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, was considered to be a candidate protein that exports plant miRNAs from the nucleus to the cytoplasm. Here, we report that HST functions in the miRNA pathway independent of its cargo-exporting activity in Arabidopsis. We found that Arabidopsis mutants with impaired HST shuttling exhibit 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 revealed 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 primary miRNA transcripts (pri-miRNAs). Taken together, these 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 the direct export of processed miRNAs from the nucleus.

由 microRNA (miRNA) 介导的转录后基因沉默调节许多发育和应激反应途径。在过去的二十年里，人类 EXPORTIN 5 的直系同源物 HASTY (HST) 被认为是将植物 miRNA 从细胞核输出到细胞质的候选蛋白质。在这里，我们报告了 HST 在 miRNA 通路中的功能独立于其在拟南芥中的货物出口活动。我们发现拟南芥HST 穿梭受损的突变体表现出正常的 miRNA 亚细胞分布。有趣的是，蛋白质-蛋白质相互作用和显微镜检测表明 HST 通过其 N 端域直接与微处理器核心组件 DCL1 相互作用。此外，质谱分析显示 HST 还独立于其 N 端结构域与介质复合体亚基 MED37 相互作用。进一步的实验表明，HST 可以充当支架，通过稳定 DCL1-MED37 复合物来促进 DCL1 募集到基因组MIRNA位点，从而促进初级 miRNA 转录物 (pri-miRNA) 的转录和正确加工。综上所述，这些结果表明 HST 可能与MIRNA处 miRNA 生物发生复合体的形成有关。 基因，促进 pri-miRNA 的转录和加工，而不是直接从细胞核输出加工过的 miRNA。