NPD      Nuclear Compartments: Paraspeckles
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Content: Archa Fox and Wendy Bickmore
Copyright 2004
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for paraspeckle components

Paraspeckles were first described as the localisation pattern of a novel protein, Paraspeckle Protein 1 (PSP1) (Fox, Lam et al. 2002) . PSP1 is found diffuse in the nucleus, but is also enriched at a number of irregularly-shaped foci that are often clustered together. There are typically 10-30 paraspeckles in a HeLa nucleus and they are found in the Inter-chromatin space, often closely adjacent to splicing speckles, hence the name: 'parallel' or 'beside' speckles. Paraspeckles are sensitive to the transcription state of the cell, Under normal conditions, the bulk of PSP1 is found in the nucleoplasm and in paraspeckles, however when transcription is inhibited, PSP1 re-accumulates at peri-nucleolar caps. It is thought that PSP1 is constantly cycling between paraspeckles and nucleoli.

Paraspeckle components

In addition to PSP1, there are at least two other known paraspeckle proteins: PSP2 and p54nrb. All three contain classical RNA recognition motifs (RRM). All three proteins re-accumulate at peri-nucleolar caps when transcription is inhibited. Recently, the mRNA 3' end processing cleavage factor CPSF6, was also reported to localise to paraspeckles (Dettwiler et al., 2004).. It also has an RRM domain.

Paraspeckle function

There is no known function for paraspeckles, or PSP1. However, p54nrb has been implicated in numerous nuclear processes including transcription and splicing (for a review see (Shav-Tal and Zipori 2002) and PSP2 is a steroid hormone receptor coactivator, also implicated in transcription and RNA splicing (Auboeuf, Dowhan et al. 2004). Further, there is evidence that the family of proteins including PSP1 and p54nrb has an evolutionary-conserved role in pre-mRNA processing as a chironomous tentans homolog of these proteins, hrp65, interacts with pre-mRNP particles in the nucleus (Miralles, Ofverstedt et al. 2000)..


Subnuclear localisation of PSP1
A) Single section of a HeLa cell expressing YFP-PSP1.
Click to enlarge
(Courtesy: A.Foxt)


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    REFERENCES

    Auboeuf, D., D. H. Dowhan, et al. (2004) CoAA, a nuclear receptor coactivator protein at the interface of transcriptional coactivation and RNA splicing." Mol Cell Biol 24(1): 442-53.

    Fox, A. H., Y. W. Lam, et al. (2002) Paraspeckles: A novel nuclear domain. Curr Biol 12: 13-25.

    Miralles, F., L. G. Ofverstedt, et al. (2000) Electron tomography reveals posttranscriptional binding of pre-mRNPs to specific fibers in the nucleoplasm. J Cell Biol 148(2): 271-82.

    Shav-Tal, Y. and D. Zipori.(2002) PSF and p54(nrb)/NonO--multi-functional nuclear proteins. FEBS Lett 531(2): 109-14.

    Dettwiler S, Aringhieri C, Cardinale S, Keller W, Barabino SM. (2004) Distinct sequence motifs within the 68-kDa subunit of cleavage factor Im mediate RNA binding, protein-protein interactions, and subcellular localization. J Biol Chem. 279(34):35788-97.