Crystal structure of murine CstF-77: dimeric association and implications for polyadenylation of mRNA precursors

Structural biology of poly(A) site definition

3' processing is an essential step in the maturation of all messenger RNAs (mRNAs) and is a tightly coupled two-step reaction: endonucleolytic cleavage at the poly(A) site is followed by the addition of a poly(A) tail, except for metazoan histone mRNAs, which are cleaved but not polyadenylated. The recognition of a poly(A) site is coordinated by the sequence elements in the mRNA 3' UTR and associated protein factors. In mammalian cells, three well-studied sequence elements, UGUA, AAUAAA, and GU-rich, are recognized by three multisubunit factors: cleavage factor I(m) (CFI(m) ), cleavage and polyadenylation specificity factor (CPSF), and cleavage stimulation factor (CstF), respectively. In the yeast Saccharomyces cerevisiae, UA repeats and A-rich sequence elements are recognized by Hrp1p and cleavage factor IA. Structural studies of protein-RNA complexes have helped decipher the mechanisms underlying sequence recognition and shed light on the role of protein factors in poly(A) site selection and 3' processing machinery assembly. In this review we focus on the interactions between the mRNA cis-elements and the protein factors (CFI(m) , CPSF, CstF, and homologous factors from yeast and other eukaryotes) that define the poly(A) site. WIREs RNA 2011 2 732-747 DOI: 10.1002/wrna.88 For further resources related to this article, please visit the WIREs website.

Hexameric architecture of CstF supported by CstF-50 homodimerization domain structure

The Cleavage stimulation Factor (CstF) complex is composed of three subunits and is essential for pre-mRNA 3'-end processing. CstF recognizes U and G/U-rich cis-acting RNA sequence elements and helps stabilize the Cleavage and Polyadenylation Specificity Factor (CPSF) at the polyadenylation site as required for productive RNA cleavage. Here, we describe the crystal structure of the N-terminal domain of Drosophila CstF-50 subunit. It forms a compact homodimer that exposes two geometrically opposite, identical, and conserved surfaces that may serve as binding platform. Together with previous data on the structure of CstF-77, homodimerization of CstF-50 N-terminal domain supports the model in which the functional state of CstF is a heterohexamer.

Pre-mRNA 3'-end processing complex assembly and function

The 3'-ends of almost all eukaryotic mRNAs are formed in a two-step process, an endonucleolytic cleavage followed by polyadenylation (the addition of a poly-adenosine or poly(A) tail). These reactions take place in the pre-mRNA 3' processing complex, a macromolecular machinery that consists of more than 20 proteins. A general framework for how the pre-mRNA 3' processing complex assembles and functions has emerged from extensive studies over the past several decades using biochemical, genetic, computational, and structural approaches. In this article, we review what we have learned about this important cellular machine and discuss the remaining questions and future challenges.

Mutation of Glu-166 blocks the substrate-induced dimerization of SARS coronavirus main protease

The maturation of SARS coronavirus involves the autocleavage of polyproteins 1a and 1ab by the main protease (Mpro) and a papain-like protease; these represent attractive targets for the development of anti-SARS drugs. The functional unit of Mpro is a homodimer, and each subunit has a His-41⋯Cys-145 catalytic dyad. Current thinking in this area is that Mpro dimerization is essential for catalysis, although the influence of the substrate binding on the dimer formation has never been explored. Here, we delineate the contributions of the peptide substrate to Mpro dimerization. Enzyme kinetic assays indicate that the monomeric mutant R298A/L exhibits lower activity but in a cooperative manner. Analytical ultracentrifugation analyses indicate that in the presence of substrates, the major species of R298A/L shows a significant size shift toward the dimeric form and the monomer-dimer dissociation constant of R298A/L decreases by 12- to 17-fold, approaching that for wild-type. Furthermore, this substrate-induced dimerization was found to be reversible after substrates were removed. Based on the crystal structures, a key residue, Glu-166, which is responsible for recognizing the Gln-P1 of the substrate and binding to Ser-1 of another protomer, will interact with Asn-142 and block the S1 subsite entrance in the monomer. Our studies indicate that mutation of Glu-166 in the R298A mutant indeed blocks the substrate-induced dimerization. This demonstrates that Glu-166 plays a pivotal role in connecting the substrate binding site with the dimer interface. We conclude that protein-ligand and protein-protein interactions are closely correlated in Mpro.

