Different strategies for carboxyl-terminal domain (CTD) recognition by serine 5-specific CTD phosphatases

CREPT serves as a biomarker of poor survival in pancreatic ductal adenocarcinoma

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is one of the most aggressive human malignancies. Cell-cycle-related and expression-elevated protein in tumor (CREPT) plays an important role in the phosphorylation of RNA Pol II, and has been implicated in the development of several types of cancer. As yet, however, there have been no reports on its role in PDAC. Here, we aimed to explore the value of CREPT as a prognostic biomarker in PDAC.

METHODS

CREPT expression was assessed by immunohistochemistry (IHC) on a tissue microarray containing samples from 375 PDAC patients. Kaplan-Meier and Cox regression analyses were performed to explore the independent prognostic value of CREPT expression for the disease-free survival (DFS) and overall survival (OS) of PDAC patients. A Cell Counting Kit-8 (CCK8) assay was used to determine the growth rates and gemcitabine sensitivities of PDAC cells, while a Transwell assay was used to determine the migration and invasion abilities of PDAC cells. Subcutaneous xenografts were used to explore the effect of CREPT expression on tumor growth in vivo.

RESULTS

We found that CREPT is highly expressed in tumor tissues and may serve as an independent prognostic biomarker for DFS and OS of PDAC patients. In vitro assays revealed that CREPT expression promotes the proliferation, migration, invasion and gemcitabine resistance of PDAC cells, and in vivo assays showed that CREPT expression knockdown led to inhibition of PDAC tumor growth.

CONCLUSIONS

We conclude that high CREPT expression enhances the proliferation, migration, invasion and gemcitabine resistance of PDAC cells. In addition, we conclude that CREPT may serve as an independent prognostic biomarker and therapeutic target for PDAC patients.

Diverse and conserved roles of the protein Ssu72 in eukaryotes: from yeast to higher organisms

Gene transcription is a complex biological process that involves a set of factors, enzymes and nucleotides. Ssu72 plays a crucial role in every step of gene transcription. RNA polymerase II (RNAPII) occupies an important position in the synthesis of mRNAs. The largest subunit of RNAPII, Rpb1, harbors its C-terminal domain (CTD), which participates in the initiation, elongation and termination of transcription. The CTD consists of heptad repeats of the consensus motif Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7 and is highly conserved among different species. The CTD is flexible in structure and undergoes conformational changes in response to serine phosphorylation and proline isomerization, which are regulated by specific kinases/phosphatases and isomerases, respectively. Ssu72 is a CTD phosphatase with catalytic activity against phosphorylated Ser5 and Ser7. The isomerization of Pro6 affects the binding of Ssu72 to its substrate. Ssu72 can also indirectly change the phosphorylation status of Ser2. In addition, Ssu72 is a member of the 3'-end cleavage and polyadenylation factor (CPF) complex. Together with other CPF components, Ssu72 regulates the 3'-end processing of premature mRNA. Recent studies have revealed other roles of Ssu72, including its roles in balancing phosphate homeostasis and controlling chromosome behaviors, which should be further explored. In conclusion, the protein Ssu72 is an enzyme worthy of attention, not confined to its role in gene transcription.

Structural Motifs for CTD Kinase Specificity on RNA Polymerase II during Eukaryotic Transcription

The phosphorylation states of RNA polymerase II coordinate the process of eukaryotic transcription by recruitment of transcription regulators. The individual residues of the repetitive heptad of the C-terminal domain (CTD) of the biggest subunit of RNA polymerase II are phosphorylated temporally at different stages of transcription. Intriguingly, despite similar flanking residues, phosphorylation of Ser2 and Ser5 in CTD heptads play dramatically different roles. The mechanism of how the kinases place phosphorylation on the correct serine is not well understood. In this paper, we use biochemical assays, mass spectrometry, molecular modeling, and structural analysis to understand the structural elements determining which serine of the CTD heptad is subject to phosphorylation. We identified three motifs in the activation/P+1 loops differentiating the intrinsic specificity of CTD in various CTD kinases. We characterized the enzyme specificity of the CTD kinases-CDK7 as Ser5-specific, Erk2 with dual specificity for Ser2 and Ser5, and Dyrk1a as a Ser2-specific kinase. We also show that the specificities of kinases are malleable and can be modified by incorporating mutations in their activation/P+1 loops that alter the interactions of the three motifs. Our results provide an important clue to the understanding of post-translational modification of RNA polymerase II temporally during active transcription.

Recent molecular insights into canonical pre-mRNA 3'-end processing

The majority of eukaryotic messenger RNA precursors (pre-mRNAs) undergo cleavage and polyadenylation at their 3' end. This canonical 3'-end processing depends on sequence elements in the pre-mRNA as well as a mega-dalton protein machinery. The cleavage site in mammalian pre-mRNAs is located between an upstream poly(A) signal, most frequently an AAUAAA hexamer, and a GU-rich downstream sequence element. This review will summarize recent advances from the studies on this canonical 3'-end processing machinery. They have revealed the molecular mechanism for the recognition of the poly(A) signal and provided the first glimpse into the overall architecture of the machinery. The studies also show that the machinery is highly dynamic conformationally, and extensive re-arrangements are necessary for its activation. Inhibitors targeting the active site of the CPSF73 nuclease of this machinery have anti-cancer, anti-inflammatory and anti-protozoal effects, indicating that CPSF73 and pre-mRNA 3'-end processing in general are attractive targets for drug discovery.

