Yeast mRNA Poly(A) tail length control can be reconstituted in vitro in the absence of Pab1p-dependent Poly(A) nuclease activity

Three-layered control of mRNA poly(A) tail synthesis in Saccharomyces cerevisiae

Biogenesis of most eukaryotic mRNAs involves the addition of an untemplated polyadenosine (pA) tail by the cleavage and polyadenylation machinery. The pA tail, and its exact length, impacts mRNA stability, nuclear export, and translation. To define how polyadenylation is controlled in S. cerevisiae, we have used an in vivo assay capable of assessing nuclear pA tail synthesis, analyzed tail length distributions by direct RNA sequencing, and reconstituted polyadenylation reactions with purified components. This revealed three control mechanisms for pA tail length. First, we found that the pA binding protein (PABP) Nab2p is the primary regulator of pA tail length. Second, when Nab2p is limiting, the nuclear pool of Pab1p, the second major PABP in yeast, controls the process. Third, when both PABPs are absent, the cleavage and polyadenylation factor (CPF) limits pA tail synthesis. Thus, Pab1p and CPF provide fail-safe mechanisms to a primary Nab2p-dependent pathway, thereby preventing uncontrolled polyadenylation and allowing mRNA export and translation.

Altered rRNA processing disrupts nuclear RNA homeostasis via competition for the poly(A)-binding protein Nab2

RNA-binding proteins (RBPs) are key mediators of RNA metabolism. Whereas some RBPs exhibit narrow transcript specificity, others function broadly across both coding and non-coding RNAs. Here, in Saccharomyces cerevisiae, we demonstrate that changes in RBP availability caused by disruptions to distinct cellular processes promote a common global breakdown in RNA metabolism and nuclear RNA homeostasis. Our data shows that stabilization of aberrant ribosomal RNA (rRNA) precursors in an enp1-1 mutant causes phenotypes similar to RNA exosome mutants due to nucleolar sequestration of the poly(A)-binding protein (PABP) Nab2. Decreased nuclear PABP availability is accompanied by genome-wide changes in RNA metabolism, including increased pervasive transcripts levels and snoRNA processing defects. These phenotypes are mitigated by overexpression of PABPs, inhibition of rDNA transcription, or alterations in TRAMP activity. Our results highlight the need for cells to maintain poly(A)-RNA levels in balance with PABPs and other RBPs with mutable substrate specificity across nucleoplasmic and nucleolar RNA processes.

The APT complex is involved in non-coding RNA transcription and is distinct from CPF

The 3'-ends of eukaryotic pre-mRNAs are processed in the nucleus by a large multiprotein complex, the cleavage and polyadenylation factor (CPF). CPF cleaves RNA, adds a poly(A) tail and signals transcription termination. CPF harbors four enzymatic activities essential for these processes, but how these are coordinated remains poorly understood. Several subunits of CPF, including two protein phosphatases, are also found in the related 'associated with Pta1' (APT) complex, but the relationship between CPF and APT is unclear. Here, we show that the APT complex is physically distinct from CPF. The 21 kDa Syc1 protein is associated only with APT, and not with CPF, and is therefore the defining subunit of APT. Using ChIP-seq, PAR-CLIP and RNA-seq, we show that Syc1/APT has distinct, but possibly overlapping, functions from those of CPF. Syc1/APT plays a more important role in sn/snoRNA production whereas CPF processes the 3'-ends of protein-coding pre-mRNAs. These results define distinct protein machineries for synthesis of mature eukaryotic protein-coding and non-coding RNAs.

The multitasking polyA tail: nuclear RNA maturation, degradation and export

A polyA (pA) tail is an essential modification added to the 3' ends of a wide range of RNAs at different stages of their metabolism. Here, we describe the main sources of polyadenylation and outline their underlying biochemical interactions within the nuclei of budding yeast Saccharomyces cerevisiae, human cells and, when relevant, the fission yeast Schizosaccharomyces pombe Polyadenylation mediated by the S. cerevisiae Trf4/5 enzymes, and their human homologues PAPD5/7, typically leads to the 3'-end trimming or complete decay of non-coding RNAs. By contrast, the primary function of canonical pA polymerases (PAPs) is to produce stable and nuclear export-competent mRNAs. However, this dichotomy is becoming increasingly blurred, at least in S. pombe and human cells, where polyadenylation mediated by canonical PAPs may also result in transcript decay.This article is part of the theme issue '5' and 3' modifications controlling RNA degradation'.

