Interactions of aCPs with Cytosine-rich Polypyrimidine Tracts Enhance Splicing of Cassette Exons

Alternative splicing comprises a robust generator of mammalian transcriptome complexity. Splice site specification and activity are controlled by interactions of cis-acting determinants on a transcript with specific RNA binding proteins. A major subset of these interactions comprises interactions localized to the intronic U-rich polypyrimidine tract located immediately 5’ to the majority of splice acceptors. alphaCPs (also referred to as polyC-binding proteins (PCBPs) and hnRNP Es) comprise a subset of KH-domain proteins with high specificity and affinity for C-rich polypyrimidine motifs. Prior studies have revealed that binding of alphaCPs to C-rich motifs can modulate splicing and 3’ processing of the human alpha-globin mRNA transcript in the nucleus as well as stabilization of the halpha-globin mRNA in the cytoplasm. In the current report, we demonstrate that alphaCPs have a positive impact on the activity of splice acceptor sites in a defined subset of mammalian transcripts via binding to polypyrimidine tracts that are predominantly C-rich. These findings lead us to conclude that the alphaCPs play a global role in determining the splicing activity and levels of cassette exon inclusion within the mammalian transcriptome. To test the impact of aCP proteins on alternative splicing, aCP proteins were knockdown from K562 cells by siRNA. Since aCP1 and aCP2 have redundent function, we therefore designed siRNAs capable of knockdown both isoform at the same time. 3 aCP1/2 combined knockdown and 3 control siRNA knockdown were performed in K562 cells. RNA-seq were then performed to identify alternative splicing pattern mediated by aCP proteins.