Cooperative interaction of transcription termination factors with the RNA polymerase II C-terminal domain

Phosphorylation of the C-terminal domain (CTD) of RNA polymerase II controls the co-transcriptional assembly of RNA processing and transcription factors. Recruitment relies on conserved CTD-interacting domains (CIDs) that recognize different CTD phosphoisoforms during the transcription cycle, but the molecular basis for their specificity remains unclear. We show that the CIDs of two transcription termination factors, Rtt103 and Pcf11, achieve high affinity and specificity both by specifically recognizing the phosphorylated CTD and by cooperatively binding to neighboring CTD repeats. Single-residue mutations at the protein-protein interface abolish cooperativity and affect recruitment at the 3' end processing site in vivo. We suggest that this cooperativity provides a signal-response mechanism to ensure that its action is confined only to proper polyadenylation sites where Ser2 phosphorylation density is highest.

The hunt for the 3' endonuclease

Pre-mRNAs are typically processed at the 3(') end by cleavage/polyadenylation. This is a two-step processing reaction initiated by endonucleolytic cleavage of pre-mRNAs downstream of the AAUAAA sequence or its variant, followed by extension of the newly generated 3(') end with a poly(A) tail. In metazoans, replication-dependent histone transcripts are cleaved by a different 3(') end processing mechanism that depends on the U7 small nuclear ribonucleoprotein and the polyadenylation step is omitted. Each of the two mechanisms occurs in a macromolecular assembly that primarily functions to juxtapose the scissile bond with the 3(') endonuclease. Remarkably, despite characterizing a number of processing factors, the identity of this most critical component remained elusive until recently. For cleavage coupled to polyadenylation, much needed help was offered by bioinformatics, which pointed to CPSF-73, a known processing factor required for both cleavage and polyadenylation, as the possible 3(') endonuclease. In silico structural analysis indicated that this protein is a member of the large metallo-β-lactamase family of hydrolytic enzymes and belongs to the β-CASP subfamily that includes several RNA and DNA-specific nucleases. Subsequent experimental studies supported the notion that CPSF-73 does function as the endonuclease in the formation of polyadenylated mRNAs, but some controversy still remains as a different cleavage and polyadenylation specificity factor (CPSF) subunit, CPSF-30, displays an endonuclease activity in vitro while recombinant CPSF-73 is inactive. Unexpectedly, CPSF-73 as the 3(') endonuclease in cleavage coupled to polyadenylation found a strong ally in U7-dependent processing of histone pre-mRNAs, which was shown to utilize the same protein as the cleaving enzyme. It thus seems likely that these two processing reactions evolved from a common mechanism, with CPSF-73 as the endonuclease.

The Arabidopsis ortholog of the 77 kDa subunit of the cleavage stimulatory factor (AtCstF-77) involved in mRNA polyadenylation is an RNA-binding protein

The 77 kDa subunit of the polyadenylation cleavage stimulation factor (CstF77) is important in messenger RNA 3' end processing. Previously, we demonstrated that AtCstF77 interacts with AtCPSF30, the Arabidopsis ortholog of the 30 kDa subunit of the Cleavage and Polyadenylation Specificity Factor. In further dissecting this interaction, it was found that the C-terminus of AtCstF77 interacts with AtCPSF30. Remarkably, we also found that the C-terminal domain of AtCstF77 possesses RNA-binding ability. These studies therefore reveal AtCstF77 to be an RNA-binding protein, adding yet another RNA-binding activity to the plant polyadenylation complex. This raises interesting questions as to the means by which RNAs are recognized during mRNA 3' end formation in plants.

Structure of the Rna15 RRM-RNA complex reveals the molecular basis of GU specificity in transcriptional 3'-end processing factors

Rna15 is a core subunit of cleavage factor IA (CFIA), an essential transcriptional 3′-end processing factor from Saccharomyces cerevisiae. CFIA is required for polyA site selection/cleavage targeting RNA sequences that surround polyadenylation sites in the 3′-UTR of RNA polymerase-II transcripts. RNA recognition by CFIA is mediated by an RNA recognition motif (RRM) contained in the Rna15 subunit of the complex. We show here that Rna15 has a strong and unexpected preference for GU containing RNAs and reveal the molecular basis for a base selectivity mechanism that accommodates G or U but discriminates against C and A bases. This mode of base selectivity is rather different to that observed in other RRM-RNA structures and is structurally conserved in CstF64, the mammalian counterpart of Rna15. Our observations provide evidence for a highly conserved mechanism of base recognition amongst the 3′-end processing complexes that interact with the U-rich or U/G-rich elements at 3′-end cleavage/polyadenylation sites.