Polyadenylation of Friend murine leukemia virus env-mRNA is affected by its splicing

As splicing was previously found to be important for increasing Friend murine leukemia virus env-mRNA stability and translation, we investigated whether splicing of env-mRNA affected the poly(A) tail length using env expression vectors that yielded unspliced or spliced env-mRNA. Incomplete polyadenylation was detected in a fraction of the unspliced env-mRNA products in an env gene-dependent manner, showing that splicing of Friend murine leukemia virus plays an important role in the efficiency of complete polyadenylation of env-mRNA. These results suggested that the promotion of complete polyadenylation of env-mRNA by splicing might partially explain up-regulation of Env protein expression as a result of splicing.

In vitro analysis of cleavage and polyadenylation in Arabidopsis

In eukaryotes, pre-messenger RNA (pre-mRNA) cleavage and polyadenylation is one of the necessary processing steps that produce a mature and functional mRNA. Regulation on pre-mRNA cleavage and polyadenylation affects other processes such as mRNA translocation, stability, and translation. The process of pre-mRNA cleavage and polyadenylation, and its relationship with RNA splicing and translation, have been extensively studied due to its importance in vivo. A successful in vitro system has provided enormous amount of information to the study of cleavage and polyadenylation in the mammalian and yeast systems. Here, we describe an in vitro pre-mRNA cleavage system that faithfully cleaves pre-mRNA substrate using Arabidopsis cell/tissue cultures.

mRNA deadenylation by Pan2-Pan3

Poly(A) tails are important regulators of mRNA stability and translational efficiency. Cytoplasmic removal of poly(A) tails by 3'→5' exonucleases (deadenylation) is the rate-limiting step in mRNA degradation. Two exonuclease complexes contribute the majority of the deadenylation activity in eukaryotes: Ccr4-Not and Pan2-Pan3. These can be specifically recruited to mRNA to regulate mRNA stability or translational efficiency, thereby fine-tuning gene expression. In the present review, we discuss the activities and roles of the Pan2-Pan3 deadenylation complex.

The human nuclear poly(a)-binding protein promotes RNA hyperadenylation and decay

Control of nuclear RNA stability is essential for proper gene expression, but the mechanisms governing RNA degradation in mammalian nuclei are poorly defined. In this study, we uncover a mammalian RNA decay pathway that depends on the nuclear poly(A)-binding protein (PABPN1), the poly(A) polymerases (PAPs), PAPα and PAPγ, and the exosome subunits RRP6 and DIS3. Using a targeted knockdown approach and nuclear RNA reporters, we show that PABPN1 and PAPα, redundantly with PAPγ, generate hyperadenylated decay substrates that are recognized by the exosome and degraded. Poly(A) tail extension appears to be necessary for decay, as cordycepin treatment or point mutations in the PAP-stimulating domain of PABPN1 leads to the accumulation of stable transcripts with shorter poly(A) tails than controls. Mechanistically, these data suggest that PABPN1-dependent promotion of PAP activity can stimulate nuclear RNA decay. Importantly, efficiently exported RNAs are unaffected by this decay pathway, supporting an mRNA quality control function for this pathway. Finally, analyses of both bulk poly(A) tails and specific endogenous transcripts reveals that a subset of nuclear RNAs are hyperadenylated in a PABPN1-dependent fashion, and this hyperadenylation can be either uncoupled or coupled with decay. Our results highlight a complex relationship between PABPN1, PAPα/γ, and nuclear RNA decay, and we suggest that these activities may play broader roles in the regulation of human gene expression.

