Inhibition of the Hepatitis C Virus Helicase-associated ATPase Activity by the Combination of ADP, NaF, MgCl2, and Poly(rU): TWO ADP BINDING SITES ON THE ENZYME-NUCLEIC ACID COMPLEX

Abstract

Hepatitis C virus (HCV) helicase has an intrinsic ATPase activity and a nucleic acid (poly(rU))-stimulated ATPase activity. The poly(rU)-stimulated ATPase activity was inhibited by F- in a time-dependent manner during ATP hydrolysis. Inhibition was the result of trapping an enzyme-bound ADP-poly(rU) ternary complex generated during the catalytic cycle and was not the result of generating enzyme-free ADP that subsequently inhibited the enzyme. However, catalysis was not required for efficient inhibition by F-. The stimulated and the intrinsic ATPase activities were also inhibited by treatment of the enzyme with F-, ADP, and poly(rU). The inhibited enzyme slowly recovered (t1/2 = 23 min) ATPase activity after a 2000-fold dilution into assay buffer. The onset of inhibition by 500 microM ADP and 15 mM F- in the absence of nucleic acid was very slow (t1/2 > 40 min). However, the sequence of addition of poly(rU) to a diluted solution of ADP/NaF-treated enzyme had a profound effect on the extent of inhibition. If the ADP/NaF-treated enzyme was diluted into an assay that lacked poly(rU) and the assay was subsequently initiated with poly(rU), the treated enzyme was not inhibited. Alternatively, if the treated enzyme was diluted into an assay containing poly(rU), the enzyme was inhibited. ATP protected the enzyme from inhibition by ADP/NaF. The stoichiometry between ADP and enzyme monomer in the inhibited enzyme complex was 2, as determined from titration of the ATPase activity ([ADP]/[E] = 2.2) and from the number of radiolabeled ADP bound to the inhibited enzyme ([ADP]/[E] = 1.7) in the presence of excess NaF, MgCl2, and poly(rU). The Hill coefficient for titration of ATPase activity with F- (n = 2.8) or MgCl2 (n = 2.1) in the presence of excess ADP and poly(rU) suggested that multiple F- and Mg2+ were involved in forming the inhibited enzyme complex. The stoichiometry between (dU)18, a defined oligomeric nucleic acid substituting for poly(rU), and enzyme monomer in the inhibited enzyme complex was estimated to be 1 ([(dU)18/[E] = 1.2) from titration of the ATPase activity in the presence of excess ADP, MgCl2, and NaF.