ABBREVIATIONS

APA: alternative polyadenylation; β-CASP: metallo-β-lactamase-associated CPSF Artemis SNM1/PSO2; CTD: C-terminal domain; CF: cleavage factor; CPF: cleavage and polyadenylation factor; CPSF: cleavage and polyadenylation specificity factor; CstF: cleavage stimulation factor; DSE: downstream element; HAT: half a TPR; HCC: histone pre-mRNA cleavage complex; mCF: mammalian cleavage factor; mPSF: mammalian polyadenylation specificity factor; mRNA: messenger RNA; nt: nucleotide; NTD: N-terminal domain; PAP: polyadenylate polymerase; PAS: polyadenylation signal; PIM: mPSF interaction motif; Poly(A): polyadenylation, polyadenylate; Pol II: RNA polymerase II; pre-mRNA: messenger RNA precursor; RRM: RNA recognition module, RNA recognition motif; snRNP: small nuclear ribonucleoprotein; TPR: tetratricopeptide repeat; UTR: untranslated region; ZF: zinc finger.

RNA Regulation in Plant Cold Stress Response

In addition to plants, all organisms react to environmental stimuli via the perception of signals and subsequently respond through alterations of gene expression. However, genes/mRNAs are usually not the functional unit themselves, and instead, resultant protein products with individual functions result in various acquired phenotypes. In order to fully characterize the adaptive responses of plants to environmental stimuli, it is essential to determine the level of proteins, in addition to the regulation of mRNA expression. This regulatory step, which is referred to as "mRNA posttranscriptional regulation," occurs subsequent to mRNA transcription and prior to translation. Although these RNA regulatory mechanisms have been well-studied in many organisms, including plants, it is not fully understood how plants respond to environmental stimuli, such as cold stress, via these RNA regulations.A recent study described several RNA regulatory factors in relation to environmental stress responses, including plant cold stress tolerance. In this chapter, the functions of RNA regulatory factors and comprehensive analyses related to the RNA regulations involved in cold stress response are summarized, such as mRNA maturation, including capping, splicing, polyadenylation of mRNA, and the quality control system of mRNA; mRNA degradation, including the decapping step; and mRNA stabilization. In addition, the putative roles of messenger ribonucleoprotein (mRNP) granules, such as processing bodies (PBs) and stress granules (SGs), which are cytoplasmic particles, are described in relation to RNA regulations under stress conditions. These RNA regulatory systems are important for adjusting or fine-tuning and determining the final levels of mRNAs and proteins in order to adapt or respond to environmental stresses. Collectively, these new areas of study revealed that plants possess precise novel regulatory mechanisms which specifically function in the response to cold stress.

The over-expression of a chrysanthemum gene encoding an RNA polymerase II CTD phosphatase-like 1 enzyme enhances tolerance to heat stress

The enzyme RNAPII CTD phosphatase-like 1 is known as a transcriptional regulator of the plant response to various abiotic stresses. Here, the isolation of CmCPL1, a chrysanthemum (Chrysanthemum morifolium) gene encoding this enzyme is described. Its predicted 955 residue gene product includes the FCPH catalytic domain, two double-stranded RNA binding motifs, and a nuclear localization signal. A sub-cellular localization assay confirmed that CmCPL1 was expressed in the nucleus. CmCPL1 transcription was shown to be significantly inducible by heat stress. The over-expression and knockdown of CmCPL1, respectively, increased and diminished the tolerance of chrysanthemum to heat stress, which maybe dependent on the regulation of CmCPL1 and on the expression of downstream heat stress-responsive genes.

Phosphorylation induces sequence-specific conformational switches in the RNA polymerase II C-terminal domain

The carboxy-terminal domain (CTD) of the RNA polymerase II (Pol II) large subunit cycles through phosphorylation states that correlate with progression through the transcription cycle and regulate nascent mRNA processing. Structural analyses of yeast and mammalian CTD are hampered by their repetitive sequences. Here we identify a region of the Drosophila melanogaster CTD that is essential for Pol II function in vivo and capitalize on natural sequence variations within it to facilitate structural analysis. Mass spectrometry and NMR spectroscopy reveal that hyper-Ser5 phosphorylation transforms the local structure of this region via proline isomerization. The sequence context of this switch tunes the activity of the phosphatase Ssu72, leading to the preferential de-phosphorylation of specific heptads. Together, context-dependent conformational switches and biased dephosphorylation suggest a mechanism for the selective recruitment of cis-proline-specific regulatory factors and region-specific modulation of the CTD code that may augment gene regulation in developmentally complex organisms.

Paf1 Has Distinct Roles in Transcription Elongation and Differential Transcript Fate

RNA polymerase II (Pol2) movement through chromatin and the co-transcriptional processing and fate of nascent transcripts is coordinated by transcription elongation factors (TEFs) such as polymerase-associated factor 1 (Paf1), but it is not known whether TEFs have gene-specific functions. Using strand-specific nucleotide resolution techniques, we show that levels of Paf1 on Pol2 vary between genes, are controlled dynamically by environmental factors via promoters, and reflect levels of processing and export factors on the encoded transcript. High levels of Paf1 on Pol2 promote transcript nuclear export, whereas low levels reflect nuclear retention. Strains lacking Paf1 show marked elongation defects, although low levels of Paf1 on Pol2 are sufficient for transcription elongation. Our findings support distinct Paf1 functions: a core general function in transcription elongation, satisfied by the lowest Paf1 levels, and a regulatory function in determining differential transcript fate by varying the level of Paf1 on Pol2.

