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 a 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 with siRNAs of various sizes, from 21-24 nucleotide in length and preferentially recruits small RNAs with a 5' terminal cytosine. Probably involved in antiviral RNA silencing. Associates with siRNAs derived from cucumber mosaic virus (CMV). Targeted by the turnip yellows virus (TuYV) protein P0 (via F-box-like domain) for probable proteasome degradation and thereby inactivating AGO5 function in RNA silencing. Belongs to the argonaute family. Ago subfamily.