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 ba lanced by that of splicing enhancers, and this is important to ensure corre ct relative levels of alternatively spliced mRNAs. Using an immunoprecipita tion and UV-cross-linking assay, we show that RNA molecules containing spli cing silencers from the human immunodeficiency virus type 1 tat exon 2 or t he human fibroblast growth factor receptor 2 R-SAM exon bind to hnRNP A1 in HeLa cell nuclear extracts better than the corresponding RNA molecule with out a silencer. Two different point mutations which abolish the K-SAM exon splicing silencer's activity reduce hnRNP Al binding twofold. Recruitment o f hnRNP Al in the form of a fusion,vith bacteriophage MS2 coat protein to a K-SAM exon whose exon splicing silencer has been replaced by a coat bindin g site efficiently represses splicing of the exon in vivo. Recruitment of o nly the glycine-rich C-terminal domain of hnRNP A1, which is capable of int eractions with other proteins, is sufficient to repress exon splicing. Our results show that hnRNP A1 can function to repress splicing, and they sugge st that at least some exon splicing silencers could work by recruiting hnRN P A1.