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A more recent version of this article appeared on October 1, 2005
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Submitted on April 15, 2005
Revised on June 24, 2005
Accepted on July 11, 2005
*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; Department of Biophysics, Institute of Experimental Physics, Warsaw University, Warsaw 02-089, Poland
Monitoring Editor: Karsten Weis
Initial steps of spliceosomal small nuclear ribonucleoprotein (snRNP) maturation take place in the cytoplasm. Following 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 survivial 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-
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