Project description: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.
Project description: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.
Project description:The exosome functions in the degradation of diverse RNA species, yet how it is negatively regulated remains largely unknown. Here, we show that NRDE2 forms a 1:1 complex with MTR4, a nuclear exosome cofactor critical for exosome recruitment, via a conserved MTR4-interacting domain (MID). Unexpectedly, NRDE2 mainly localizes in nuclear speckles, where it inhibits MTR4 recruitment and RNA degradation, and thereby ensures efficient mRNA nuclear export. Structural and biochemical data revealed that NRDE2 interacts with MTR4's key residues, locks MTR4 in a closed conformation, and inhibits MTR4 interaction with the exosome as well as proteins important for MTR4 recruitment, such as the cap-binding complex (CBC) and ZFC3H1. Functionally, MID deletion results in the loss of self-renewal of mouse embryonic stem cells. Together, our data pinpoint NRDE2 as a nuclear exosome negative regulator that ensures mRNA stability and nuclear export.
Project description:The RNA exosome is an essential 3’ to 5’ processing exoribonuclease complex that mediates degradation, processing, and quality control of virtually all eukaryotic RNAs. The nucleolar RNA exosome, consisting of a 9-subunit core and a distributive 3ʹ to 5ʹ exonuclease EXOSC10, plays a critical role in processing and degrading nucleolar RNAs, including pre-ribosomal RNA (pre-rRNA). However, how the RNA exosome is regulated in the nucleolus is poorly understood. Here, we report that the nucleolar ubiquitin-specific protease USP36 is a novel regulator of the nucleolar RNA exosome. USP36 binds to the RNA exosome through direct interaction with EXOSC10. Interestingly, USP36 does not significantly regulate the levels of EXOSC10 and other exosome subunits. Instead, it mediates EXOSC10 SUMOylation at Lys (K) 583. Mutating K583 impaired the binding of EXOSC10 to pre-rRNAs and the K583R mutant failed to rescue the defects in rRNA processing and cell growth inhibition caused by knockdown of endogenous EXOSC10, indicating that EXOSC10 SUMOylation is critical for the exosome function in rRNA processing. Furthermore, USP36 itself is a rRNA-binding protein that associates pre-rRNA. These results suggest that USP36 acts as a novel SUMO ligase to mediate EXOSC10 SUMOylation critical for the RNA exosome function in rRNA processing and ribosome biogenesis.
Project description:we analyzed globally the effect of exosome processing on the nuclear pre-mRNA transcripts by inactivating either the RRP41 or DIS3 subunit of the exosome. Using SOLiD RNA sequencing technology, we report 30-120 million mapped cellular compartment specific reads per sample allowing the detection of unspliced pre-mRNAs. We show that RRP41 and DIS3 knockdowns stabilize an overlapping set of U12-type introns. Studying the global effect of the exosome (Rrp41 or Dis3 subunit) knockdown comparing to the control sample.
Project description:The nuclear exosome performs critical functions in non-coding RNA processing, and in diverse surveillance functions including the quality control of mRNP formation, and in the removal of pervasive transcripts. Most non-coding RNAs and pervasive nascent transcripts are targeted by the Nrd1p-Nab3p-Sen1p (NNS) complex to terminate Pol II transcription coupled to nuclear exosome degradation or 3´-end trimming. Prior to nuclear exosome activity, the Trf4p-Air2p-Mtr4p polyadenylation complex adds an oligo-A tail to exosome substrates. Inactivating exosome activity stabilizes and lengthens these A-tails. We utilized high-throughput 3´-end poly(A)+ sequencing to identify at nucleotide resolution the 3´ ends targeted by the nuclear exosome, and determine the sites of NNS-dependent termination genome-wide.
Project description:Degradation of transcripts in mammalian nuclei is primarily facilitated by the RNA exosome. To obtain substrate specificity, the exosome is aided by adaptors; in the nucleoplasm, the Nuclear EXosome Targeting (NEXT) complex and the PolyA (pA) eXsome Targeting (PAXT) connection. However, how exact targeting is achieved remains enigmatic. Employing high-resolution 3’end sequencing of both steady state and newly produced pA+ and pA- RNA, we demonstrate that NEXT substrates arise from heterogenous and predominantly pA- 3’ends often covering kb-wide genomic regions. In contrast, PAXT targets harbor well-defined pA+ 3’ends defined by canonical pA site usage. Irrespective this clear division, NEXT and PAXT act redundantly in two ways: i) Regional redundancy: The majority of exosome-targeted transcription units produce both NEXT- and PAXT-sensitive RNA isoforms; and ii) Isoform redundancy: The PAXT connection ensures the fail-safe decay of post-transcriptionally polyadenylated NEXT targets. In conjunction, this provides for the efficient nuclear removal of superfluous RNA.
Project description:In this study, an exoribonuclease was analyzed by iCLIP. The data documents the role of the archaeal exosome as an exoribonuclease and RNA-tailing enzyme interacting with all RNA classes. Mapping of most reads to mRNAs underlines the role of exosome in mRNA turnover, which is important for adaptation of prokaryotic cells to changing environmental conditions. The clustering of crosslink sites near 5’-ends of genes suggests simultaneous binding of both RNA ends by the S. solfataricus exosome. This may serve to prevent translation of mRNAs designated to degradation in 3’-5’ direction.