Dephosphorylating eukaryotic RNA polymerase II

The phosphorylation state of the C-terminal domain of RNA polymerase II is required for the temporal and spatial recruitment of various factors that mediate transcription and RNA processing throughout the transcriptional cycle. Therefore, changes in CTD phosphorylation by site-specific kinases/phosphatases are critical for the accurate transmission of information during transcription. Unlike kinases, CTD phosphatases have been traditionally neglected as they are thought to act as passive negative regulators that remove all phosphate marks at the conclusion of transcription. This over-simplified view has been disputed in recent years and new data assert the active and regulatory role phosphatases play in transcription. We now know that CTD phosphatases ensure the proper transition between different stages of transcription, balance the distribution of phosphorylation for accurate termination and re-initiation, and prevent inappropriate expression of certain genes. In this review, we focus on the specific roles of CTD phosphatases in regulating transcription. In particular, we emphasize how specificity and timing of dephosphorylation are achieved for these phosphatases and consider the various regulatory factors that affect these dynamics.

RNA polymerase II CTD phospho-sites Ser5 and Ser7 govern phosphate homeostasis in fission yeast

Phosphorylation of the tandem YSPTSPS repeats of the RNA polymerase II CTD inscribes an informational code that orchestrates eukaryal mRNA synthesis. Here we interrogate the role of the CTD in phosphate homeostasis in fission yeast. Expression of Pho1 acid phosphatase, which is repressed during growth in phosphate-rich medium and induced by phosphate starvation, is governed strongly by CTD phosphorylation status, but not by CTD repeat length. Inability to place a Ser7-PO4 mark (as in S7A) results in constitutive derepression of Pho1 expression in phosphate-replete medium. In contrast, indelible installation of a Ser7-PO4 mimetic (as in S7E) hyper-represses Pho1 in phosphate-replete cells and inhibits Pho1 induction during starvation. Pho1 phosphatase is derepressed by ablation of the CTD Ser5-PO4 mark, achieved either by mutating Ser5 in all consensus heptads to alanine, or replacing all Pro6 residues with alanine. We find that Ser5 status is a tunable determinant of Pho1 regulation, i.e., serial decrements in the number of consensus Ser5 heptads from seven to two elicits a progressive increase in Pho1 expression in phosphate-replete medium. Pho1 is also derepressed by hypomorphic mutations of the CTD kinase Cdk9. Inactivation of the CTD phosphatase Ssu72 attenuates Pho1 induction in wild-type cells and blocks Pho1 derepression in S7A cells. These experiments implicate Ser5, Pro6, and Ser7 as component letters of a CTD coding "word" that transduces a repressive transcriptional signal via serine phosphorylation.

Genetic and structural analysis of the essential fission yeast RNA polymerase II CTD phosphatase Fcp1

Protein phosphatases regulate mRNA synthesis and processing by remodeling the carboxy-terminal domain (CTD) of RNA polymerase II (Pol2) to dynamically inscribe a Pol2 CTD code. Fission yeast Fcp1 (SpFcp1) is an essential 723-amino acid CTD phosphatase that preferentially hydrolyzes Ser2-PO4 of the YS(2)PTSPS repeat. The SpFcp1 catalytic domain (aa 140-580) is composed of a DxDxT acyl-phosphatase module (FCPH) and a BRCT module. Here we conducted a genetic analysis of SpFcp1, which shows that (i) phosphatase catalytic activity is required for vegetative growth of fission yeast; (ii) the flanking amino-terminal domain (aa 1-139) and its putative metal-binding motif C(99)H(101)Cys(109)C(112) are essential; (iii) the carboxy-terminal domain (aa 581-723) is dispensable; (iv) a structurally disordered internal segment of the FCPH domain (aa 330-393) is dispensable; (v) lethal SpFcp1 mutations R271A and R299A are rescued by shortening the Pol2 CTD repeat array; and (vi) CTD Ser2-PO4 is not the only essential target of SpFcp1 in vivo. Recent studies highlight a second CTD code involving threonine phosphorylation of a repeat motif in transcription elongation factor Spt5. We find that Fcp1 can dephosphorylate Thr1-PO4 of the fission yeast Spt5 CTD nonamer repeat T(1)PAWNSGSK. We identify Arg271 as a governor of Pol2 versus Spt5 CTD substrate preference. Our findings implicate Fcp1 as a versatile sculptor of both the Pol2 and Spt5 CTD codes. Finally, we report a new 1.45 Å crystal structure of SpFcp1 with Mg(2+) and AlF3 that mimics an associative phosphorane transition state of the enzyme-aspartyl-phosphate hydrolysis reaction.

The Ssu72 phosphatase mediates the RNA polymerase II initiation-elongation transition

Transitions between the different stages of the RNAPII transcription cycle involve the recruitment and exchange of factors, including mRNA capping enzymes, elongation factors, splicing factors, 3'-end-processing complexes, and termination factors. These transitions are coordinated by the dynamic phosphorylation of the C-terminal domain (CTD) of the largest subunit of RNAPII (Rpb1). The CTD is composed of reiterated heptapeptide repeats (Y(1)S(2)P(3)T(4)S(5)P(6)S(7)) that undergo phosphorylation and dephosphorylation as RNAPII transitions through the transcription cycle. An essential phosphatase in this process is Ssu72, which exhibits catalytic specificity for Ser(P)(5) and Ser(P)(7). Ssu72 is unique in that it is specific for Ser(P)(5) in one orientation of the CTD and for Ser(P)(7) when bound in the opposite orientation. Moreover, Ssu72 interacts with components of the initiation machinery and affects start site selection yet is an integral component of the CPF 3'-end-processing complex. Here we provide a comprehensive view of the effects of Ssu72 with respect to its Ser(P)(5) phosphatase activity. We demonstrate that Ssu72 dephosphorylates Ser(P)(5) at the initiation-elongation transition. Furthermore, Ssu72 indirectly affects the levels of Ser(P)(2) during the elongation stage of transcription but does so independent of its catalytic activity.

