Involved in RNA-mediated post-transcriptional gene silencing (PTGS). Main component of the RNA-induced silencing complex (RISC) that binds to a short guide RNA such as microRNA (miRNA) or small interfering RNA (siRNA). RISC uses the mature miRNA or siRNA as a guide for slicer-directed cleavage of homologous mRNAs to repress gene expression. Associates mainly with siRNAs of 21 nucleotide in length and preferentially recruits small RNAs with a 5' terminal adenosine. Probably involved in antiviral RNA silencing. Associates with siRNA derived from cucumber mosaic virus (CMV). Targeted by turnip yellows virus (TuYV) protein P0 (via F-box-like domain) for probable proteasome degradation and thereby inactivating AGO2 function in RNA silencing. Required to direct NERD-dependent DNA methylation and silencing