In eukaryotes, mRNAs include a stretch of adenosine nucleotides at their 3′ end termed the poly(A) tail. In the cytoplasm, the poly(A) tail stimulates translation of the mRNA into protein, and protects the transcript from degradation. Evidence suggests that poly(A) tails may play distinct roles in RNA metabolism in the nucleus, but little is known about these functions and mechanisms. We show here that poly(A) tails can stimulate transcript decay in the nucleus, a function mediated by the ubiquitous nuclear poly(A) binding protein PABPN1. We find that PABPN1 is required for the degradation of a viral nuclear noncoding RNA as well as an inefficiently exported human mRNA. Importantly, the targeting of RNAs to this decay pathway requires the PABPN1 and poly(A) polymerase-dependent extension of the poly(A) tail. Nuclear transcripts with longer poly(A) tails are then selectively degraded by components of the nuclear exosome. These studies elucidate mechanisms that mammalian cells use to ensure proper mRNA “quality control” and may be important to regulate the expression of nuclear noncoding RNAs. Furthermore, our results suggest that the poly(A) tail has diverse and context-specific roles in gene expression.

Disengaging polymerase: terminating RNA polymerase II transcription in budding yeast

Termination of transcription by RNA polymerase II requires two distinct processes: The formation of a defined 3′ end of the transcribed RNA, as well as the disengagement of RNA polymerase from its DNA template. Both processes are intimately connected and equally pivotal in the process of functional messenger RNA production. However, research in recent years has elaborated how both processes can additionally be employed to control gene expression in qualitative and quantitative ways. This review embraces these new findings and attempts to paint a broader picture of how this final step in the transcription cycle is of critical importance to many aspects of gene regulation. This article is part of a Special Issue entitled: RNA polymerase II Transcript Elongation.

The architecture of eukaryotic translation

Translation in baker’s yeast involves the coordinated interaction of 200 000 ribosomes, 3 000 000 tRNAs and between 15 000 and 60 000 mRNAs. It is currently unknown whether this specific constellation of components has particular relevance for the requirements of the yeast proteome, or whether this is simply a frozen accident. Our study uses a computational simulation model of the genome-wide translational apparatus of yeast to explore quantitatively which combinations of mRNAs, ribosomes and tRNAs can produce viable proteomes. Surprisingly, we find that if we only consider total translational activity over time without regard to composition of the proteome, then there are many and widely differing combinations that can generate equivalent synthesis yields. In contrast, translational activity required for generating specific proteomes can only be achieved within a much more constrained parameter space. Furthermore, we find that strongly ribosome limited regimes are optimal for cells in that they are resource efficient and simplify the dynamics of the system.

Rrp6p controls mRNA poly(A) tail length and its decoration with poly(A) binding proteins

Poly(A) (pA) tail binding proteins (PABPs) control mRNA polyadenylation, stability, and translation. In a purified system, S. cerevisiae PABPs, Pab1p and Nab2p, are individually sufficient to provide normal pA tail length. However, it is unknown how this occurs in more complex environments. Here we find that the nuclear exosome subunit Rrp6p counteracts the in vitro and in vivo extension of mature pA tails by the noncanonical pA polymerase Trf4p. Moreover, PABP loading onto nascent pA tails is controlled by Rrp6p; while Pab1p is the major PABP, Nab2p only associates in the absence of Rrp6p. This is because Rrp6p can interact with Nab2p and displace it from pA tails, potentially leading to RNA turnover, as evidenced for certain pre-mRNAs. We suggest that a nuclear mRNP surveillance step involves targeting of Rrp6p by Nab2p-bound pA-tailed RNPs and that pre-mRNA abundance is regulated at this level.

The long and the short of it: the role of the zinc finger polyadenosine RNA binding protein, Nab2, in control of poly(A) tail length

In eukaryotic cells, addition of poly(A) tails to transcripts by 3'-end processing/polyadenylation machinery is a critical step in gene expression. The length of the poly(A) tail influences the stability, nuclear export and translation of mRNA transcripts. Control of poly(A) tail length is thus an important mechanism to regulate the abundance and ultimate translation of transcripts. Understanding the global regulation of poly(A) tail length will require dissecting the contributions of enzymes, regulatory factors, and poly(A) binding proteins (Pabs) that all cooperate to regulate polyadenylation. A recent addition to the Pab family is the CCCH-type zinc finger class of Pabs that includes S. cerevisiae Nab2 and its human counterpart, ZC3H14. In S. cerevisiae, Nab2 is an essential nuclear Pab implicated in both poly(A) RNA export from the nucleus and control of poly(A) tail length. Consistent with an important role in regulation of poly(A) tail length, depletion of Nab2 from yeast cells results in hyperadenylation of poly(A) RNA. In this review, we focus on the role of Nab2 in poly(A) tail length control and speculate on potential mechanisms by which Nab2 could regulate poly(A) tail length based on reported physical and genetic interactions. We present models, illustrating how Nab2 could regulate poly(A) tail length by limiting polyadenylation and/or enhancing trimming. Given that mutation of the gene encoding the human Nab2 homologue, ZC3H14, causes a form of autosomal recessive intellectual disability, we also speculate on how mutations in a gene encoding a ubiquitously expressed Pab lead specifically to neurological defects. This article is part of a Special Issue entitled: Nuclear Transport and RNA Processing.

