Signal recognition particle assembly in relation to the function of amplified nucleoli in Xenopus oocytes

Division of Cell and Molecular Biology, School of Biology, University of St Andrews, KY16 9TS, UK.
Journal of Cell Science (Impact Factor: 5.43). 04/2005; 118(Pt 6):1299-307. DOI: 10.1242/jcs.01726
Source: PubMed

ABSTRACT The signal recognition particle (SRP) is a ribonucleoprotein machine that controls the translation and intracellular sorting of membrane and secreted proteins. The SRP contains a core RNA subunit with which six proteins are assembled. Recent work in both yeast and mammalian cells has identified the nucleolus as a possible initial site of SRP assembly. In the present study, SRP RNA and protein components were identified in the extrachromosomal, amplified nucleoli of Xenopus laevis oocytes. Fluorescent SRP RNA microinjected into the oocyte nucleus became specifically localized in the nucleoli, and endogenous SRP RNA was also detected in oocyte nucleoli by RNA in situ hybridization. An initial step in the assembly of SRP involves the binding of the SRP19 protein to SRP RNA. When green fluorescent protein (GFP)-tagged SRP19 protein was injected into the oocyte cytoplasm it was imported into the nucleus and became concentrated in the amplified nucleoli. After visiting the amplified nucleoli, GFP-tagged SRP19 protein was detected in the cytoplasm in a ribonucleoprotein complex, having a sedimentation coefficient characteristic of the SRP. These results suggest that the amplified nucleoli of Xenopus oocytes produce maternal stores not only of ribosomes, the classical product of nucleoli, but also of SRP, presumably as a global developmental strategy for stockpiling translational machinery for early embryogenesis.

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    • "Fungal SRP resembles its mammalian counterpart in that it also consists of six proteins (SRP72p, SRP68p, SRP54p, SRP14p and Sec65p, which are homologous to the mammalian proteins, and Srp21) and a single RNA molecule called srp7 in S. pombe (45–48). Studies in yeasts (49,50), Xenopus laevis oocytes (51,52) and mammalian cells (53,54) support a model in which all SRP proteins, except SRP54, are imported to the nucleolus for assembly with 7S RNA. The resulting pre-particle is then exported to the cytoplasm where it incorporates SRP54, leading to the formation of a mature and functional RNP. "
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    ABSTRACT: Spinal muscular atrophy is a severe motor neuron disease caused by reduced levels of the ubiquitous Survival of MotoNeurons (SMN) protein. SMN is part of a complex that is essential for spliceosomal UsnRNP biogenesis. Signal recognition particle (SRP) is a ribonucleoprotein particle crucial for co-translational targeting of secretory and membrane proteins to the endoplasmic reticulum. SRP biogenesis is a nucleo-cytoplasmic multistep process in which the protein components, except SRP54, assemble with 7S RNA in the nucleolus. Then, SRP54 is incorporated after export of the pre-particle into the cytoplasm. The assembly factors necessary for SRP biogenesis remain to be identified. Here, we show that 7S RNA binds to purified SMN complexes in vitro and that SMN complexes associate with SRP in cellular extracts. We identified the RNA determinants required. Moreover, we report a specific reduction of 7S RNA levels in the spinal cord of SMN-deficient mice, and in a Schizosaccharomyces pombe strain carrying a temperature-degron allele of SMN. Additionally, microinjected antibodies directed against SMN or Gemin2 interfere with the association of SRP54 with 7S RNA in Xenopus laevis oocytes. Our data show that reduced levels of the SMN protein lead to defect in SRP steady-state level and describe the SMN complex as the first identified cellular factor required for SRP biogenesis.
    Nucleic Acids Research 12/2012; 41(2). DOI:10.1093/nar/gks1224 · 9.11 Impact Factor
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    • "Rather than RNA processing or modification, it may be that certain miRNAs visit the nucleolus to assemble with the appropriate proteins and/or RNAs necessary for transport of the active miRNP from the nucleus. The signal recognition particle RNA traffics through the nucleolus to assemble with specific proteins (Jacobson and Pederson 1998; Politz et al. 2000; Sommerville et al. 2005) and, in the process, accumulates in the GC (Politz et al. 2002), just as do the nucleolus-concentrated miRNAs. We also speculate that some miRNAs could combine with target messenger RNAs in the nucleolus to be exported as ''presuppressed'' mRNAs, a somatic cell analogy with masked maternal mRNAs in oocytes. "
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    ABSTRACT: There is increasing evidence that noncoding RNAs play a functional role in the nucleus. We previously reported that the microRNA (miRNA), miR-206, is concentrated in the nucleolus of rat myoblasts, as well as in the cytoplasm as expected. Here we have extended this finding. We show by cell/nuclear fractionation followed by microarray analysis that a number of miRNAs can be detected within the nucleolus of rat myoblasts, some of which are significantly concentrated there. Pronounced nucleolar localization is a specific phenomenon since other miRNAs are present at only very low levels in the nucleolus and occur at much higher levels in the nucleoplasm and/or the cytoplasm. We have further characterized a subset of these miRNAs using RT-qPCR and in situ hybridization, and the results suggest that some miRNAs are present in the nucleolus in precursor form while others are present as mature species. Furthermore, we have found that these miRNAs are clustered in specific sites within the nucleolus that correspond to the classical granular component. One of these miRNAs is completely homologous to a portion of a snoRNA, suggesting that it may be processed from it. In contrast, the other nucleolar-concentrated miRNAs do not show homology with any annotated rat snoRNAs and thus appear to be present in the nucleolus for other reasons, such as modification/processing, or to play roles in the late stages of ribosome biosynthesis or in nonribosomal functions that have recently been ascribed to the granular component of the nucleolus.
    RNA 08/2009; 15(9):1705-15. DOI:10.1261/rna.1470409 · 4.94 Impact Factor
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    • "There are four Y RNAs encoded in the human genome, three of which are found in PNCs in addition to the cytoplasm (57). The signal recognition particle (SRP) RNA (also known as 7SL RNA) is found in human nucleoli and PNC and rat and Xenopus nucleoli (101,105–107). Microinjected SRP RNA is nucleolar in Xenopus nuclei and transiently nucleolar in rat nuclei (103,107). "
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    ABSTRACT: RNA polymerase III (pol III) transcribes many essential, small, noncoding RNAs, including the 5S rRNAs and tRNAs. While most pol III-transcribed genes are found scattered throughout the linear chromosome maps or in multiple linear clusters, there is increasing evidence that many of these genes prefer to be spatially clustered, often at or near the nucleolus. This association could create an environment that fosters the coregulation of transcription by pol III with transcription of the large ribosomal RNA repeats by RNA polymerase I (pol I) within the nucleolus. Given the high number of pol III-transcribed genes in all eukaryotic genomes, the spatial organization of these genes is likely to affect a large portion of the other genes in a genome. In this Survey and Summary we analyze the reports regarding the spatial organization of pol III genes and address the potential influence of this organization on transcriptional regulation.
    Nucleic Acids Research 02/2006; 34(17):4826-36. DOI:10.1093/nar/gkl656 · 9.11 Impact Factor
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