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


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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    • "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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    • "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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