonstrated that the HCV NS3 helicase
could bind to either 5'- or 3'-tailed substrates but not to substrates
lacking a single-stranded region, indicating that the polarity of the RNA
strand to which the helicase bound was a more important enzymatic activity
determinant. In addition to double- stranded RNA substrates, HCV NS3
helicase activity could displace both RNA and DNA oligonucleotides on a DNA
template, suggesting that HCV NS3 too was disposed to DNA helicase
activity. This study also demonstrated that RNA helicase activity was
dramatically inhibited by the single- stranded polynucleotides. Taken
altogether, our results indicate that the HCV NS3 helicase is unique among
the RNA helicases characterized so far.
Copyright © 1996, American Society for Microbiology
The helicase activity associated with hepatitis C virus nonstructural protein 3 (NS3)
Graduate Institute of Microbiology, National Taiwan University, Taipei.
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