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.
Exon definition complexes contain the tri-snRNP and can be directly converted into B-like precatalytic splicing complexes
Schneider, M.; Will, C. L.; Anokhina, M.; Tazi, J.; Urlaub, H.; Luhrmann, R.
2010-04-23 / vol 38 / pages 223-35
S1097-2765(10)00212-1 [pii] 10.1016/j.molcel.2010.02.027
1097-4164 (Electronic) 1097-2765 (Linking)
Humans; *Exons; RNA, Messenger/*metabolism; *RNA Splicing; Ribonucleoproteins, Small Nuclear/genetics/*metabolism; RNA Precursors/genetics/*metabolism