Reconstitution of CF IA from overexpressed subunits reveals stoichiometry and provides insights into molecular topology

Yeast cleavage factor I (CF I) is an essential complex of five proteins that binds signal sequences at the 3' end of yeast mRNA. CF I is required for correct positioning of a larger protein complex, CPF, which contains the catalytic subunits executing mRNA cleavage and polyadenylation. CF I is composed of two parts, CF IA and Hrp1. The CF IA has only four subunits, Rna14, Rna15, Pcf11, and Clp1, but the structural organization has not been fully established. Using biochemical and biophysical methods, we demonstrate that CF IA can be reconstituted from bacterially expressed proteins and that it has 2:2:1:1 stoichiometry of its four proteins, respectively. We also describe mutations that disrupt the dimer interface of Rna14 while preserving the other subunit interactions. On the basis of our results and existing interaction data, we present a topological model for heterohexameric CF IA and its association with RNA and Hrp1.

An essential role for Clp1 in assembly of polyadenylation complex CF IA and Pol II transcription termination

Polyadenylation is a co-transcriptional process that modifies mRNA 3′-ends in eukaryotes. In yeast, CF IA and CPF constitute the core 3′-end maturation complex. CF IA comprises Rna14p, Rna15p, Pcf11p and Clp1p. CF IA interacts with the C-terminal domain of RNA Pol II largest subunit via Pcf11p which links pre-mRNA 3′-end processing to transcription termination. Here, we analysed the role of Clp1p in 3′ processing. Clp1p binds ATP and interacts in CF IA with Pcf11p only. Depletion of Clp1p abolishes transcription termination. Moreover, we found that association of mutations in the ATP-binding domain and in the distant Pcf11p-binding region impair 3′-end processing. Strikingly, these mutations prevent not only Clp1p-Pcf11p interaction but also association of Pcf11p with Rna14p-Rna15p. ChIP experiments showed that Rna15p cross-linking to the 3′-end of a protein-coding gene is perturbed by these mutations whereas Pcf11p is only partially affected. Our study reveals an essential role of Clp1p in CF IA organization. We postulate that Clp1p transmits conformational changes to RNA Pol II through Pcf11p to couple transcription termination and 3′-end processing. These rearrangements likely rely on the correct orientation of ATP within Clp1p.

Locked tether formation by cooperative folding of Rna14p monkeytail and Rna15p hinge domains in the yeast CF IA complex

The removal of the 3' region of pre-mRNA followed by polyadenylation is a key step in mRNA maturation. In the yeast Saccharomyces cerevisiae, one component of the processing machinery is the cleavage/polyadenylation factor IA (CF IA) complex, composed of four proteins (Clp1p, Pcf11p, Rna14p, Rna15p) that recognize RNA sequences adjacent to the cleavage site and recruit additional processing factors. To gain insight into the molecular architecture of CF IA we solved the solution structure of the heterodimer composed of the interacting regions between Rna14p and Rna15p. The C-terminal monkeytail domain from Rna14p and the hinge region from Rna15p display a coupled binding and folding mechanism, where both peptides are initially disordered. Mutants with destabilized monkeytail-hinge interactions prevent association of Rna15p within CF IA. Conservation of interdomain residues reveals that the structural tethering is preserved in the homologous mammalian cleavage stimulation factor (CstF)-77 and CstF-64 proteins of the CstF complex.

