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PABPN1 Shuts Down Alternative Poly(A) Sites

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PABPN1 Shuts Down Alternative Poly(A) Sites Cell Research (2012) 22:1419-1421. npg © 2012 IBCB, SIBS, CAS All rights reserved 1001-0602/12 $ 32.00 RESEARCH HIGHLIGHT www.nature.com/cr PABPN1 shuts down alternative poly(A) sites Martine Simonelig1 1mRNA Regulation and Development, Institute of Human Genetics, CNRS UPR1142, 141 rue de la Cardonille, 34396 Montpellier Cedex 5, France Cell Research (2012) 22:1419-1421. doi:10.1038/cr.2012.86; published online 29 May 2012 Although overlooked for many by two sequences, the canonical poly(A) It has been known for many years years, alternative cleavage and poly- signal AAUAAA localized upstream that, in addition to specific regula- adenylation (APA) is now emerging of the cleavage site and a downstream tors of APA, an important mechanism as a major mechanism of gene regula- U/GU-rich motif; both motifs coop- underlying APA involves changes in tion. A recent study identifies poly(A)- eratively recruit the polyadenylation the concentration of general cleavage binding protein nuclear 1 (PABPN1), machinery through direct interactions and polyadenylation factors, CstF be- a general factor of polyadenylation, with CPSF and CstF, respectively. ing the first complex to be implicated as a suppressor of alternative poly(A) Recent studies using genome-wide in this regulation [4-7]. From these sites. approaches have revealed alternative analyses, a general view emerged that mRNA 3′-end processing is a cotran- cleavage and polyadenylation (APA) promoter-proximal poly(A) sites tend to scriptional reaction that leads to the to be very widespread. More than 50% be weaker than distal poly(A) sites, and addition of a poly(A) tail – polyade- of human genes generate multiple tran- that increased levels of the core cleav- nylation – to virtually all eukaryotic scripts with different 3′ UTRs resulting age and polyadenylation machinery mRNAs. The reaction occurs in two from APA. Alternative poly(A) site us- favor weak proximal sites (Figure 1). steps: the endonucleolytic cleavage of age can lead to the production of differ- Consistent with this, short 3′ UTRs in the pre-mRNA at the poly(A) site and ent protein isoforms, or the production undifferentiated and cancer cells cor- the synthesis of a poly(A) tail onto the of different transcripts encoding the relate with upregulation of cleavage upstream cleaved molecule [1]. The same protein with 3′ UTRs of varying and polyadenylation factors, including poly(A) tail has essential functions in lengths. Because the 3′ UTRs represent CPSF, CstF and symplekin, whereas the many aspects of the life of mRNAs binding platforms for RNA-binding lengthening of 3′ UTRs in differentiated including export to the cytoplasm, proteins and microRNAs, changes in cells correlates with the downregulation stability, intracellular localization and the length of 3′ UTRs can strongly affect of the same factors [2]. translation. mRNA 3′-end processing the cytoplasmic regulation of mRNAs The recent study by Jenal et al. [8] requires four multiprotein cleavage fac- including their stability, localization and now showed that PABPN1 plays a role tors (CPSF: cleavage and polyadenyla- translation rate, with longer 3′ UTRs be- in APA. Strikingly, in contrast to other tion specificity factor; CstF: cleavage ing more likely to suffer a negative regu- cleavage and polyadenylation factors, stimulation factor; CF I and II: cleavage lation [2]. Genome-wide analyses have PABPN1 prevents the usage of proximal factors I and II), additional accessory revealed global shortening of 3′ UTRs in poly(A) sites. proteins such as RNA polymerase II proliferating cells, undifferentiated cells PABPN1 was first identified for its and symplekin, the poly(A) polymerase including induced pluripotent stem cells role during the second step of the cleav- (PAP) that synthesizes the poly(A) tail reprogrammed from somatic cells, and age and polyadenylation reaction, poly- and the poly(A)-binding protein nuclear cancer cell lines, whereas 3′ UTRs were adenylation. In vitro experiments have 1 (PABPN1). Poly(A) sites are defined found to lengthen during embryonic shown that PABPN1 has two functions development and cell differentiation [2]. during polyadenylation: it firstly binds A recent study in Drosophila showed nascent poly(A) tails and stimulates PAP a general trend for very long 3′ UTRs together with CPSF by stabilizing the Correspondence: Martine Simonelig Tel: +33 4 34 35 99 59; Fax: +33 4 34 35 99 57 (up to 18 kb) in the central nervous interaction of PAP with RNA [9], then E-mail: [email protected] system [3]. it stops elongation when the poly(A) tail npg 1420 has reached a length of 250 nucleotides with these regions. Binding of PABPN1 of the proximal poly(A) site. by disrupting PAP-CPSF association to proximal poly(A) site regions was Based on these results, the authors [10]. A