Targeted 3' processing of antisense transcripts triggers Arabidopsis FLC chromatin silencing

Noncoding RNA is emerging as an important regulator of gene expression in many organisms. We are characterizing RNA-mediated chromatin silencing of the Arabidopsis major floral repressor gene, FLC. Through suppressor mutagenesis, we identify a requirement for CstF64 and CstF77, two conserved RNA 3'-end-processing factors, in FLC silencing. However, FLC sense transcript 3' processing is not affected in the mutants. Instead, CstF64 and CstF77 are required for 3' processing of FLC antisense transcripts. A specific RNA-binding protein directs their activity to a proximal antisense polyadenylation site. This targeted processing triggers localized histone demethylase activity and results in reduced FLC sense transcription. Targeted 3' processing of antisense transcripts may be a common mechanism triggering transcriptional silencing of the corresponding sense gene.

The hinge domain of the cleavage stimulation factor protein CstF-64 is essential for CstF-77 interaction, nuclear localization, and polyadenylation

Because polyadenylation is essential for cell growth, in vivo examination of polyadenylation protein function has been difficult. Here we describe a new in vivo assay that allows structure-function assays on CstF-64, a protein that binds to pre-mRNAs downstream of the cleavage site for accurate and efficient polyadenylation. In this assay (the stem-loop luciferase assay for polyadenylation, SLAP), expression of a luciferase pre-mRNA with a modified downstream sequence element was made dependent upon co-expression of an MS2-CstF-64 fusion protein. We show here that SLAP accurately reflects CstF-64-dependent polyadenylation, confirming the validity of this assay. Using SLAP, we determined that CstF-64 domains involved in RNA binding, interaction with CstF-77 (the "Hinge" domain), and coupling to transcription are critical for polyadenylation. Further, we showed that the Hinge domain is necessary for CstF-64 interaction with CstF-77 and consequent nuclear localization, suggesting that nuclear import of a preformed CstF complex is an essential step in polyadenylation.

Crystal structure of the HEAT domain from the Pre-mRNA processing factor Symplekin

The majority of eukaryotic pre-mRNAs are processed by 3'-end cleavage and polyadenylation, although in metazoa the replication-dependent histone mRNAs are processed by 3'-end cleavage but not polyadenylation. The macromolecular complex responsible for processing both canonical and histone pre-mRNAs contains the approximately 1160-residue protein Symplekin. Secondary-structural prediction algorithms identified putative HEAT domains in the 300 N-terminal residues of all Symplekins of known sequence. The structure and dynamics of this domain were investigated to begin elucidating the role Symplekin plays in mRNA maturation. The crystal structure of the Drosophila melanogaster Symplekin HEAT domain was determined to 2.4 A resolution with single-wavelength anomalous dispersion phasing methods. The structure exhibits five canonical HEAT repeats along with an extended 31-amino-acid loop (loop 8) between the fourth and fifth repeat that is conserved within closely related Symplekin sequences. Molecular dynamics simulations of this domain show that the presence of loop 8 dampens correlated and anticorrelated motion in the HEAT domain, therefore providing a neutral surface for potential protein-protein interactions. HEAT domains are often employed for such macromolecular contacts. The Symplekin HEAT region not only structurally aligns with several established scaffolding proteins, but also has been reported to contact proteins essential for regulating 3'-end processing. Together, these data support the conclusion that the Symplekin HEAT domain serves as a scaffold for protein-protein interactions essential to the mRNA maturation process.