Arabidopsis CPL4 is an essential C-terminal domain phosphatase that suppresses xenobiotic stress responses

Eukaryotic gene expression is both promoted and inhibited by the reversible phosphorylation of the C-terminal domain of RNA polymerase II (pol II CTD). More than 20 Arabidopsis genes encode CTD phosphatase homologs, including four CTD phosphatase-like (CPL) family members. Although in vitro CTD phosphatase activity has been established for some CPLs, none have been shown to be involved in the phosphoregulation of pol II in vivo. Here we report that CPL4 is a CTD phosphatase essential for the viability of Arabidopsis thaliana. Mass spectrometry analysis identified the pol II subunits RPB1, RPB2 and RPB3 in the affinity-purified CPL4 complex. CPL4 dephosphorylates both Ser2- and Ser5-PO(4) of the CTD in vitro, with a preference for Ser2-PO(4). Arabidopsis plants overexpressing CPL4 accumulated hypophosphorylated pol II, whereas RNA interference-mediated silencing of CPL4 promoted hyperphosphorylation of pol II. A D128A mutation in the conserved DXDXT motif of the CPL4 catalytic domain resulted in a dominant negative form of CPL4, the overexpression of which inhibited transgene expression in transient assays. Inhibition was abolished by truncation of the phosphoprotein-binding Breast Cancer 1 C-terminal domain of CPL4, suggesting that both catalytic function and protein-protein interaction are essential for CPL4-mediated regulation of gene expression. We were unable to recover a homozygous cpl4 mutant, probably due to the zygotic lethality of this mutation. The reduction in CPL4 levels in CPL4(RNAi) plants increased transcript levels of a suite of herbicide/xenobiotic-responsive genes and improved herbicide tolerance, thus suggesting an additional role for CPL4 as a negative regulator of the xenobiotic detoxification pathway.

RPRD1A and RPRD1B are human RNA polymerase II C-terminal domain scaffolds for Ser5 dephosphorylation

The RNA polymerase II (RNAPII) C-terminal domain (CTD) heptapeptide repeats (1-YSPTSPS-7) undergo dynamic phosphorylation and dephosphorylation during the transcription cycle to recruit factors that regulate transcription, RNA processing and chromatin modification. We show here that RPRD1A and RPRD1B form homodimers and heterodimers through their coiled-coil domains and interact preferentially via CTD-interaction domains (CIDs) with RNAPII CTD repeats phosphorylated at S2 and S7. Crystal structures of the RPRD1A, RPRD1B and RPRD2 CIDs, alone and in complex with RNAPII CTD phosphoisoforms, elucidate the molecular basis of CTD recognition. In an example of cross-talk between different CTD modifications, our data also indicate that RPRD1A and RPRD1B associate directly with RPAP2 phosphatase and, by interacting with CTD repeats where phospho-S2 and/or phospho-S7 bracket a phospho-S5 residue, serve as CTD scaffolds to coordinate the dephosphorylation of phospho-S5 by RPAP2.

novel modifications on C-terminal domain of RNA polymerase II can fine-tune the phosphatase activity of Ssu72

The C-terminal domain of RNA polymerase II (CTD) modulates the process of transcription through sequential phosphorylation/dephosphorylation of its heptide repeats, through which it recruits various transcription regulators. Ssu72 is the first characterized cis-specific CTD phosphatase that dephosphorylates Ser5 with a requirement for the adjacent Pro6 in a cis conformation. The recent discovery of Thr4 phosphorylation in the CTD calls into question whether such a modification can interfere with Ssu72 binding via the elimination of a conserved intramolecular hydrogen bond in the CTD that is potentially essential for recognition. To test if Thr4 phosphorylation will abolish Ser5 dephosphorylation by Ssu72, we determined the kinetic and structural properties of Drosophila Ssu72-symplekin in complex with the CTD peptide with consecutive phosphorylated Thr4 and Ser5. Our mass spectrometric and kinetic data established that Ssu72 does not dephosphorylate Thr4, but the existence of phosphoryl-Thr4 next to Ser5 reduces the activity of Ssu72 toward the CTD peptide by 4-fold. To our surprise, even though the intramolecular hydrogen bond is eliminated due to the phosphorylation of Thr4, the CTD adopts an almost identical conformation to be recognized by Ssu72 with Ser5 phosphorylated alone or both Thr4/Ser5 phosphorylated. Our results indicate that Thr4 phosphorylation will not abolish the essential Ssu72 activity, which is needed for cell survival. Instead, the phosphatase activity of Ssu72 is fine-tuned by Thr4 phosphorylation and eventually may lead to changes in transcription. Overall, we report the first case of structural and kinetic effects of phosphorylated Thr4 on CTD modifying enzymes. Our results support a model in which a combinatorial cascade of CTD modification can modulate transcription.

