Differential alternative splicing activity of isoforms of polypyrimidine tract binding protein (PTB)
Polypyrimidine tract binding protein (PTB) is an RNA-binding protein that regulates splicing by repressing specific splicing events. It also has roles in 3′-end processing, internal initiation of translation, and RNA localization. PTB exists in three alternatively spliced isoforms, PTB1, PTB2, and PTB4, which differ by the insertion of 19 or 26 amino acids, respectively, between the second and third RNA recognition motif domains. Here we show that the PTB isoforms have distinct activities upon [alpha]-tropomyosin (TM) alternative 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 repressive hierarchy was observed in all in vivo and in vitro assays with TM, but the isoforms were equally active in inducing skipping of [alpha]-actinin exons and showed the opposite hierarchy of activity when tested for activation of IRES-driven translation. These findings establish that the ratio of PTB isoforms could form part of a cellular code that in turn controls the splicing of various other pre-mRNAs.(Received February 19 2001)
(Revised March 13 2001)
(Accepted March 22 2001)
Key Words: alternative splicing; gene regulation; nPTB; pre-mRNA splicing; PTB.
c1 Reprint requests to: Christopher W.J. Smith, Department of Biochemistry, 80 Tennis Court Road, Old Addenbrookes Site, University of Cambridge, Cambridge CB2 1GA, United Kingdom; e-mail: email@example.com.