Control of poly(A) tail length

Poly(A) tails have long been known as stable 3' modifications of eukaryotic mRNAs, added during nuclear pre-mRNA processing. It is now appreciated that this modification is much more diverse: A whole new family of poly(A) polymerases has been discovered, and poly(A) tails occur as transient destabilizing additions to a wide range of different RNA substrates. We review the field from the perspective of poly(A) tail length. Length control is important because (1) poly(A) tail shortening from a defined starting point acts as a timer of mRNA stability, (2) changes in poly(A) tail length are used for the purpose of translational regulation, and (3) length may be the key feature distinguishing between the stabilizing poly(A) tails of mRNAs and the destabilizing oligo(A) tails of different unstable RNAs. The mechanism of length control during nuclear processing of pre-mRNAs is relatively well understood and is based on the changes in the processivity of poly(A) polymerase induced by two RNA-binding proteins. Developmentally regulated poly(A) tail extension also generates defined tails; however, although many of the proteins responsible are known, the reaction is not understood mechanistically. Finally, destabilizing oligoadenylation does not appear to have inherent length control. Rather, average tail length results from the balance between polyadenylation and deadenylation.

Recognition of polyadenosine RNA by the zinc finger domain of nuclear poly(A) RNA-binding protein 2 (Nab2) is required for correct mRNA 3'-end formation

Proteins bound to the poly(A) tail of mRNA transcripts, called poly(A)-binding proteins (Pabs), play critical roles in regulating RNA stability, translation, and nuclear export. Like many mRNA-binding proteins that modulate post-transcriptional processing events, assigning specific functions to Pabs is challenging because these processing events are tightly coupled to one another. To investigate the role that a novel class of zinc finger-containing Pabs plays in these coupled processes, we defined the mode of polyadenosine RNA recognition for the conserved Saccharomyces cerevisiae Nab2 protein and assessed in vivo consequences caused by disruption of RNA binding. The polyadenosine RNA recognition domain of Nab2 consists of three tandem Cys-Cys-Cys-His (CCCH) zinc fingers. Cells expressing mutant Nab2 proteins with decreased binding to polyadenosine RNA show growth defects as well as defects in poly(A) tail length but do not accumulate poly(A) RNA in the nucleus. We also demonstrate genetic interactions between mutant nab2 alleles and mutant alleles of the mRNA 3'-end processing machinery. Together, these data provide strong evidence that Nab2 binding to RNA is critical for proper control of poly(A) tail length.

Assembly of an export-competent mRNP is needed for efficient release of the 3'-end processing complex after polyadenylation

Before polyadenylated mRNA is exported from the nucleus, the 3'-end processing complex is removed by a poorly described mechanism. In this study, we asked whether factors involved in mRNP maturation and export are also required for disassembly of the cleavage and polyadenylation complex. An RNA immunoprecipitation assay monitoring the amount of the cleavage factor (CF) IA component Rna15p associated with poly(A)(+) RNA reveals defective removal of Rna15p in mutants of the nuclear export receptor Mex67p as well as other factors important for assembly of an export-competent mRNP. In contrast, Rna15p is not retained in mutants of export factors that function primarily on the cytoplasmic side of the nuclear pore. Consistent with a functional interaction between Mex67p and the 3'-end processing complex, a mex67 mutant accumulates unprocessed SSA4 transcripts and exhibits a severe growth defect when this mutation is combined with mutation of Rna15p or another CF IA subunit, Rna14p. RNAs that become processed in a mex67 mutant have longer poly(A) tails both in vivo and in vitro. This influence of Mex67p on 3'-end processing is conserved, as depletion of its human homolog, TAP/NXF1, triggers mRNA hyperadenylation. Our results indicate a function for nuclear mRNP assembly factors in releasing the 3'-end processing complex once polyadenylation is complete.

