Biochemical properties of a minimal functional domain with ATP-binding activity of the NTPase/helicase of hepatitis C virus

The RNA-stimulated nucleoside triphosphatase (NTPase) and helicase of hepatitis C virus (HCV) consists of three domains with highly conserved NTP binding motifs located in the first domain. The ATP-binding domain was obtained by limited proteolysis of a greater fragment of the HCV polyprotein, and it was purified to homogenity by column chromatography. The identity of the domain, comprising amino acids 1203 to 1364 of the HCV polyprotein, was confirmed by N- and C-terminal sequencing and by its capability to bind 5'- fluorosulfonylbenzoyladenosine (FSBA). The analyses of the kinetics of ATP binding revealed a single class of binding site with the K(d) of 43.6 μM. The binding is saturable and dependent on Mn²⁺ or Mg²⁺ ions. Poly(A) and poly(dA) show interesting properties as regulators of the ATP-binding capacity of the domain. Polynucleotides bind to the domain and enhance its affinity for ATP. In addition, ATP enhances the affinity of the domain for the polynucleotides. Different compounds, which are known to interact with nucleotide binding sites of various classes of enzymes, were tested for their ability to inhibit the binding of ATP to the domain. Of the compounds tested, two agents behaved as inhibitors: paclitaxel, which inhibits the ATP binding competitively (IC₅₀ = 22 μM), and trifluopemzine, which inhibits the ATP binding by a noncompetitive mechanism (IC₅₀ = 98 μM). Kinetic experiments with the NTPase/helicase indicate that both compounds inhibit the NTPase activity of the holoenzyme by interacting with its ATP-binding domain.