Structure-based mutagenesis study of hepatitis C virus NS3 helicase

Area of Science:

• Biochemistry
• Virology
• Molecular Biology

Background:

• Hepatitis C virus (HCV) NS3 protein is crucial for viral replication, possessing both protease and helicase functions.
• The helicase domain, located in the C-terminal two-thirds, is essential for unwinding nucleic acids.
• Understanding the NS3 helicase mechanism is vital for developing antiviral therapies.

Purpose of the Study:

• To characterize the purified HCV NS3 helicase domain's biochemical activities.
• To elucidate the functional roles of conserved residues in HCV helicase activity using site-directed mutagenesis.
• To investigate the mechanism of nucleic acid unwinding by the HCV NS3 helicase.

Main Methods:

• Expression and purification of a hexahistidine-tagged HCV NS3 helicase domain in E. coli.
• Biochemical assays to measure ATPase and nucleic acid unwinding activities.
• Site-directed mutagenesis to alter specific conserved residues within the helicase domain.
• Analysis of mutant enzyme activity, including RNA binding, ATPase stimulation, and unwinding.

Main Results:

• The purified HCV helicase domain exhibits basal ATPase, polynucleotide-stimulated ATPase, nucleic acid unwinding, and single-stranded polynucleotide binding activities.
• Mutations at Thr269, Thr411, and Trp501 severely reduced RNA binding and abolished unwinding and poly(U)-stimulated ATPase activities.
• Mutations at Ser231 and Ser370 did not affect helicase activity, while a Trp501-Phe mutant showed altered poly(U) but not poly(C) stimulation.

Conclusions:

• Specific conserved residues (e.g., Thr269, Thr411, Trp501) are critical for HCV NS3 helicase function, particularly RNA binding and unwinding.
• These findings provide insights into the mechanism of nucleic acid unwinding by viral helicases.
• The study identifies key residues for potential drug targeting in HCV therapy.