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MBC in Press, published online ahead of print March 26, 2008
Mol. Biol. Cell 10.1091/mbc.E07-12-1259

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Submitted on December 18, 2007
Revised on March 10, 2008
Accepted on March 13, 2008

Spliceosomal snRNPs Repeatedly Cycle through Cajal Bodies

David Stanek,* Jarmila Pridalová,* Ivan Novotny,* Martina Huranová,* Michaela Blazíková,{dagger} Xin Wen,{ddagger} Aparna K. Sapra,{sect} and Karla M. Neugebauer{sect}

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

Monitoring Editor: Wendy Bickmore

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 photo-activatable 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 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 siRNA treatment. Surprisingly, depletion of either protein led to the accumulation of U4/U6 snRNPs in CBs, suggesting that reassembly of the U4/U6•U5 trisnRNP 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. Taken together, the data show that particular phases of the spliceosome cycle are compartmentalized in living cells, with reassembly of the trisnRNP occurring in CBs.

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






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