A splicing enhancer in the human fibronectin alternate ED1 exon interacts with SR proteins and stimulates U2 snRNP binding.

The inclusion of the 270-nucleotide human fibronectin ED1 exon in HeLa cells requires the presence of a centrally located 81-nucleotide exon sequence. We have conducted a series of in vitro experiments aimed at understanding the structural and functional features associated with this splicing enhancer (SE). Using hybrid model pre-mRNA substrates, we show that the SE element markedly stimulates the use of the 3' splice site of ED1. Deletion and replacement analysis identifies the stimulating sequences as a purine-rich stretch of 9 nucleotides (GAAGAAGAC). The SE element stimulates splicing to the ED1 3' splice site from various positions within the exon except when placed beyond 293 nucleotides downstream from that 3' splice site. The action of the enhancer is not limited to the ED1 acceptor site because the SE element stimulates human beta-globin splicing and also induces the use of a 3' splice site in a prokaryotic sequence in vitro. We have explored the mechanism of action of the fibronectin splicing enhancer and found that the SE element is required for efficient assembly of early splicing complexes, allowing a more efficient interaction of the U2 snRNP with branch site sequences. In competition experiments, an RNA containing mainly SE sequences specifically abolished the action of the SE element, suggesting that factors bind the enhancer element to mediate stimulation of splicing. Using RNA mobility shift assays we show that SR proteins interact specifically with the SE element. Our results demonstrate that exon sequences lying in the SE element play a crucial role in specifying splice site recognition through interactions with factors binding to the 3' splice site.