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Review
. 2001 May;21(10):3281-8.
doi: 10.1128/MCB.21.10.3281-3288.2001.

Polypyrimidine tract binding protein antagonizes exon definition

E J Wagner  1 , M A Garcia-Blanco
Affiliations
Review

Polypyrimidine tract binding protein antagonizes exon definition

E J Wagner et al. Mol Cell Biol. 2001 May.
No abstract available

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Figures

FIG. 1
FIG. 1
Functional domains and motifs of PTB. (A) Three variants of human PTB exist: PTB1 (protein accession no. CAA43973), PTB2 (protein accession no. CAA46443), and PTB4 (protein accession no. CAA46444). The PTB4 isoform was also identified as a member of the hnRNP family and named hnRNP I (23). Each of the isoforms has four RRMs (24, 54), as well as an N-terminal nuclear localization signal (56). RRM2, as well as the flanking sequence, has been demonstrated to be necessary for PTB oligomerization (53, 56). RRM3 has been shown to be required for RNA binding (53, 56). RRM4 appears to be required for function but not for RNA binding and thus may bind to an unknown corepressor (Grabowski, personal communication). (B) Alignment of PTB protein sequences from seven species using the MacVector CLUSTALW Alignment function. Dark-gray regions within the alignment represent identity, while the light-gray regions denote conserved amino acid changes. Areas of purple shading are the RRMs, and the segment shaded in light red highlights the region of presumed alternative splicing. The NCBI protein accession numbers for these proteins are as follows: Homo sapiens PTB4, CAA46444; S. scrofa PTB4, CAA63597; Mus musculis PTB1, P17225; Rattus norvegicus PTB4, Q00438; Xenopus laevis PTB4, AAF00041; D. melanogaster PTB4, AAF22979; and Caenorhabditis elegans PTB4, T20381.
FIG. 1
FIG. 1
Functional domains and motifs of PTB. (A) Three variants of human PTB exist: PTB1 (protein accession no. CAA43973), PTB2 (protein accession no. CAA46443), and PTB4 (protein accession no. CAA46444). The PTB4 isoform was also identified as a member of the hnRNP family and named hnRNP I (23). Each of the isoforms has four RRMs (24, 54), as well as an N-terminal nuclear localization signal (56). RRM2, as well as the flanking sequence, has been demonstrated to be necessary for PTB oligomerization (53, 56). RRM3 has been shown to be required for RNA binding (53, 56). RRM4 appears to be required for function but not for RNA binding and thus may bind to an unknown corepressor (Grabowski, personal communication). (B) Alignment of PTB protein sequences from seven species using the MacVector CLUSTALW Alignment function. Dark-gray regions within the alignment represent identity, while the light-gray regions denote conserved amino acid changes. Areas of purple shading are the RRMs, and the segment shaded in light red highlights the region of presumed alternative splicing. The NCBI protein accession numbers for these proteins are as follows: Homo sapiens PTB4, CAA46444; S. scrofa PTB4, CAA63597; Mus musculis PTB1, P17225; Rattus norvegicus PTB4, Q00438; Xenopus laevis PTB4, AAF00041; D. melanogaster PTB4, AAF22979; and Caenorhabditis elegans PTB4, T20381.
FIG. 2
FIG. 2
Schematic representation of six pre-mRNAs that are regulated by PTB through exon definition antagonism. Blue exons represent constitutively spliced exons, while the gray exons are regulated. Dark-blue segments represent PTB binding sites; red rectangles are the branch point-associated polypyrimidine tract, while the black dots represent identified or putative branch points. (c-src) The N1 exon is repressed in nonneural cell types by intronic PTB binding sites flanking the exon (9, 10, 12). (α-actinin) The SM exon may be repressed in tissues other than smooth muscle through a network of PTB binding sites located on both sides of the SM exon (65). It should be noted that in vitro the intron downstream of SM is not required for PTB repression. (FGF-R2) Exon IIIb is also repressed in mesenchymal tissues through multiple PTB binding sites found on either side of IIIb (; E. J. Wagner and M. A. Garcia-Blanco, unpublished results). (Calcitonin/CGRP) The calcitonin-CGRP fourth exon is included and subsequently polyadenylated in the majority of tissues, possibly due to PTB-mediated repression of a zero-length exon located in the downstream intron. Repression of this exon blocks a potential recursive-splicing pathway to exon 5 (, –42). Splicing to exon 5 occurs in neural cell types where PTB levels may be reduced. (GABAAγ2) The small 24-nucleotide exon is repressed in nonneural cell types through PTB binding near the branch point sequence (1, 72, 73). (α-tropomyosin). Exon 3 is specifically repressed in smooth muscle tissue by PTB or a novel PTB variant (25, 26, 55) (Smith, unpublished).
FIG. 3
FIG. 3
Two potential mechanisms to define a zone of silencing. The left model predicts that PTB-PTB interactions between binding sites flanking an exon sequesters an exon, thus precluding the definition of this exon. The model on the right suggests that PTB can oligomerize across an exon, resulting in the coating of the exon, which will also antagonize its definition. The model on the right could also explain the silencing of exons that have a PTB site only on one flank; PTB could multimerize, covering a region of the RNA determined by interactions with other factors.
FIG. 4
FIG. 4
Two possible mechanisms of PTB derepression. (c-src N1 Derepression) In neural cell types it has been suggested that an activity that requires ATP is capable of displacing PTB from the intronic binding sites. This could allow the formation of a neuron-specific complex on the downstream control sequence enhancer resulting in the further activation of the exon (, , –49). (FGF-R2 IIIb Derepression) In epithelial cells, exon IIIb is included. The repressive effect of PTB is overcome at least in part through the counteracting activity of yet-to-be-identified proteins, which could potentially stabilize a secondary structure between the two cis elements ISAR and IAS2 (, , , ; Wagner and Garcia-Blanco, unpublished).
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References

    1. Ashiya M, Grabowski P J. A neuron-specific splicing switch mediated by an array of pre-mRNA repressor sites: evidence of a regulatory role for the polypyrimidine tract binding protein and a brain-specific PTB counterpart. RNA. 1997;3:996–1015. - PMC - PubMed
    1. Berget S M. Exon recognition in vertebrate splicing. J Biol Chem. 1995;270:2411–2414. - PubMed
    1. Berglund J A, Abovich N, Rosbash M. A cooperative interaction between U2AF65 and mBBP/SF1 facilitates branchpoint region recognition. Genes Dev. 1998;12:858–867. - PMC - PubMed
    1. Blanchette M, Chabot B. Modulation of exon skipping by high-affinity hnRNP A1-binding sites and by intron elements that repress splice site utilization. EMBO J. 1999;18:1939–1952. - PMC - PubMed
    1. Burge C T, Tuschl T, Sharp P A. Splicing of precursors to mRNAs by the spliceosomes. In: Gesteland R, editor. The RNA world. 2nd ed. Cold Spring Harbor, N.Y: Cold Spring Harbor Laboratory Press; 1999. pp. 525–560.

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