| Summary: | The hepatitis C virus (HCV) was identified in 1989 as the major causative agent of transfusion-associated non-A, non-B hepatitis and today represents a worldwide health crisis with prevalence estimates of 2.2%. HCV-specific therapeutics have never been more urgently needed. One of the validated drug targets is the non-structural (NS) protein 3 (NS3) membrane-bound protease. The major aim of this thesis was characterization of NS3 allosteric activation by its viral cofactor, NS4A. We hypothesized that there would be specific residues that dominate the interaction between NS3 and NS4A, and further hypothesized that binding and activation may be separate events mediated by different residues.
This thesis details the development of novel cell-based assays for detection of NS3-4A protease activity and heterocomplex formation. The protease assay substrate was a membrane-targeted intracellular protein, which upon proteolysis released a red fluorescent protein (FP) reporter, DsRed-Express, into the cytoplasm; this change was detected by microscopy or quantified by Western blotting. The complex formation assay detected fluorescence resonance energy transfer (FRET) between yellow and cyan FP-tagged NS3 and NS4A, respectively.
Our data shows binding can be functionally separated from activation. We identified two NS4A residues (I25 and I29) important for NS3 binding and two NS4A residues (V23 and I25) important for NS3 activation. Therefore the binding-pockets of these residues are prime targets for small-molecule therapeutic development.
In addition, I have compared the NS3-4A substrate sequence cleavage efficiencies in vivo. I have been able to show that the activation-dependent NS4B/NS5A junction is processed efficiently and the NS4A/NS4B junction is not. I have also shown NS3-4A substrate specificity is not modulated by replicase components; however the specific activity of this enzyme is increased.
The strength of this thesis work stems from the novel and creative development of cell-based assays that can easily be modified to study other membrane-associated proteases. In vitro assays fall short in that they do not take into account the unique micro-environment in which these proteases are found.
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