role in poly(A) tail lengthening confirmedin vitro and depended on the proposed that PABPN1 plays an ac- has been validated in vivo using muta- integrity of the non-canonical poly(A) tive role in the repression of proximal tions in the Drosophila homologue of signal. In addition, enhanced cleavage at poly(A) site usage by direct binding PABPN1 (Pabp2). Analysis of these proximal poly(A) sites upon reduction to non-canonical poly(A) signals; mutants revealed an additional func- of PABPN1 was recapitulated in in vitro PABPN1 binding would compete tion of PABP2/PABPN1 in cytoplasmic cleavage assays. Importantly, this study with the recruitment of CPSF on these mRNA poly(A) tail length regulation also showed using cyclin D1 mRNA as a weak poly(A) signals. Not only does during early development [11]. Recent model, that increased proximal poly(A) this study identify a new function of studies in S. pombe identified even site usage upon reduction of PABPN1 PABPN1 in APA, prior to cleavage, but broader functions of the PABPN1 ho- prevented mRNA regulation by microR- it also changes the current view of regu- mologue via its interaction with the NAs targeting the 3′ UTR downstream lation of poly(A) site usage by general nuclear exosome, in the maturation of non-coding small nucleolar RNAs and in the nuclear degradation of unspliced pre-mRNAs [12, 13]. Adding to the already diverse func- tional repertoire of PABPN1 in RNA metabolism, Jenal et al. [8] identified yet another novel function for this protein. Using a RNAi screen for RNA- binding proteins in U2OS cells to iden- tify regulators of APA, PABPN1 was found to be the top-scoring candidate. Decreased amounts of PABPN1 favored usage of a proximal poly(A) site. 3' end deep-sequencing was applied to address the generality of this effect. Of a little over 9 000 sequenced transcripts, 32% had more than one cleavage site in their 3′ UTR. A decrease in PABPN1 levels resulted in a shift in poly(A) site usage for 572 transcripts, with increased us- age of the proximal site in most cases (91%), thus revealing extensive 3′ UTR shortening upon PABPN1 reduction. Proximal poly(A) sites are generally weaker due to non-canonical poly(A) signals and accordingly variants to the canonical AAUAAA poly(A) signal were enriched in proximal poly(A) site regions. These results suggested that reduction in PABPN1 enhanced usage of non-canonical proximal poly(A) Figure 1 Proposed model for the regulation of APA integrating the role of sites and therefore that PABPN1 might PABPN1 and of CPSF and CstF levels. Top panel: Low levels of CPSF/CstF actively prevent their usage. Address- favor usage of strong poly(A) sites with canonical poly(A) signals; PABPN1 bound to non-canonical poly(A) signals competes with binding by CPSF and ing the mechanism behind this regula- actively prevents usage of weak proximal poly(A) sites. Bottom panel: High tion, the authors went on to show that levels of CPSF/CstF, or reduction of PABPN1 such as is observed in OPMD, PABPN1 could indeed be crosslinked favor usage of weak proximal poly(A) sites through the binding of CPSF/CstF to proximal poly(A) site regions, indi- to non-canonical motifs. The dotted line indicates that cleavage at the proximal cating direct interactions of PABPN1 poly(A) site might occur before transcription of the distal poly(A) site. Cell Research | Vol 22 No 10 | October 2012 npg 1421 cleavage and polyadenylation factors, important role of APA in OPMD. How 771. suggesting that the low affinity of these APA indeed contributes to OPMD pa- 7 Takagaki Y, Seipelt RL, Peterson ML, factors for non-canonical motifs in weak thology is now a very interesting ques- Manley JL. The polyadenylation factor CstF-64 regulates alternative process- poly(A) sites might not be sufficient to tion to address. ing of IgM heavy chain pre-mRNA prevent their usage and that an active during B cell differentiation. Cell 1996; role of PABPN1 is also required (Figure Acknowledgments 87:941-952. 1). Binding of PABPN1 to canonical 8 Jenal M, Elkon R, Loayza-Puch F, et poly(A) signals in strong poly(A) sites I thank Aymeric Chartier, Isabelle Busseau al. The poly(a)-binding protein nucle- would not affect usage of these sites due and Elmar Wahle for discussions. Work in ar 1 suppresses alternative cleavage the Simonelig lab is supported by the CNRS and polyadenylation sites. Cell 2012; to the enhanced recruitment of CPSF UPR1142, ANR (ANR-2010-BLAN-1201 and CstF to canonical motifs. 149:538-553. 01 and ANR-2009-GENO-025-01), FRM 9 Kerwitz Y, Kuhn U, Lilie H, et al. It would now be of interest to address ("Equipe FRM 2007" and "Projets Inno- Stimulation of poly(A) polymerase the contribution of PABPN1 in APA in vants ING20101221078"), ARC (ARC Libre through a direct interaction with the physiological or pathological conditions 2009 N°3192) and AFM (Research Program nuclear poly(A) binding protein allos- such as differentiation or cancer where 15123).
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