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First published online 1 March 2005
doi: 10.1242/jcs.01726

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

John Sommerville^1,*, Craig L. Brumwell^2,, Joan C. Ritland Politz² and Thoru Pederson²

¹ Division of Cell and Molecular Biology, School of Biology, University of St Andrews, KY16 9TS, UK
² Department of Biochemistry and Molecular Pharmacology, and Program in Cell Dynamics, University of Massachusetts Medical School, Worcester, MA 01605, USA

^* Author for correspondence (e-mail: js15{at}st-and.ac.uk)

Accepted 13 January 2005

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.

Key words: Immunohistochemistry, In situ hybridization, Microinjection, Nuclear organization, Nucleolar function

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