Viewing serine/threonine protein phosphatases through the eyes of drug designers

Protein phosphatases, as the counterpart to protein kinases, are essential for homeostatic balance of cell signaling. Small chemical compounds that modulate the specific activity of phosphatases can be powerful tools to elucidate the biological functions of these enzymes. More importantly, many phosphatases are central players in the development of pathological pathways where inactivation can reverse or delay the onset of human diseases. Therefore, potent inhibitors for such phosphatases can be of great therapeutic benefit. In contrast to the seemingly identical enzymatic mechanism and structural characterization of eukaryotic protein kinases, protein phosphatases evolved from diverse ancestors, resulting in different domain architectures, reaction mechanisms and active site properties. In this review, we discuss for each family of serine/threonine protein phosphatases their involvement in biological processes and corresponding strategies for small chemical intervention. Recent advances in modern drug discovery technologies have markedly facilitated the identification of selective inhibitors for some members of the phosphatase family. Furthermore, the rapid growth in knowledge about structure-activity relationships related to possible new drug targets has aided the discovery of natural product inhibitors for the phosphatase family. This review summarizes the current state of investigation of the small molecules that regulate the function of serine/threonine phosphatases, the challenges presented and also strategies to overcome these obstacles.

The arabidopsis RNA binding protein with K homology motifs, SHINY1, interacts with the C-terminal domain phosphatase-like 1 (CPL1) to repress stress-inducible gene expression

The phosphorylation state of the C-terminal domain (CTD) of the RNA polymerase II plays crucial roles in transcription and mRNA processing. Previous studies showed that the plant CTD phosphatase-like 1 (CPL1) dephosphorylates Ser-5-specific CTD and regulates abiotic stress response in Arabidopsis. Here, we report the identification of a K-homology domain-containing protein named SHINY1 (SHI1) that interacts with CPL1 to modulate gene expression. The shi1 mutant was isolated from a forward genetic screening for mutants showing elevated expression of the luciferase reporter gene driven by a salt-inducible promoter. The shi1 mutant is more sensitive to cold treatment during vegetative growth and insensitive to abscisic acid in seed germination, resembling the phenotypes of shi4 that is allelic to the cpl1 mutant. Both SHI1 and SHI4/CPL1 are nuclear-localized proteins. SHI1 interacts with SHI4/CPL1 in vitro and in vivo. Loss-of-function mutations in shi1 and shi4 resulted in similar changes in the expression of some stress-inducible genes. Moreover, both shi1 and shi4 mutants display higher mRNA capping efficiency and altered polyadenylation site selection for some of the stress-inducible genes, when compared with wild type. We propose that the SHI1-SHI4/CPL1 complex inhibits transcription by preventing mRNA capping and transition from transcription initiation to elongation.

Plants, including important economic crops, frequently grow under unfavorable conditions that largely reduce their production potential. Plants respond to these stress conditions by adjusting physiological status resulting from changes in gene expression. Many genes that are repressed at normal growth conditions are activated in response to stresses. The presented work here attempted to answer the question as to how stress inducible genes are repressed at normal growth conditions. We established a genetic system to identify genes that are essential for such repression and found a protein complex that plays crucial roles in regulating stress inducible gene expression. Our work also revealed important molecular processes that are modulated by the identified protein complex. These findings will help our understanding about gene expression and regulation in general and the molecular mechanisms governing plant stress response in particular.

An unexpected binding mode for a Pol II CTD peptide phosphorylated at Ser7 in the active site of the CTD phosphatase Ssu72

Ssu72, an RNA polymerase II C-terminal domain (CTD) phospho-Ser5 (pSer5) phosphatase, was recently reported to have pSer7 phosphatase activity as well. We report here the crystal structure of a ternary complex of the N-terminal domain of human symplekin, human Ssu72, and a 10-mer pSer7 CTD peptide. Surprisingly, the peptide is bound in the Ssu72 active site with its backbone running in the opposite direction compared with a pSer5 peptide. The pSer7 phosphatase activity of Ssu72 is ∼4000-fold lower than its pSer5 phosphatase activity toward a peptide substrate, consistent with the structural observations.

Punctuation and syntax of the RNA polymerase II CTD code in fission yeast

The primary structure and phosphorylation pattern of the tandem Y(1)S(2)P(3)T(4)S(5)P(6)S(7) repeats of the RNA polymerase II carboxyl-terminal domain (CTD) convey information about the transcription apparatus--a CTD code--to a large ensemble of CTD-binding receptor proteins. Four of the seven coding "letters" of the fission yeast CTD (Tyr1, Pro3, Ser5, Pro6) are essential in vivo, but the grammatical rules of the code are obscure. Here we show that the minimal fission yeast CTD coding unit is a decapeptide Y(1)S(2)P(3)T(4)S(5)P(6)S(7)Y(1)S(2)P(3) and the spacing between coding units is flexible; the coding unit must contain two Tyr1 residues and the spacing between consecutive tyrosines is important; Ser5-PO(4)-Pro6 comprises an essential two-letter code "word" that is read by the mRNA capping apparatus; and a threshold number of Ser5-PO(4)-Pro6 words are needed to comprise a readable "sentence" of CTD information. Bypassing the essentiality of the Ser5 and Pro6 letters by fusion of capping enzymes to the CTD helped reveal how CTD phosphorylation circuits are wired in vivo. We found that the Ser2-PO(4) mark is independent of Ser5, Pro6, Ser7, and Thr4, whereas the Ser5-PO(4) mark is independent of Ser2, Ser7, and Thr4. These results provide unique insights to the reading and writing of the CTD code.

