Strand-separating activity of hepatitis C virus helicase in the absence of ATP.
HCV helicase [E(wt)] catalyzed strand separation of a short DNA duplex (F21:HF31) formed from a 5'-hexachlorofluorescein-tagged 31-mer (HF31) and a 3'-fluorescein-tagged 21-mer (F21) complementary to the 5'-end of HF31. Strand separation was monitored by the fluorescence increase associated with the formation of F21 from F21:HF31. In the presence of ATP, the strand-separating activity was catalytic. In the absence of ATP and with E(wt) concentrations greater than that of F21:HF31, a biphasic fluorescence increase was observed at 25 degrees C. The late phase of this reaction was assigned to the separation of F21 from F21:HF31. The ATP-independent strand-separating reaction occurred more rapidly in the absence of Mg(2+) than in its presence. This result correlated with a lower T(m) value of F21:HF31 in the absence of 3.5 mM Mg(2+) than in its presence (45 vs 63 degrees C). The stoichiometry for the strand-separating reaction in the absence of ATP was 8 mol of E(wt) per mole of F21:HF31 separated into single-stranded F21 and HF31. The dissociation constants of HCV helicase for F21, HF31, and F21:HF31 in the absence of Mg(2+) were 0.6 +/- 0.4, 6 +/- 1, and 7.3 +/- 0.9 nM, respectively. Histidinyl-tagged E(wt) [hE(wt)] and a mutant enzyme [hE(V432A)] were prepared. hE(wt) and E(wt) bound F21 and HF31 with similar affinities and had similar ATP-dependent helicase activities, whereas hE(V432A) bound F21 and HF31 with affinities similar to that of E(wt) but had greatly reduced ATP-dependent helicase activities. In contrast to E(wt) and hE(wt), hE(V432A) did not support the ATP-independent strand-separating reaction. Consequently, the ATP-independent strand-separating reaction was not only the result of the high affinity of the enzyme for single-stranded DNA. The enzyme preferentially used duplex DNA with a 3'-tail for the ATP-dependent helicase reaction. In contrast, the enzyme strand-separated blunt-ended, 5'-tailed, and 3'-tailed duplex DNA equally effectively in the ATP-independent strand-separating reaction.
Porter
D J
DJ
Glaxo Wellcome, 5 Moore Drive, Research Triangle Park, North Carolina 27709, USA.
Preugschat
F
F
D000255:Adenosine Triphosphate / Q000378:metabolism*; D004247:DNA / Q000737:chemistry / Q000378:metabolism*; D004789:Enzyme Activation; D016174:Hepacivirus / Q000201:enzymology*; D006801:Humans; D013379:Substrate Specificity; D017361:Viral Nonstructural Proteins / Q000378:metabolism*; D012697:Serine Endopeptidases; D043583:Nucleoside-Triphosphatase; D053487:DEAD-box RNA Helicases; D000085242:Viral Proteases
D000255:Adenosine Triphosphate; D004247:DNA; D017361:Viral Nonstructural Proteins; C084422:NS3 protein, hepatitis C virus; D012697:Serine Endopeptidases; D043583:Nucleoside-Triphosphatase; D053487:DEAD-box RNA Helicases; D000085242:Viral Proteases