Multiple interactions between the splicing substrate and small nuclear ribonucleoproteins in spliceosomes.

Protection experiments with antibodies against small nuclear ribonucleoproteins (snRNPs) have elucidated the location of and requirements for interactions between snRNPs and human beta-globin transcripts during splicing in vitro. U2 snRNP association with the intron branch site continues after branch formation, requires intact U2 RNA, and is affected by some alterations of the 3' splice site sequence. U2 snRNP binding to the branched intermediate and U1 snRNP protection of an extended 5' splice region are detected exclusively in spliceosome fractions, indicating that both snRNPs are spliceosome components. While each snRNP associates specifically with the pre-mRNA, they also appear to interact with each other. The recovery of fragments mapping upstream of the 5' splice site suggests how the excised exon is held in the spliceosome.