The biochemical basis for the cooperative action of microRNAs

Significance MicroRNAs (miRNAs) are short RNAs that guide the repression of most human messenger RNAs (mRNAs) and play many important roles in physiology and development. To function, each miRNA associates with an Argonaute (AGO) protein to form a complex in which the miRNA pairs to sites in mRNAs, thereby targeting these mRNAs for repression, and AGO associates with TNRC6, a protein that recruits the mRNA-repression machinery. For previously unknown reasons, repression is often more effective when miRNAs pair to sites close to one another. We find that in the presence of the TNRC6 AGO-binding domain, which can simultaneously bind multiple AGO proteins, miRNA-AGO complexes bind a two-site target RNA with high cooperatively, providing a biochemical explanation for cooperative action in cells.