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

Abstract

Polypyrimidine tract binding protein (PTB) is an RNA-binding protein that regulates splicing by repressing specific splicing events. It also has roles in 39-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 a-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 a-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.

Control of Insulin mRNA Stability in Rat Pancreatic Islets

“…The PTB-binding site in insulin mRNA is located just upstream of the polyadenylation signal, and it is not unlikely that PTB, in addition to regulation of mRNA stability, is involved in polyadenylation, transport, and/or translation. Moreover, PTB exists in three different isoforms, with differential alternative splicing activity, and PTB homologues with cell type-specific expression patterns have recently been identified (36). Consequently, the possibility exists that PTB isoforms and the different PTB homologues have overlapping but distinct RNA binding specificities and that a specific PTB isoform or homologue is expressed in ␤-cells with the purpose to modulate insulin mRNA levels.…”

Activation of picornaviral IRESs by PTB shows differential dependence on each PTB RNA-binding domain

“…In contrast, RBDs 3 and 4 are separated by a short linker and seem to act as a pair or didomain (Oberstrass et al 2005). In addition to the prototypic species (PTB-1), there are two isoforms resulting from alternative splicing (Wollerton et al 2001) which differ from PTB-1 by the insertion of 19 (PTB-2) or 26 amino acids in the linker between RBDs2 and 3.…”

The importance of RNA binding proteins in preproinsulin mRNA stability

“…The primary transcript of this gene is alternatively spliced giving rise to four transcript variants, PTBP1a -PTBP1d, which are also known as PTB1, PTB2, PTB3, PTB4 and PTB-T (Gil et al, 1991;Hamilton et al, 2003;Wollerton et al, 2001). In addition, there is also a neuronal form of A c c e p t e d M a n u s c r i p t 4 PTB (PTBP2) (Ashiya and Grabowski, 1997) and the gene coding for PTBP2 is localized to 1p22.1.…”