KETCH1 imports HYL1 to nucleus for miRNA biogenesis in <i>Arabidopsis</i>

Abstract

MicroRNA (miRNA) is processed from primary transcripts with hairpin structures (pri-miRNAs) by microprocessors in the nucleus. How cytoplasmic-borne microprocessor components are transported into the nucleus to fulfill their functions remains poorly understood. Here, we report KETCH1 (karyopherin enabling the transport of the cytoplasmic HYL1) as a partner of hyponastic leaves 1 (HYL1) protein, a core component of microprocessor in <i>Arabidopsis</i> and functional counterpart of DGCR8/Pasha in animals. Null mutation of <i>ketch1</i> is embryonic-lethal, whereas knockdown mutation of <i>ketch1</i> caused morphological defects, reminiscent of mutants in the miRNA pathway. <i>ketch1</i> knockdown mutation also substantially reduced miRNA accumulation, but did not alter nuclear-cytoplasmic shuttling of miRNAs. Rather, the mutation significantly reduced nuclear portion of HYL1 protein and correspondingly compromised the pri-miRNA processing in the nucleus. We propose that KETCH1 transports HYL1 from the cytoplasm to the nucleus to constitute functional microprocessor in <i>Arabidopsis</i> This study provides insight into the largely unknown nuclear-cytoplasmic trafficking process of miRNA biogenesis components through eukaryotes.