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Vol. 16, Issue 10, 4660-4671, October 2005
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* Department of Genetics, School of Medicine, Case Western Reserve University, Cleveland, OH 44106-4955;
Departments of Cell Biology and Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037; and
Department of Biophysics, Institute of Experimental Physics, Warsaw University, Warsaw 02-089, Poland
Submitted April 15, 2005;
Revised June 24, 2005;
Accepted July 11, 2005
Monitoring Editor: Karsten Weis
The initial steps of spliceosomal small nuclear ribonucleoprotein (snRNP) maturation take place in the cytoplasm. After formation of an Sm-core and a trimethylguanosine (TMG) cap, the RNPs are transported into the nucleus via the import adaptor snurportin1 (SPN) and the import receptor importin-
. To better understand this process, we identified SPN residues that are required to mediate interactions with TMG caps, importin-, and the export receptor, exportin1 (Xpo1/Crm1). Mutation of a single arginine residue within the importin- binding domain (IBB) disrupted the interaction with importin-, but preserved the ability of SPN to bind Xpo1 or TMG caps. Nuclear transport assays showed that this IBB mutant is deficient for snRNP import but that import can be rescued by addition of purified survival of motor neurons (SMN) protein complexes. Conserved tryptophan residues outside of the IBB are required for TMG binding. However, SPN can be imported into the nucleus without cargo. Interestingly, SPN targets to Cajal bodies when U2 but not U1 snRNPs are imported as cargo. SPN also relocalizes to Cajal bodies upon treatment with leptomycin B. Finally, we uncovered an interaction between the N- and C-terminal domains of SPN, suggesting an autoregulatory function similar to that of importin-
.
The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).
Address correspondence to: A. Gregory Matera (a.matera{at}case.edu).
This article has been cited by other articles:
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 G. B. Gonsalvez, K. Praveen, A. J. Hicks, L. Tian, and A. G. Matera Sm protein methylation is dispensable for snRNP assembly in Drosophila melanogaster RNA, May 1, 2008; 14(5): 878 - 887. [Abstract] [Full Text] [PDF]
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 G. Mitrousis, A. S. Olia, N. Walker-Kopp, and G. Cingolani Molecular Basis for the Recognition of Snurportin 1 by Importin {beta} J. Biol. Chem., March 21, 2008; 283(12): 7877 - 7884. [Abstract] [Full Text] [PDF]
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 N. J. Watkins, I. Lemm, and R. Luhrmann Involvement of Nuclear Import and Export Factors in U8 Box C/D snoRNP Biogenesis Mol. Cell. Biol., October 15, 2007; 27(20): 7018 - 7027. [Abstract] [Full Text] [PDF]
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