Regulation of NAB2 mRNA 3'-end formation requires the core exosome and the Trf4p component of the TRAMP complex

The nuclear exosome functions in a variety of pathways catalyzing formation of mature RNA 3'-ends or the destruction of aberrant RNA transcripts. The RNA 3'-end formation activity of the exosome appeared restricted to small noncoding RNAs. However, the nuclear exosome controls the level of the mRNA encoding the poly(A)-binding protein Nab2p in a manner requiring an A(26) sequence in the mRNA 3' untranslated regions (UTR), and the activities of Nab2p and the exosome-associated exoribonuclease Rrp6p. Here we show that the A(26) sequence inhibits normal 3'-end processing of NAB2 mRNA in vivo and in vitro, and makes formation of the mature 3'-end dependent on trimming of the transcript by the core exosome and the Trf4p component of the TRAMP complex from a downstream site. The detection of mature, polyadenylated transcripts ending at, or within, the A(26) sequence indicates that exosome trimming sometimes gives way to polyadenylation of the mRNA. Alternatively, Rrp6p and the TRAMP-associated Mtr4p degrade these transcripts thereby limiting the amount of Nab2p in the cell. These findings suggest that NAB2 mRNA 3'-end formation requires the exosome and TRAMP complex, and that competition between polyadenylation and Rrp6p-dependent degradation controls the level of this mRNA.

Structure of yeast poly(A) polymerase in complex with a peptide from Fip1, an intrinsically disordered protein

In yeast, the mRNA processing enzyme poly(A) polymerase is tethered to the much larger 3'-end processing complex via Fip1, a 36 kDa protein of unknown structure. We report the 2.6 A crystal structure of yeast poly(A) polymerase in complex with a peptide containing residues 80-105 of Fip1. The Fip1 peptide binds to the outside surface of the C-terminal domain of the polymerase. On the basis of this structure, we designed a mutant of the polymerase (V498Y, C485R) that is lethal to yeast. The mutant is unable to bind Fip1 but retains full polymerase activity. Fip1 is found in all eukaryotes and serves to connect poly(A) polymerase to pre-mRNA processing complexes in yeast, plants, and mammals. However, the Fip1 sequence is highly divergent, and residues on both Pap1 and Fip1 at the observed interaction surface are poorly conserved. Herein we demonstrate using analytical ultracentrifugation, circular dichroism, proteolytic studies, and other techniques that, in the absence of Pap1, Fip1 is largely, if not completely, unfolded. We speculate that flexibility may be important for Fip1's function as a molecular scaffold.

Molecular dissection of mRNA poly(A) tail length control in yeast

In eukaryotic cells, newly synthesized mRNAs acquire a poly(A) tail that plays several fundamental roles in export, translation and mRNA decay. In mammals, PABPN1 controls the processivity of polyadenylation and the length of poly(A) tails during de novo synthesis. This regulation is less well-detailed in yeast. We have recently demonstrated that Nab2p is necessary and sufficient for the regulation of polyadenylation and that the Pab1p/PAN complex may act at a later stage in mRNA metabolism. Here, we show that the presence of both Pab1p and Nab2p in reconstituted pre-mRNA 3′-end processing reactions has no stimulating nor inhibitory effect on poly(A) tail regulation. Importantly, the poly(A)-binding proteins are essential to protect the mature mRNA from being subjected to a second round of processing. We have determined which domains of Nab2p are important to control polyadenylation and found that the RGG-box work in conjunction with the two last essential CCCH-type zinc finger domains. Finally, we have tried to delineate the mechanism by which Nab2p performs its regulation function during polyadenylation: it likely forms a complex with poly(A) tails different from a simple linear deposit of proteins as it has been observed with Pab1p.

PAB1 self-association precludes its binding to poly(A), thereby accelerating CCR4 deadenylation in vivo

The mRNA deadenylation process, catalyzed by the CCR4 deadenylase, is known to be the major factor controlling mRNA decay rates in Saccharomyces cerevisiae. We have identified the proline-rich region and RRM1 domains of poly(A) binding protein (PAB1) as necessary for CCR4 deadenylation. Deletion of either of these regions but not other regions of PAB1 significantly reduced PAB1-PAB1 protein interactions, suggesting that PAB1 oligomerization is a required step for deadenylation. Moreover, defects in these two regions inhibited the formation of a novel, circular monomeric PAB1 species that forms in the absence of poly(A). Removal of the PAB1 RRM3 domain, which promoted PAB1 oligomerization and circularization, correspondingly accelerated CCR4 deadenylation. Circular PAB1 was unable to bind poly(A), and PAB1 multimers were severely deficient or unable to bind poly(A), implicating the PAB1 RNA binding surface as critical in making contacts that allow PAB1 self-association. These results support the model that the control of CCR4 deadenylation in vivo occurs in part through the removal of PAB1 from the poly(A) tail following its self-association into multimers and/or a circular species. Known alterations in the P domains of different PAB proteins and factors and conditions that affect PAB1 self-association would, therefore, be expected to be critical to controlling mRNA turnover in the cell.

