AGO2 protects Drosophila siRNAs and microRNAs from target-directed degradation, even in the absence of 2′-O-methylation

Target-directed degradation (TDD) of microRNAs (miRNAs) plays an important role in shaping miRNA abundances across bilateria. Some endogenous small interfering RNAs (siRNAs) of Drosophila cells have target sites resembling those that trigger miRNA TDD, raising the question as to whether they too might undergo such regulation. Here, we find that at least seven of these siRNAs are indeed sensitive to TDD when loaded into AGO1, the Argonaute paralog that preferentially associates with miRNAs. Despite this sensitivity when loaded into AGO1, these siRNAs are not detectably regulated by TDD because most molecules are loaded into AGO2, the Argonaute paralog that preferentially associates with siRNAs, and we find that siRNAs and miRNAs loaded into AGO2 are insensitive to TDD. One explanation for the protection of these small RNAs loaded into AGO2 is that these small RNAs are 2′-O-methylated at their 3′ termini. However, contrary to previous proposals, 2′-O-methylation does not protect these RNAs from TDD, which indicates that their protection is instead conferred by features of the AGO2 protein itself. Together, these observations clarify the requirements for regulation by TDD and revise our understanding of the role of 2′-O-methylation in small-RNA biology. Measurement of miRNA and siRNA abundances in AGO1 and/or AGO2 in wild-type, dora, hen1, or hen1/dora Drosophila S2 cell lines