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In vivo kinetics of U4/U6·U5 tri-snRNP formation in Cajal bodies

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      A combination of mathematical modeling and live-cell measurements was applied to determine the dynamics of small nuclear ribonucleoprotein (snRNP) formation in Cajal bodies of living cells. Our results indicate that a substantial fraction of tri-snRNPs is formed in Cajal bodies in cells with many Cajal bodies per nucleus.


      The U4/U6·U5 tri-small nuclear ribonucleoprotein particle (tri-snRNP) is an essential pre-mRNA splicing factor, which is assembled in a stepwise manner before each round of splicing. It was previously shown that the tri-snRNP is formed in Cajal bodies (CBs), but little is known about the dynamics of this process. Here we created a mathematical model of tri-snRNP assembly in CBs and used it to fit kinetics of individual snRNPs monitored by fluorescence recovery after photobleaching. A global fitting of all kinetic data determined key reaction constants of tri-snRNP assembly. Our model predicts that the rates of di-snRNP and tri-snRNP assemblies are similar and that ∼230 tri-snRNPs are assembled in one CB per minute. Our analysis further indicates that tri-snRNP assembly is approximately 10-fold faster in CBs than in the surrounding nucleoplasm, which is fully consistent with the importance of CBs for snRNP formation in rapidly developing biological systems. Finally, the model predicted binding between SART3 and a CB component. We tested this prediction by Förster resonance energy transfer and revealed an interaction between SART3 and coilin in CBs.

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      Most cited references 81

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          Speckles are subnuclear structures that are enriched in pre-messenger RNA splicing factors and are located in the interchromatin regions of the nucleoplasm of mammalian cells. At the fluorescence-microscope level they appear as irregular, punctate structures, which vary in size and shape, and when examined by electron microscopy they are seen as clusters of interchromatin granules. Speckles are dynamic structures, and both their protein and RNA-protein components can cycle continuously between speckles and other nuclear locations, including active transcription sites. Studies on the composition, structure and behaviour of speckles have provided a model for understanding the functional compartmentalization of the nucleus and the organization of the gene-expression machinery.

            Author and article information

            aInstitute of Molecular Genetics, Academy of Sciences of the Czech Republic, 142 20 Prague 4, Czech Republic
            bInstitute of Experimental Medicine, Academy of Sciences of the Czech Republic, 142 20 Prague 4, Czech Republic
            cFaculty of Mathematics and Physics, Charles University, 121 16 Prague 2, Czech Republic
            University of British Columbia
            Author notes
            †Address correspondence to: David Staneˇk ( stanek@ ) and Petr Herman (

            *These authors contributed equally to this work.

            Role: Monitoring Editor
            Mol Biol Cell
            Mol. Bio. Cell
            Molecular Biology of the Cell
            The American Society for Cell Biology
            15 February 2011
            : 22
            : 4
            : 513-523
            21177826 3038649 E10-07-0560 10.1091/mbc.E10-07-0560
            © 2011 Novotný et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (

            “ASCB®,“ “The American Society for Cell Biology®,” and “Molecular Biology of the Cell®” are registered trademarks of The American Society of Cell Biology.

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