Regulation of the nuclear poly(A)-binding protein by arginine methylation in fission yeast

Two structurally different poly(A)-binding proteins (PABP) bind the poly(A) tract of mRNAs in most mammalian cells: PABPC in the cytoplasm and PABP2/PABPN1 in the nucleus. Whereas yeast orthologs of the cytoplasmic PABP are characterized, a gene product homologous to mammalian PABP2 has not been identified in yeast. We report here the identification of a homolog of PABP2 as an arginine methyltransferase 1 (RMT1)-associated protein in fission yeast. The product of the Schizosaccharomyces pombe pab2 gene encodes a nonessential nuclear protein and demonstrates specific poly(A) binding in vitro. Consistent with a functional role in poly(A) tail metabolism, mRNAs from pab2-null cells displayed hyperadenylated 3'-ends. We also show that arginine residues within the C-terminal arginine-rich domain of Pab2 are modified by RMT1-dependent methylation. Whereas the arginine methylated and unmethylated forms of Pab2 behaved similarly in terms of subcellular localization, poly(A) binding, and poly(A) tail length control; Pab2 oligomerization levels were markedly increased when Pab2 was not methylated. Significantly, Pab2 overexpression reduced growth rate, and this growth inhibitory effect was exacerbated in rmt1-null cells. Our results indicate that the main cellular function of Pab2 is in poly(A) tail length control and support a biological role for arginine methylation in the regulation of Pab2 oligomerization.

Tpa1p is part of an mRNP complex that influences translation termination, mRNA deadenylation, and mRNA turnover in Saccharomyces cerevisiae

In this report, we show that the Saccharomyces cerevisiae protein Tpa1p (for termination and polyadenylation) influences translation termination efficiency, mRNA poly(A) tail length, and mRNA stability. Tpa1p is encoded by the previously uncharacterized open reading frame YER049W. Yeast strains carrying a deletion of the TPA1 gene (tpa1Delta) exhibited increased readthrough of stop codons, and coimmunoprecipitation assays revealed that Tpa1p interacts with the translation termination factors eRF1 and eRF3. In addition, the tpa1Delta mutation led to a 1.5- to 2-fold increase in the half-lives of mRNAs degraded by the general 5'-->3' pathway or the 3'-->5' nonstop decay pathway. In contrast, this mutation did not have any affect on the nonsense-mediated mRNA decay pathway. Examination of mRNA poly(A) tail length revealed that poly(A) tails are longer than normal in a tpa1Delta strain. Consistent with a potential role in regulating poly(A) tail length, Tpa1p was also found to coimmunoprecipitate with the yeast poly(A) binding protein Pab1p. These results suggest that Tpa1p is a component of a messenger ribonucleoprotein complex bound to the 3' untranslated region of mRNAs that affects translation termination, deadenylation, and mRNA decay.

Regulation of yeast mRNA 3' end processing by phosphorylation

Recent studies have found that the phosphatase Glc7 associates with the yeast cleavage/polyadenylation factor (CPF), but the role of Glc7 in 3' end processing has not been investigated. Here, we report that depletion of Glc7 causes shortened poly(A) tails in vivo and accumulation of phosphorylated Pta1, a CPF subunit. Removal of Glc7 also gives extract defective for poly(A) addition but normal for cleavage at the poly(A) site. Polyadenylation is rescued by addition of Glc7 or Pta1, but not by phosphorylated Pta1. Moreover, Ypi1, a Glc7-specific inhibitor, or the Cka1 kinase blocks poly(A) addition in wild-type (wt) extract. Pta1 interacts physically and genetically with Glc7, suggesting that Pta1 may also regulate Glc7 or recruit it to CPF. A weakened association of Fip1 with phosphorylated CPF may explain the specific effect on polyadenylation. These results support a model in which poly(A) synthesis is controlled by cycles of phosphorylation and dephosphorylation that require the action of Glc7.