URB1 ensures 3' end processing of pre-rRNAs to prevent exosome-dependent degradation and is required for development

The nucleolus composes hundreds of proteins that play distinct roles in ribosomal RNA (rRNA) processing within Fibrillar Center/Dense Fibrillar Component (FC/DFC) units and ribosome assembly in Granular Component (GC). The sub-nucleolar localization of most proteins and how their unique localization facilitates rRNA processing have remained elusive. By screening 200 nucleolar candidate proteins with high-resolution, live-cell microscopy, we identified a previously undescribed sub-nucleolar compartment, named the periphery of DFC (PDFC). Among the 12 proteins identified in PDFC, URB1 (unhealthy ribosome biogenesis 1) is required for PDFC organization and proper 3' end processing of 47S pre-rRNA. URB1 binds between the 28S rRNA and the 3' external transcribed spacer (3' ETS) to ensure 3' ETS removal, which occurs at the DFC/PDFC boundary. Loss of URB1 leads to accumulation of aberrant 3' ETS-retained 32S pre-rRNA variants, which activates exosome-dependent nucleolar surveillance. This causes decreased 28S rRNA production, reduced cell proliferation and retarded mouse embryonic pre-implementation development. Furthermore, urb1 depletion results in developmental craniofacial disorder in zebrafish, which can be at least partially rescued by further depleting exosome components. Together, this study provides new insights into functional sub-nucleolar organizations, identifies a physiologically essential step in rRNA maturation and emphasizes the exosome-dependent pre-rRNA surveillance pathway. Overall design: iCLIP-seq of Flag-tagged URB1 protein in HeLa cells and mapped with pre-rRNA. Each experiment was performed in biological duplicate. Control libraries for size-match inputs (SMI) were prepared in parallel.