Small Molecule Inhibition of RISC Loading

Argonaute proteins are the core components of the microRNP/RISC. The biogenesis and function of microRNAs and endo- and exo- siRNAs are regulated by Ago2, an Argonaute protein with RNA binding and nuclease activities. Currently, there are no in vitro assays suitable for large-scale screening of microRNP/RISC loading modulators. We describe a novel in vitro assay that is based on fluorescence polarization of TAMRA-labeled RNAs loaded to human Ago2. Using this assay, we identified potent small-molecule inhibitors of RISC loading, including aurintricarboxylic acid (IC50 = 0.47 mu M), suramin (IC50 = 0.69 mu M), and oxidopamine HCL (IC50 = 1.61 mu M). Small molecules identified by this biochemical screening assay also inhibited siRNA loading to endogenous Ago2 in cultured cells.