Arabidopsis thaliana AGO1 immunoprecipitated small RNA from Col0 plants transformed with a transgene expressing wild-type or a nuclear localized form of AGO1 fused to GFP.

Plant microRNAs undergo stepwise-nuclear maturation before engaging cytosolic sequence-complementary transcripts in association with the silencing-effector protein ARGONAUTE1(AGO1). How plant miRNAs translocate to the cytosol remains mysterious as does their cellular loading site(s) into AGO1. Here, we show that the N-termini of plant AGO1s contain a nuclear-localization (NLS) and nuclear-export signal (NES), which, in Arabidopsis thaliana (At) enables AtAGO1 nucleo-cytosolic shuttling in a Leptomycin-B-inhibited manner, diagnostic of CRM1(Expo1)/NES-dependent nuclear export. Nuclear-only AtAGO1 contains the same 2’O-methylated miRNA cohort as its nucleo-cytosolic counterpart, but specifically interacts with the miRNA loading-chaperone HSP90. AtAGO1 nucleo-cytosolic shuttling is required for mature miRNA translocation and for miRNA-mediated silencing. We propose that plant miRNAs are matured, methylated, loaded into AGO1 in the nucleus, and exported cytoplasmically as AGO1:miRNA complexes in a CRM1(Expo1)/NES-dependent manner. 2 samples examined: Arabidopsis thaliana AGO1 immunoprecipitated small RNA Col0 from plants expressing pAGO1:GFP-AGO1 or pAGO1:GFP-AGO1ΔNES (2 replicates for each)