Dynamic Nucleocytoplasmic Trafficking of Argonaute 4 Enables RNA-Directed DNA Methylation

Argonaute (AGO) proteins are found in all major eukaryotic lineages, where they bind small RNA (sRNA) to facilitate RNA silencing pathways. The best-studied AGO proteins function in the cytoplasm to post-transcriptionally regulate mature mRNAs, but many AGO proteins instead function in the nucleus, where they mediate chromatin modification or genome elimination. Among these nuclear-acting AGO proteins is Arabidopsis thaliana AGO4, which binds 24 nucleotide siRNAs and triggers DNA methylation of homologous sequences.

Recently, control of nuclear import has emerged as a common thread in regulation of nuclear AGOs, however little is known regarding nuclear export of these proteins. Here we demonstrate that AGO4 contains multiple nuclear trafficking signals and is rapidly transferred between the nucleus and cytoplasm. This trafficking is required for RNA-directed DNA methylation, in part by promoting AGO4 stability in the cytoplasm.