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: 5.33). 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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Available from: Joan Ritland, Aug 16, 2015
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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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    • "A key question that therefore has arisen is how these newly recognized functions and proteins are spatially localized in relation to the well defined intranucleolar sites of ribosome synthesis. One newly suggested function of the nucleolus is a role in the assembly of the signal recognition particle (Jacobson and Pederson, 1998; Ciufo and Brown, 2000; Pederson and Politz, 2000; Politz et al., 2000, 2002; Grosshans et al., 2001; Alavian et al., 2004; Sommerville et al., 2005). However, signal recognition particle (SRP) RNA does not spatially overlap substantially with 28S rRNA in the mammalian cell nucleolus, nor is it present in the regions in which rRNA transcription or initial processing take place (Politz et al., 2002). "
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