Dissection Of The Role Of DOR In Ribosomal RNA Transcription

Cells control their metabolism through regulation of both the anabolic and catabolic pathways. Ribosome biogenesis, one of the most important anabolic pathways in eukaryotes, mainly takes place in the nucleolus, where ribosomal RNA (rRNA) is synthesized and assembled into ribosomal subunits.The nuclear protein DOR (diabetes and obesity regulated protein), first identified as a co-activator of thyroid hormone receptors, regulates catabolic pathway by stimulating thyroid hormone-associated gene expression. DOR is highly expressed in metabolically active tissues such as cardiac, skeletal muscle and brain, and muscle-specific deletion of DOR resulted in muscle hypertrophy. It has been reported that DOR is also required for autophagy--another important catabolic pathway owned by eukaryotic cells. In response to nutrient deprivation, DOR exits the nucleus and participates in autophagosomes formation. Nevertheless, apart from localizing to the nucleoplasm, DOR was recently reported to localize to the nucleolus and nucleolar disruption does not alter DOR function in autophagy, which raises the possibility that nucleolus-localized DOR participates in other biological processes other than autophagy.Here we demonstrate a novel function of nucleolus-localized DOR in ribosome biogenesis. We found that DOR is localized to the nucleolus through its nucleolar-targeting sequence located at the C-terminal of the protein. ChIP analysis indicated that DOR bound throughout the rDNA repeats, with particular affinity for promoter region. Knockdown of DOR resulted in a decrease in rDNA promoter activity and consequent rRNA biogenesis and protein synthesis. In addition, occupancy of the pre-initiation complex subunits UBF, TIF-IA and RPA194 along rDNA promoter is abated in the absence of DOR. Mechanism analyses demonstrated that DOR was associated with the pre-initiation complex for RNA polymerase I, knockdown of DOR dramatically diminished the interaction between UBF and other core components of the pre-initiation complex. These data suggest an important role of DOR in rDNA transcription through facilitating the assembly of the pre-initiation complex at rDNA promoter. The sub-cellular localization of DOR may be involved in its distinct functions in cell metabolism regulation. Under nutrient-rich conditions, DOR localizes to the nucleolus and stimulates ribosome biogenesis and subsequent protein synthesis; under nutrient deprivation conditions, DOR is released from the nucleolus and exported to the cytosol to promote catabolic metabolism through downregulating protein synthesis and upregulating autophagy.