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Vol. 19, Issue 6, 2534-2543, June 2008

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Spliceosomal Small Nuclear Ribonucleoprotein Particles Repeatedly Cycle through Cajal Bodies

David Stank^*, Jarmila Pidalová-Hnilicová^*, Ivan Novotn^*, Martina Huranová^*, Michaela Blaíková, Xin Wen, Aparna K. Sapra, and Karla M. Neugebauer

*Institute of Molecular Genetics and Institute of Experimental Medicine, Academy of Sciences of the Czech Republic, 142 20 Prague 4, Czech Republic; Center for Craniofacial Molecular Biology, School of Dentistry, University of Southern California, Los Angeles, CA 90033; and Max Planck Institute of Molecular Cell Biology and Genetics, 01307 Dresden, Germany

Submitted December 18, 2007; Revised March 10, 2008; Accepted March 13, 2008
Monitoring Editor: Wendy Bickmore

InCytes from MBC

The Cajal body (CB) is a nuclear structure closely associated with import and biogenesis of small nuclear ribonucleoprotein particles (snRNPs). Here, we tested whether CBs also contain mature snRNPs and whether CB integrity depends on the ongoing snRNP splicing cycle. Sm proteins tagged with photoactivatable and color-maturing variants of fluorescent proteins were used to monitor snRNP behavior in living cells over time; mature snRNPs accumulated in CBs, traveled from one CB to another, and they were not preferentially replaced by newly imported snRNPs. To test whether CB integrity depends on the snRNP splicing cycle, two human orthologues of yeast proteins involved in distinct steps in spliceosome disassembly after splicing, hPrp22 and hNtr1, were depleted by small interfering RNA treatment. Surprisingly, depletion of either protein led to the accumulation of U4/U6 snRNPs in CBs, suggesting that reassembly of the U4/U6·U5 tri-snRNP was delayed. Accordingly, a relative decrease in U5 snRNPs compared with U4/U6 snRNPs was observed in CBs, as well as in nuclear extracts of treated cells. Together, the data show that particular phases of the spliceosome cycle are compartmentalized in living cells, with reassembly of the tri-snRNP occurring in CBs.

Address correspondence to: David Stank (stanek{at}img.cas.cz).

Abbreviations used: CB, Cajal body; snRNA, small nuclear RNA; snRNP, small nuclear ribonucleoprotein particle.

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