High levels of TopBP1 induce ATR-dependent shut-down of rRNA transcription and nucleolar segregation.
Bottom Line: We found that a basal level of TopBP1 protein associates with ribosomal DNA repeat.Our findings demonstrate that TopBP1 and ATR are able to inhibit the synthesis of rRNA and to activate nucleolar stress pathway; yet the p53-mediated cell cycle arrest is thwarted in cells expressing high levels of TopBP1.We suggest that inhibition of rRNA transcription by different stress regulators is a general mechanism for cells to initiate nucleolar stress pathway.
Affiliation: Department of Biology, University of Eastern Finland, FI-80101 Joensuu, Finland Institute of Biomedicine, University of Eastern Finland, FI-70211 Kuopio, Finland email@example.com.Show MeSH
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Mentions: Next we looked the nucleoli at the ultrastructural level using immuno-EM. In non-induced cells nucleoli were visible as strongly contrasted, round regions in the nuclei (Figure 2A). Interestingly, we found endogenous TopBP1 co-localizing with RNA Pol I, UBF and NCL inside the nucleoli in non-induced cells (Figure 2B, upper panels).The levels of endogenous TopBP1 staining by immuno-EM in the nucleolus appeared even to exceed the general staining in the nucleus. When induced to express eGFP-TopBP1, we found TopBP1 concentrating in regions that were closely associated with the nucleolus, but which appeared distinct from the more electron-dense body of nucleolus (Figure 2A and B, lower panels). These regions obviously correspond to the nucleolar TopBP1 foci visible in fluorescence microscopy.
Affiliation: Department of Biology, University of Eastern Finland, FI-80101 Joensuu, Finland Institute of Biomedicine, University of Eastern Finland, FI-70211 Kuopio, Finland firstname.lastname@example.org.