Exon definition complexes contain the tri-snRNP and can be directly converted into B-like precatalytic splicing complexes.
The first step in splicing of pre-mRNAs with long introns is exon definition, where U1 and U2 snRNPs bind at opposite ends of an exon. After exon definition, these snRNPs must form a complex across the upstream intron to allow splicing catalysis. Exon definition and conversion of cross-exon to cross-intron spliceosomal complexes are poorly understood. Here we demonstrate that, in addition to U1 and U2 snRNPs, cross-exon complexes contain U4, U5, and U6 (which form the tri-snRNP). Tri-snRNP docking involves the formation of U2/U6 helix II. This interaction is stabilized by a 5' splice site (SS)-containing oligonucleotide, which can bind the tri-snRNP and convert the cross-exon complex into a cross-intron, B-like complex. Our data suggest that the switch from cross-exon to cross-intron complexes can occur directly when an exon-bound tri-snRNP interacts with an upstream 5'SS, without prior formation of a cross-intron A complex, revealing an alternative spliceosome assembly pathway.
Schneider
Marc
M
Department of Cellular Biochemistry, MPI of Biophysical Chemistry, Göttingen, Germany.
Will
Cindy L
CL
Anokhina
Maria
M
Tazi
Jamal
J
Urlaub
Henning
H
Lührmann
Reinhard
R
D005091:Exons; D006801:Humans; D012322:RNA Precursors / Q000235:genetics / Q000378:metabolism*; D012326:RNA Splicing; D012333:RNA, Messenger / Q000378:metabolism*; D017411:Ribonucleoproteins, Small Nuclear / Q000235:genetics / Q000378:metabolism*
D012322:RNA Precursors; D012333:RNA, Messenger; D017411:Ribonucleoproteins, Small Nuclear