Quantitative Evaluation of Viral Protein Binding to Phosphoinositide Receptors and Pharmacological Inhibition.

There is significant interest in developing analytical methods to characterize molecular recognition events between proteins and phosphoinositides, which are a medically important class of carbohydrate-functionalized lipids. Within this scope, one area of high priority involves quantitatively evaluating drug candidates that pharmacologically inhibit protein-phosphoinositide interactions. As full-length proteins are often difficult to produce, establishing methods to study these interactions with shorter, bioactive peptides would be advantageous. Herein, we report an atomic force microscopy (AFM)-based force spectroscopic approach to detect the specific interaction between an amphipathic, α-helical (AH) peptide derived from the hepatitis C virus NS5A protein and its biological target, the phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P] phosphoinositide receptor. After optimization of the peptide tethering strategy and measurement parameters, the binding specificity of AH peptide for PI(4,5)P receptors was comparatively evaluated across a panel of phosphoinositides and the influence of ionic strength on AH-PI(4,5)P binding strength was tested. Importantly, these capabilities were translated into the development of a novel experimental methodology to determine the inhibitory activity of a small-molecule drug candidate acting against the AH-PI(4,5)P interaction, and extracted kinetic parameters agree well with literature values obtained by conventional biochemical methods. Taken together, our findings provide a nanomechanical basis for explaining the high binding specificity of the NS5A AH to PI(4,5)P receptors, in turn establishing an analytical framework to study phosphoinositide-binding viral peptides and proteins as well as a broadly applicable approach to evaluate candidate inhibitors of protein-phosphoinositide interactions.