A structural model for the HAT domain of Utp6 incorporating bioinformatics and genetics

The half-a-tetratricopeptide (HAT) repeat motif is of interest because it is found exclusively in proteins that are involved in RNA metabolism. Little is known about structure-function relationships in this class of repeat motif. Here, we present the results of a combined bioinformatics, modeling and mutagenesis study of the HAT domain of Utp6. We have derived a new HAT consensus, delineated its structure-defining residues and, by homology modeling, have placed these residues in a structural context. By considering only HAT motifs from Utp6 proteins, we identified residues that are shared by, and unique to, only this subset of HAT motifs, suggesting a key functional role. Employing both random and directed mutagenesis of the HAT domain in yeast Utp6, we have identified residues whose mutation results in loss of function. By examining these residues in the context of the homology model, we have delineated those that act by perturbing structure and those that more likely have a direct effect on function. Importantly, the residues we predict to have a direct effect on function map together on the tertiary structure, thus defining a potential functional interaction surface.

A family of splice variants of CstF-64 expressed in vertebrate nervous systems

Background

Alternative splicing and polyadenylation are important mechanisms for creating the proteomic diversity necessary for the nervous system to fulfill its specialized functions. The contribution of alternative splicing to proteomic diversity in the nervous system has been well documented, whereas the role of alternative polyadenylation in this process is less well understood. Since the CstF-64 polyadenylation protein is known to be an important regulator of tissue-specific polyadenylation, we examined its expression in brain and other organs.

Results

We discovered several closely related splice variants of CstF-64 – collectively called βCstF-64 – that could potentially contribute to proteomic diversity in the nervous system. The βCstF-64 splice variants are found predominantly in the brains of several vertebrate species including mice and humans. The major βCstF-64 variant mRNA is generated by inclusion of two alternate exons (that we call exons 8.1 and 8.2) found between exons 8 and 9 of the CstF-64 gene, and contains an additional 147 nucleotides, encoding 49 additional amino acids. Some variants of βCstF-64 contain only the first alternate exon (exon 8.1) while other variants contain both alternate exons (8.1 and 8.2). In mice, the predominant form of βCstF-64 also contains a deletion of 78 nucleotides from exon 9, although that variant is not seen in any other species examined, including rats. Immunoblot and 2D-PAGE analyses of mouse nuclear extracts indicate that a protein corresponding to βCstF-64 is expressed in brain at approximately equal levels to CstF-64. Since βCstF-64 splice variant family members were found in the brains of all vertebrate species examined (including turtles and fish), this suggests that βCstF-64 has an evolutionarily conserved function in these animals. βCstF-64 was present in both pre- and post-natal mice and in different regions of the nervous system, suggesting an important role for βCstF-64 in neural gene expression throughout development. Finally, experiments in representative cell lines suggest that βCstF-64 is expressed in neurons but not glia.

Conclusion

This is the first report of a family of splice variants encoding a key polyadenylation protein that is expressed in a nervous system-specific manner. We propose that βCstF-64 contributes to proteomic diversity by regulating alternative polyadenylation of neural mRNAs.

Detection of alpha-rod protein repeats using a neural network and application to huntingtin

A growing number of solved protein structures display an elongated structural domain, denoted here as alpha-rod, composed of stacked pairs of anti-parallel alpha-helices. Alpha-rods are flexible and expose a large surface, which makes them suitable for protein interaction. Although most likely originating by tandem duplication of a two-helix unit, their detection using sequence similarity between repeats is poor. Here, we show that alpha-rod repeats can be detected using a neural network. The network detects more repeats than are identified by domain databases using multiple profiles, with a low level of false positives (<10%). We identify alpha-rod repeats in approximately 0.4% of proteins in eukaryotic genomes. We then investigate the results for all human proteins, identifying alpha-rod repeats for the first time in six protein families, including proteins STAG1-3, SERAC1, and PSMD1-2 & 5. We also characterize a short version of these repeats in eight protein families of Archaeal, Bacterial, and Fungal species. Finally, we demonstrate the utility of these predictions in directing experimental work to demarcate three alpha-rods in huntingtin, a protein mutated in Huntington's disease. Using yeast two hybrid analysis and an immunoprecipitation technique, we show that the huntingtin fragments containing alpha-rods associate with each other. This is the first definition of domains in huntingtin and the first validation of predicted interactions between fragments of huntingtin, which sets up directions toward functional characterization of this protein. An implementation of the repeat detection algorithm is available as a Web server with a simple graphical output: http://www.ogic.ca/projects/ard. This can be further visualized using BiasViz, a graphic tool for representation of multiple sequence alignments.