The yeast regulator of transcription protein Rtr1 lacks an active site and phosphatase activity

The activity of RNA polymerase II (Pol II) is controlled in part by the phosphorylation state of the C-terminal domain (CTD) of its largest subunit. Recent reports have suggested that yeast regulator of transcription protein, Rtr1, and its human homologue RPAP2, possess Pol II CTD Ser5 phosphatase activity. Here we report the crystal structure of Kluyveromyces lactis Rtr1, which reveals a new type of zinc finger protein and does not have any close structural homologues. Importantly, the structure does not show evidence of an active site, and extensive experiments to demonstrate its CTD phosphatase activity have been unsuccessful, suggesting that Rtr1 has a non-catalytic role in CTD dephosphorylation.

Fcp1 dephosphorylation of the RNA polymerase II C-terminal domain is required for efficient transcription of heat shock genes

Fcp1 dephosphorylates the C-terminal domain of the largest subunit of RNA polymerase II (Pol II) to recycle it into a form that can initiate a new round of transcription. Previously, we identified Drosophila Fcp1 as an important factor in optimal Hsp70 mRNA accumulation after heat shock. Here, we examine the role of Fcp1 in transcription of heat shock genes in vivo. We demonstrate that Fcp1 localizes to active sites of transcription including the induced Hsp70 gene. The reduced Hsp70 mRNA accumulation seen by RNA interference (RNAi) depletion of Fcp1 in S2 cells is a result of a loss of Pol II in the coding region of highly transcribed heat shock-induced genes: Hsp70, Hsp26, and Hsp83. Moreover, Fcp1 depletion dramatically increases phosphorylation of the non-chromatin-bound Pol II. Reexpression of either wild-type or catalytically dead versions of Fcp1 demonstrates that both the reduced Pol II levels on heat shock genes and the increased levels of phosphorylated free Pol II are dependent on the catalytic activity of Fcp1. Our results indicate that Fcp1 is required to maintain the pool of initiation-competent unphosphorylated Pol II, and this function is particularly important for the highly transcribed heat shock genes.

Deciphering the RNA polymerase II CTD code in fission yeast

The RNA polymerase II carboxy-terminal domain (CTD) consists of tandem Y(1)S(2)P(3)T(4)S(5)P(6)S(7) repeats. Dynamic remodeling of the CTD, especially its serine phosphorylation pattern, conveys informational cues about the transcription apparatus to a large ensemble of CTD-binding proteins. Our genetic dissection of fission yeast CTD function provides insights to the "CTD code." Two concepts stand out. First, the Ser2 requirement for transcription during sexual differentiation is bypassed by subtracting Ser7, signifying that imbalance in the phosphorylation array, not absence of a phospho-CTD cue, underlies a CTD-associated pathology. Second, the essentiality of Ser5 for vegetative growth is circumvented by covalently tethering mRNA capping enzymes to the CTD, thus proving that capping enzyme recruitment is a chief function of the Ser5-PO(4) mark. This illustrates that a key "letter" in the CTD code can be neutralized by delivering its essential cognate receptor to the transcription complex via an alternative route.

Structural insights to how mammalian capping enzyme reads the CTD code

Physical interaction between the phosphorylated RNA polymerase II carboxyl-terminal domain (CTD) and cellular capping enzymes is required for efficient formation of the 5' mRNA cap, the first modification of nascent mRNA. Here, we report the crystal structure of the RNA guanylyltransferase component of mammalian capping enzyme (Mce) bound to a CTD phosphopeptide. The CTD adopts an extended β-like conformation that docks Tyr1 and Ser5-PO(4) onto the Mce nucleotidyltransferase domain. Structure-guided mutational analysis verified that the Mce-CTD interface is a tunable determinant of CTD binding and stimulation of guanylyltransferase activity, and of Mce function in vivo. The location and composition of the CTD binding site on mammalian capping enzyme is distinct from that of a yeast capping enzyme that recognizes the same CTD primary structure. Thus, capping enzymes from different taxa have evolved different strategies to read the CTD code.

Crystal structure of Ssu72, an essential eukaryotic phosphatase specific for the C-terminal domain of RNA polymerase II, in complex with a transition state analogue

Reversible phosphorylation of the CTD (C-terminal domain) of the eukaryotic RNA polymerase II largest subunit represents a critical regulatory mechanism during the transcription cycle and mRNA processing. Ssu72 is an essential phosphatase conserved in eukaryotes that dephosphorylates phosphorylated Ser5 of the CTD heptapeptide. Its function is implicated in transcription initiation, elongation and termination, as well as RNA processing. In the present paper we report the high resolution X-ray crystal structures of Drosophila melanogaster Ssu72 phosphatase in the apo form and in complex with an inhibitor mimicking the transition state of phosphoryl transfer. Ssu72 facilitates dephosphorylation of the substrate through a phosphoryl-enzyme intermediate, as visualized in the complex structure of Ssu72 with the oxo-anion compound inhibitor vanadate at a 2.35 Å (1 Å=0.1 nm) resolution. The structure resembles the transition state of the phosphoryl transfer with vanadate exhibiting a trigonal bi-pyramidal geometry covalently bonded to the nucleophilic cysteine residue. Interestingly, the incorporation of oxo-anion compounds greatly stabilizes a flexible loop containing the general acid, as detected by an increase of melting temperature of Ssu72 detected by differential scanning fluorimetry. The Ssu72 structure exhibits a core fold with a similar topology to that of LMWPTPs [low-molecular-mass PTPs (protein tyrosine phosphatases)], but with an insertion of a unique ‘cap’ domain to shelter the active site from the solvent with a deep groove in between where the CTD substrates bind. Mutagenesis studies in this groove established the functional roles of five residues (Met17, Pro46, Asp51, Tyr77 and Met85) that are essential specifically for substrate recognition.

cis-Proline-mediated Ser(P)5 dephosphorylation by the RNA polymerase II C-terminal domain phosphatase Ssu72

