Differential alternative splicing activity of isoforms of polypyrimidine tract binding protein (PTB)

Polypyrimidine tract binding protein (PTB) is an RNA-binding protein that r egulates splicing by repressing specific splicing events. It also has roles in 3'-end processing, internal initiation of translation, and RNA localiza tion. PTB exists in three alternatively spliced isoforms, PTB1, PTB2, and P TB4, which differ by the insertion of 19 or 26 amino acids, respectively, b etween the second and third RNA recognition motif domains. Here we show tha t the PTB isoforms have distinct activities upon alpha -tropomyosin (TM) al ternative splicing. PTB1 reduced the repression of TM exon 3 in transfected smooth muscle cells, whereas PTB4 enhanced TM exon 3 skipping in vivo and in vitro. PTB2 had an intermediate effect. The PTB4 > PTB2 > PTB1 repressiv e hierarchy was observed in all in vivo and in vitro assays with TM, but th e isoforms were equally active in inducing skipping of alpha -actinin exons and showed the opposite hierarchy of activity when tested for activation o f IRES-driven translation. These findings establish that the ratio of PTB i soforms could form part of a cellular code that in turn controls the splici ng of various other pre-mRNAs.