There has been much recent research on the contribution of microRNA (miRNA) in plant organogenesis and hormone action. In plants, it has been reported that Dicer-like 1 (DCL1), HYPONASTIC LEAVES1 (HYL1) and SERRATE (SE) are involved in the production of miRNAs. The means by which miRNAs are processed and transported is not well understood in detail, however. In this study, we investigated the intracellular localization and intermolecular interaction of these molecules using imaging techniques, including bimolecular fluorescence complementation and fluorescence resonance energy transfer techniques, making use of various enhanced fluorescent proteins. We found that DCL1, HYL1 and SE formed bodies which localized in the nuclei. We were also able to locate the miRNA primary transcript using an MS2-tagged method on these bodies. It appears very likely that the observed DCL1–HYL1–SE nuclear body is involved in miRNA production. Co-expression of SmD3 or SmB proteins revealed the localization of DCL1–HYL1–SE complexes in the SmD3/SmB nuclear bodies.

Wnt signaling pathways are tightly regulated by ubiquitination, and dysregulation of these pathways promotes tumorigenesis. It has been reported that the ubiquitin ligase RNF43 plays an important role in frizzled-dependent regulation of the Wnt/βcatenin pathway. Here, we show that RNF43 suppresses both Wnt/βcatenin signaling and noncanonical Wnt signaling by distinct mechanisms. The suppression of Wnt/βcatenin signaling requires interaction between the extracellular PA domain and the CRD of frizzled and the intracellular RING-finger domain of RNF43. In contrast, these N-terminal domains of RNF43 are not required for inhibition of noncanonical Wnt signaling, but interaction between the C-terminal cytoplasmic region of RNF43 and the PDZ domain of dishevelled is essential for this suppression. We further show the mechanism by which missense mutations in the extracellular portion of RNF43 identified in patients with tumors activate Wnt/βcatenin signaling. Missense mutations of RNF43 change their localization from the endosome to the ER, resulting in the failure of frizzled-dependent suppression of Wnt/βcatenin signaling. However, these mutants retain the ability to suppress noncanonical Wnt signaling probably due to interaction with dishevelled. RNF43 is also one of the potential target genes of Wnt/βcatenin signaling. Our results reveal the molecular role of RNF43 and provide an insight into tumorigenesis.