RNA polymerase II coordinates co-transcriptional events by recruiting distinct sets of nuclear factors to specific stages of transcription via changes of phosphorylation patterns along its C-terminal domain (CTD). Although it has become increasingly clear that proline isomerization also helps regulate CTD-associated processes, the molecular basis of its role is unknown. Here, we report the structure of the Ser(P)(5) CTD phosphatase Ssu72 in complex with substrate, revealing a remarkable CTD conformation with the Ser(P)(5)-Pro(6) motif in the cis configuration. We show that the cis-Ser(P)(5)-Pro(6) isomer is the minor population in solution and that Ess1-catalyzed cis-trans-proline isomerization facilitates rapid dephosphorylation by Ssu72, providing an explanation for recently discovered in vivo connections between these enzymes and a revised model for CTD-mediated small nuclear RNA termination. This work presents the first structural evidence of a cis-proline-specific enzyme and an unexpected mechanism of isomer-based regulation of phosphorylation, with broad implications for CTD biology.

Crystal structure of the human symplekin-Ssu72-CTD phosphopeptide complex

Symplekin (Pta1 in yeast) is a scaffold in the large protein complex that is required for 3′-end cleavage and polyadenylation of eukaryotic messenger RNA precursors (pre-mRNAs) 1–4, and also participates in transcription initiation and termination by RNA polymerase II (Pol II) 5,6. Symplekin mediates interactions among many different proteins in this machinery 1,2,7–9, although the molecular basis for its function is not known. Here we report the crystal structure at 2.4 Å resolution of the N-terminal domain (residues 30–340) of human symplekin (Symp-N) in a ternary complex with the Pol II C-terminal domain (CTD) Ser⁵ phosphatase Ssu72 7,10–17 and a CTD Ser⁵ phosphopeptide. The N-terminal domain of symplekin has the ARM or HEAT fold, with seven pairs of anti-parallel α-helices arranged in the shape of an arc. The structure of Ssu72 has some similarity to that of low-molecular-weight phosphotyrosine protein phosphatase 18,19, although Ssu72 has a unique active site landscape as well as extra structural features at the C-terminus that is important for interaction with symplekin. Ssu72 is bound to the concave face of symplekin, and engineered mutations in this interface can abolish interactions between the two proteins. The CTD peptide is bound in the active site of Ssu72, unexpectedly with the pSer⁵-Pro⁶ peptide bond in the cis configuration, which contrasts with all other known CTD peptide conformations 20,21. While the active site of Ssu72 is about 25 Å away from the interface with symplekin, we found that the symplekin N-terminal domain stimulates Ssu72 CTD phosphatase activity in vitro. Furthermore, the N-terminal domain of symplekin inhibits polyadenylation in vitro, but importantly only when coupled to transcription. As catalytically active Ssu72 overcomes this inhibition, our results demonstrate a role for mammalian Ssu72 in transcription-coupled pre-mRNA 3′-end processing.

How eukaryotic genes are transcribed

Regulation of eukaryotic gene expression is far more complex than one might have imagined 30 years ago. However, progress towards understanding gene regulatory mechanisms has been rapid and comprehensive, which has made the integration of detailed observations into broadly connected concepts a challenge. This review attempts to integrate the following concepts: (1) a well-defined organization of nucleosomes and modification states at most genes; (2) regulatory networks of sequence-specific transcription factors; (3) chromatin remodeling coupled to promoter assembly of the general transcription factors and RNA polymerase II; and (4) phosphorylation states of RNA polymerase II coupled to chromatin modification states during transcription. The wealth of new insights arising from the tools of biochemistry, genomics, cell biology, and genetics is providing a remarkable view into the mechanics of gene regulation.

Involvement of Pta1, Pcf11 and a KlCYC1 AU-rich element in alternative RNA 3'-end processing selection in yeast

This work reports the involvement of yeast RNA processing factors Pta1 and Pcf11 in alternative 3'-end RNA processing. The pta1-1 and pcf11-2 mutations changed the predominance of KlCYC1 1.14 and 1.5 kb transcript isoforms. Mutation of the KlCYC1 3'-UTR AU-rich sequence at positions 679-690 (mutant M1) altered transcript predominance. Moreover, expression of M1 in the yeast mutants partially suppressed their effects in the predominance pattern. The combination of the M1 and M2 (694-698 deletion) mutations abolished the alternative processing. Pta1 involvement in this selection was confirmed using the Pta1-td degron strain.

Functional interaction of the Ess1 prolyl isomerase with components of the RNA polymerase II initiation and termination machineries

The C-terminal domain (CTD) of the largest subunit of RNA polymerase II (Pol II) is a reiterated heptad sequence (Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7) that plays a key role in the transcription cycle, coordinating the exchange of transcription and RNA processing factors. The structure of the CTD is flexible and undergoes conformational changes in response to serine phosphorylation and proline isomerization. Here we report that the Ess1 peptidyl prolyl isomerase functionally interacts with the transcription initiation factor TFIIB and with the Ssu72 CTD phosphatase and Pta1 components of the CPF 3'-end processing complex. The ess1(A144T) and ess1(H164R) mutants, initially described by Hanes and coworkers (Yeast 5:55-72, 1989), accumulate the pSer5 phosphorylated form of Pol II; confer phosphate, galactose, and inositol auxotrophies; and fail to activate PHO5, GAL10, and INO1 reporter genes. These mutants are also defective for transcription termination, but in vitro experiments indicate that this defect is not caused by altering the processing efficiency of the cleavage/polyadenylation machinery. Consistent with a role in initiation and termination, Ess1 associates with the promoter and terminator regions of the PMA1 and PHO5 genes. We propose that Ess1 facilitates pSer5-Pro6 dephosphorylation by generating the CTD structural conformation recognized by the Ssu72 phosphatase and that pSer5 dephosphorylation affects both early and late stages of the transcription cycle.

