hnRNP A1 recruited to an exon in vivo can function as an exon splicing silencer

Some exons contain exon splicing silencers. Their activity is frequently balanced by that of splicing enhancers, and this is important to ensure correct relative levels of alternatively spliced mRNAs. Using an immunoprecipitation and UV-cross-linking assay, we show that RNA molecules containing splicing silencers from the human immunodeficiency virus type 1 tat exon 2 or the human fibroblast growth factor receptor 2 K-SAM exon bind to hnRNP A1 in HeLa cell nuclear extracts better than the corresponding RNA molecule without a silencer. Two different point mutations which abolish the K-SAM exon splicing silencer's activity reduce hnRNP A1 binding twofold. Recruitment of hnRNP A1 in the form of a fusion with bacteriophage MS2 coat protein to a K-SAM exon whose exon splicing silencer has been replaced by a coat binding site efficiently represses splicing of the exon in vivo. Recruitment of only the glycine-rich C-terminal domain of hnRNP A1, which is capable of interactions with other proteins, is sufficient to repress exon splicing. Our results show that hnRNP A1 can function to repress splicing, and they suggest that at least some exon splicing silencers could work by recruiting hnRNP A1.