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MBC in Press, published online ahead of print September 20, 2006
Mol. Biol. Cell 10.1091/mbc.E06-06-0513

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Submitted on June 9, 2006
Revised on July 26, 2006
Accepted on September 8, 2006

Enhancement of U4/U6 snRNP Association in Cajal Bodies Predicted by Mathematical Modeling

Mirko Klingauf,* David Stanek,{dagger} and Karla M. Neugebauer

Max Planck Institute of Molecular Cell Biology and Genetics, 01307 Dresden, Germany

Monitoring Editor: A. Gregory Matera

Spliceosomal snRNPs undergo specific assembly steps in Cajal bodies (CBs), nonmembrane bound compartments within cell nuclei. An example is the U4/U6 di-snRNP, assembled from U4 and U6 monomers. These snRNPs can also assemble in the nucleoplasm when cells lack CBs. Here we address the hypothesis that snRNP concentration in CBs facilitates assembly, by comparing the predicted rates of U4 and U6 snRNP association in nuclei with and without CBs. This was accomplished by a random walk-and-capture simulation applied to a three-dimensional model of the HeLa cell nucleus, derived from measurements of living cells. Results of the simulations indicated that snRNP capture is optimal when nuclei contain 3-4 CBs. Interestingly, this is the observed number of CBs in most cells. Microinjection experiments showed that U4 snRNA targeting to CBs was U6 snRNP-independent and that snRNA concentration in CBs is ~20-fold higher than in nucleoplasm. Finally, combination of the simulation with calculated association rates predicted that the presence of CBs enhances U4 and U6 snRNP association by up to 11-fold, largely due to this concentration difference. This provides a chemical foundation for the proposal that these and other cellular compartments promote molecular interactions, by increasing the local concentration of individual components.

Present addresses: *Institute of Biochemistry, ETH-Zurich, Schafmattstrasse 18, 8093 Zurich, Switzerland; {dagger}Department of Cellular Biology and Pathology, First Medical Faculty of Charles University in Prague, Albertov 4, 128 00 Prague, Czech Republic.

Address correspondence to: Karla M. Neugebauer (neugebau{at}mpi-cbg.de)




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D. Stanek, J. Pridalova-Hnilicova, I. Novotny, M. Huranova, M. Blazikova, X. Wen, A. K. Sapra, and K. M. Neugebauer
Spliceosomal Small Nuclear Ribonucleoprotein Particles Repeatedly Cycle through Cajal Bodies
Mol. Biol. Cell, June 1, 2008; 19(6): 2534 - 2543.
[Abstract] [Full Text] [PDF]


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