Snapshots of the RNA processing factor SCAF8 bound to different phosphorylated forms of the carboxyl-terminal domain of RNA polymerase II

Concomitant with RNA polymerase II (Pol II) transcription, RNA maturation factors are recruited to the carboxyl-terminal domain (CTD) of Pol II, whose phosphorylation state changes during a transcription cycle. CTD phosphorylation triggers recruitment of functionally different factors involved in RNA processing and transcription termination; most of these factors harbor a conserved CTD interacting domain (CID). Orchestration of factor recruitment is believed to be conducted by CID recognition of distinct phosphorylated forms of the CTD. We show that the human RNA processing factor SCAF8 interacts weakly with the unphosphorylated CTD of Pol II. Upon phosphorylation, affinity for the CTD is increased; however, SCAF8 is promiscuous to the phosphorylation pattern on the CTD. Employing a combined structural and biophysical approach, we were able to distinguish motifs within CIDs that are involved in a generic CTD sequence recognition from items that confer phospho-specificity.

Cracking the RNA polymerase II CTD code

The carboxyl-terminal domain (CTD) of the largest subunit of RNA polymerase II comprises multiple tandem conserved heptapeptide repeats, unique to this eukaryotic RNA polymerase. This unusual structure provides a docking platform for factors involved in various co-transcriptional events. Recruitment of the appropriate factors at different stages of the transcription cycle is achieved through changing patterns of post-translational modification of the CTD repeats, which create a readable 'code'. A new phosphorylation mark both expands the CTD code and provides the first example of a CTD signal read in a gene type-specific manner. How and when is the code written and read? How does it contribute to transcription and coordinate RNA processing?

Protein factors in pre-mRNA 3'-end processing

Most eukaryotic mRNA precursors (premRNAs) must undergo extensive processing, including cleavage and polyadenylation at the 3'-end. Processing at the 3'-end is controlled by sequence elements in the pre-mRNA (cis elements) as well as protein factors. Despite the seeming biochemical simplicity of the processing reactions, more than 14 proteins have been identified for the mammalian complex, and more than 20 proteins have been identified for the yeast complex. The 3'-end processing machinery also has important roles in transcription and splicing. The mammalian machinery contains several sub-complexes, including cleavage and polyadenylation specificity factor, cleavage stimulation factor, cleavage factor I, and cleavage factor II. Additional protein factors include poly(A) polymerase, poly(A)-binding protein, symplekin, and the C-terminal domain of RNA polymerase II largest subunit. The yeast machinery includes cleavage factor IA, cleavage factor IB, and cleavage and polyadenylation factor.

Role for the Ssu72 C-terminal domain phosphatase in RNA polymerase II transcription elongation

The RNA polymerase II (RNAP II) transcription cycle is accompanied by changes in the phosphorylation status of the C-terminal domain (CTD), a reiterated heptapeptide sequence (Y(1)S(2)P(3)T(4)S(5)P(6)S(7)) present at the C terminus of the largest RNAP II subunit. One of the enzymes involved in this process is Ssu72, a CTD phosphatase with specificity for serine-5-P. Here we report that the ssu72-2-encoded Ssu72-R129A protein is catalytically impaired in vitro and that the ssu72-2 mutant accumulates the serine-5-P form of RNAP II in vivo. An in vitro transcription system derived from the ssu72-2 mutant exhibits impaired elongation efficiency. Mutations in RPB1 and RPB2, the genes encoding the two largest subunits of RNAP II, were identified as suppressors of ssu72-2. The rpb1-1001 suppressor encodes an R1281A replacement, whereas rpb2-1001 encodes an R983G replacement. This information led us to identify the previously defined rpb2-4 and rpb2-10 alleles, which encode catalytically slow forms of RNAP II, as additional suppressors of ssu72-2. Furthermore, deletion of SPT4, which encodes a subunit of the Spt4-Spt5 early elongation complex, also suppresses ssu72-2, whereas the spt5-242 allele is suppressed by rpb2-1001. These results define Ssu72 as a transcription elongation factor. We propose a model in which Ssu72 catalyzes serine-5-P dephosphorylation subsequent to addition of the 7-methylguanosine cap on pre-mRNA in a manner that facilitates the RNAP II transition into the elongation stage of the transcription cycle.

A role for the CPF 3'-end processing machinery in RNAP II-dependent gene looping

The prevailing view of the RNA polymerase II (RNAP II) transcription cycle is that RNAP II is recruited to the promoter, transcribes a linear DNA template, then terminates transcription and dissociates from the template. Subsequent rounds of transcription are thought to require de novo recruitment of RNAP II to the promoter. Several recent findings, including physical interaction of 3'-end processing factors with both promoter and terminator regions, challenge this concept. Here we report a physical association of promoter and terminator regions of the yeast BUD3 and SEN1 genes. These interactions are transcription-dependent, require the Ssu72 and Pta1 components of the CPF 3'-end processing complex, and require the phosphatase activity of Ssu72. We propose a model for RNAP II transcription in which promoter and terminator regions are juxtaposed, and that the resulting gene loops facilitate transcription reinitiation by the same molecule of RNAP II in a manner dependent upon Ssu72-mediated CTD dephosphorylation.