NIPBL is an essential loader of cohesin to mediate sister chromatid cohesion and chromatin loop organization. NIPBL mutations cause Cornelia de Lange Syndrome. How NIPBL's genomic localization is specified is not fully understood. We found that NIPBL localizes to the nucleolus in an RNA-dependent manner and binds directly to ribosomal RNA (rRNA). We identified two RNA binding domains in NIPBL in vitro, both of which are required for efficient rRNA binding in vivo. NIPBL binds to ribosomal DNA (rDNA) in an RNA-stimulated manner, recruits PAF1 and promotes pre-rRNA transcription. Stress that inhibits rRNA synthesis displaces NIPBL from the nucleolus and rDNA. Interestingly, treacle, mutated in Treacher Collins syndrome, tightly binds to and recruits NIPBL to the nucleolus, nucleolar organizer regions, and the stress-induced nucleolar cap. The results reveal that a subpopulation of NIPBL is recruited to the nucleolus through its interaction with RNA and treacle and regulates pre-rRNA transcription.6
Results
NIPBL localizes to the Fibrillar Center of the nucleolusThe nucleolus has a "tripartite structure" consisting of the Fibrillar Center (FC), the Dense Fibrillar Component (DFC), and the Granular Component (GC), each with distinct functions (Fig. 1A) (Boulon et al., 2010a). The FC is the site for pre-rRNA transcription and is enriched in RNA polymerase I (Pol I) machinery. Pre-rRNA is processed in the DFC, which contains many RNA processing factors such as snoRNA and fibrillarin. The GC is the site where pre-ribosome assembly takes place, and contains ribosome assembly factors such as B23 (nucleophosmin). Consistent with cohesin binding to the rDNA region (Zeng et al., 2009a), biochemical fractionation analysis indicates that Nipbl and cohesin are present in both nuclear and nucleolar fractions (Fig. 1B). Using an affinity-purified polyclonal antibody directed against the C-terminus of human NIPBL, we observed clear localization of both mouse Nipbl and human NIPBL in the nucleolus (mouse embryonic fibroblasts (MEFs) in Fig. 1C and D; human 293T cells in Supplemental Fig. S1, respectively). Costaining with antibodies specific for B23 (a marker for the GC) and fibrillarin (a marker for the DFC) revealed that Nipbl tightly clusters to the inside of the DFC corresponding to the FC (Fig. 1C and D). Furthermore, nucleolar Nipbl staining signals were reduced significantly in a Nipbl heterozygous mutant MEFs (modeling NIPBL haploinsufficiency in CdLS (Kawauchi et al., 2009;Newkirk et al., 2017)) and Nipbl siRNAdepleted MEFs compared to the wild type and control siRNA-treated MEFs, respectively (Fig. 1E). These results confirmed the specificity of immunostaining and also indicate that Nipbl in the nucleolus is sensitive to partial protein depletion (haploinsufficiency).
NIPBL localizes to the nucleolus in an RNA-dependent manner and binds to preribosomal RNARNA is highly enriched and is an important constituent of nucleoli. Interestingly, RNase treatment dispersed Nipbl nucleolar foci while bulk Nipbl remained in the nucl...