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Structural basis for m 3 G‐cap‐mediated nuclear import of spliceosomal UsnRNPs by snurportin1
Author(s) -
Strasser Anja,
Dickmanns Achim,
Lührmann Reinhard,
Ficner Ralf
Publication year - 2005
Publication title -
the embo journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 7.484
H-Index - 392
eISSN - 1460-2075
pISSN - 0261-4189
DOI - 10.1038/sj.emboj.7600701
Subject(s) - biology , basis (linear algebra) , computational biology , genetics , mathematics , geometry
In higher eukaryotes the biogenesis of spliceosomal UsnRNPs involves a nucleocytoplasmic shuttling cycle. After the m 7 G‐cap‐dependent export of the snRNAs U1, U2, U4 and U5 to the cytoplasm, each of these snRNAs associates with seven Sm proteins. Subsequently, the m 7 G‐cap is hypermethylated to the 2,2,7‐trimethylguanosine (m 3 G)‐cap. The import adaptor snurportin1 recognises the m 3 G‐cap and facilitates the nuclear import of the UsnRNPs by binding to importin‐β. Here we report the crystal structure of the m 3 G‐cap‐binding domain of snurportin1 with bound m 3 GpppG at 2.4 Å resolution, revealing a structural similarity to the mRNA‐guanyly‐transferase. Snurportin1 binds both the hypermethylated cap and the first nucleotide of the RNA in a stacked conformation. This binding mode differs significantly from that of the m 7 G‐cap‐binding proteins Cap‐binding protein 20 (CBP20), eukaryotic initiation factor 4E (eIF4E) and viral protein 39 (VP39). The specificity of the m 3 G‐cap recognition by snurportin1 was evaluated by fluorescence spectroscopy, demonstrating the importance of a highly solvent exposed tryptophan for the discrimination of m 7 G‐capped RNAs. The critical role of this tryptophan and as well of a tryptophan continuing the RNA base stack was confirmed by nuclear import assays and cap‐binding activity tests using several snurportin1 mutants.