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Functional interaction of a novel 15.5kD [U4/U6.U5] tri-snRNP protein with the 5' stem-loop of U4 snRNA.
Authors:S Nottrott  K Hartmuth  P Fabrizio  H Urlaub  I Vidovic  R Ficner  and R Lührmann
Institution:Institut für Molekularbiologie und Tumorforschung, Philipps-Universität Marburg, Emil-Mannkopff-Strasse 2, D-35037 Marburg, Germany.
Abstract:Activation of the spliceosome for splicing catalysis requires the dissociation of U4 snRNA from the U4/U6 snRNA duplex prior to the first step of splicing. We characterize an evolutionarily conserved 15.5 kDa protein of the HeLa U4/U6.U5] tri-snRNP that binds directly to the 5' stem-loop of U4 snRNA. This protein shares a novel RNA recognition motif with several RNP-associated proteins, which is essential, but not sufficient for RNA binding. The 15.5kD protein binding site on the U4 snRNA consists of an internal purine-rich loop flanked by the stem of the 5' stem-loop and a stem comprising two base pairs. Addition of an RNA oligonucleotide comprising the 5' stem-loop of U4 snRNA (U4SL) to an in vitro splicing reaction blocked the first step of pre-mRNA splicing. Interestingly, spliceosomal C complex formation was inhibited while B complexes accumulated. This indicates that the 15.5kD protein, and/or additional U4 snRNP proteins associated with it, play an important role in the late stage of spliceosome assembly, prior to step I of splicing catalysis. Our finding that the 15.5kD protein also efficiently binds to the 5' stem-loop of U4atac snRNA indicates that it may be shared by the U4atac/U6atac.U5] tri-snRNP of the minor U12-type spliceosome.
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