LAUSR

Post-transcriptional gene silencing mediated by microRNAs (miRNAs) modulates numerous developmental and stress response pathways. For the last two decades, HASTY (HST), the ortholog of human Exportin-5, has been considered as a candidate protein that exports plant miRNAs from the nucleus to the cytoplasm. Here, we report HST functions in the miRNA pathway independent of its cargo-exporting activity in Arabidopsis. We found that Arabidopsis mutants with impaired HST shuttling present normal subcellular distribution of miRNAs. Interestingly, protein-protein interaction and microscopy assays showed that HST directly interacts with the microprocessor core component DCL1 through its N-terminal domain. Moreover, mass-spectrometry analysis revealed that HST also interacts, independently of its N-terminal domain, with the mediator complex subunit MED37. Further experiments showed that HST could act as a scaffold to facilitate the recruitment of DCL1 to genomic MIRNA loci by stabilizing the DCL1-MED37 complex, which in turn promotes the transcription and proper processing of pri-miRNAs. In summary, our results suggest that HST is likely associated with the formation of the miRNA biogenesis complex at MIRNA genes, promoting the transcription and processing of pri-miRNAs, rather than with the direct export of processed miRNAs from the nucleus.