Molecular architecture of the human pre-mRNA 3' processing complex

Pre-mRNA 3' end formation is an essential step in eukaryotic gene expression. Over half of human genes produce alternatively polyadenylated mRNAs, suggesting that regulated polyadenylation is an important mechanism for posttranscriptional gene control. Although a number of mammalian mRNA 3' processing factors have been identified, the full protein composition of the 3' processing machinery has not been determined, and its structure is unknown. Here we report the purification and subsequent proteomic and structural characterization of human mRNA 3' processing complexes. Remarkably, the purified 3' processing complex contains approximately 85 proteins, including known and new core 3' processing factors and over 50 proteins that may mediate crosstalk with other processes. Electron microscopic analyses show that the core 3' processing complex has a distinct "kidney" shape and is approximately 250 A in length. Together, our data has revealed the complexity and molecular architecture of the pre-mRNA 3' processing complex.

A direct interaction between the Utp6 half-a-tetratricopeptide repeat domain and a specific peptide in Utp21 is essential for efficient pre-rRNA processing

The small subunit (SSU) processome is a ribosome biogenesis intermediate that assembles from its subcomplexes onto the pre-18S rRNA with yet unknown order and structure. Here, we investigate the architecture of the UtpB subcomplex of the SSU processome, focusing on the interaction between the half-a-tetratricopeptide repeat (HAT) domain of Utp6 and a specific peptide in Utp21. We present a comprehensive map of the interactions within the UtpB subcomplex and further show that the N-terminal domain of Utp6 interacts with Utp18 while the HAT domain interacts with Utp21. Using a panel of point and deletion mutants of Utp6, we show that an intact HAT domain is essential for efficient pre-rRNA processing and cell growth. Further investigation of the Utp6-Utp21 interaction using both genetic and biophysical methods shows that the HAT domain binds a specific peptide ligand in Utp21, the first example of a HAT domain peptide ligand, with a dissociation constant of 10 muM.

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.

Finishing touches: post-translational modification of protein factors involved in mammalian pre-mRNA 3' end formation

In eukaryotes, a pre-messenger RNA (pre-mRNA) must undergo several processing reactions before it is exported to the cytoplasm for translation. One of these reactions, endonucleolytic 3' cleavage at the polyadenylation site, prepares the pre-mRNA for addition of the poly(A) tail and defines the 3' untranslated region (UTR), which typically contains important gene expression regulatory sequences. While the protein factors responsible for the endonucleolytic cleavage have been largely identified, the means by which their action is limited to the 3' end of the transcription unit and coordinated with other co-transcriptional events remains unclear. In this review, we summarize and review recent findings revealing that the mammalian 3' cleavage factors undergo extensive post-translational modification. These modifications include: arginine methylation, lysine sumoylation, lysine acetylation, and the phosphorylation of serine, threonine and tyrosine residues. Every cleavage factor, though not every subunit, is affected. Human Fip1 and the 59 kDa subunit of cleavage factor I emerge as the most frequently modified core cleavage factor subunits. We outline and compare the various proteomic methods that have uncovered these modifications, and review emerging hypotheses concerning their function. The roles of these covalent but reversible modifications in other systems suggest that 3' end formation in mammals relies upon post-translational modification for proper function and regulation.

Structure of mouse ADP-ribosylhydrolase 3 (mARH3)

ADP-ribosylation is a reversible and covalent post-translational modification in which the attachment of ADP-ribose is catalyzed by ADP-ribosyltransferases and the removal of ADP-ribose is catalyzed by ADP-ribosylhydrolases. ADP-ribosylhydrolase 3 from mouse, consisting of 347 amino-acid residues, has been cloned, purified and crystallized. The three-dimensional structure has been resolved at a resolution of 1.8 A. The structure constitutes a compact all-alpha-helical protein with two Mg(2+) ions located in the active-site crevice. A structural comparison of mouse ADP-ribosylhydrolase 3 with its human orthologue shows a high degree of structural similarity. Furthermore, four prokaryotic proteins deposited in the PDB could be identified as being structurally related.

Correlation between dissociation and catalysis of SARS-CoV main protease

The dimeric interface of severe acute respiratory syndrome coronavirus main protease is a potential target for the anti-SARS drug development. We have generated C-terminal truncated mutants by serial truncations. The quaternary structure of the enzyme was analyzed using both sedimentation velocity and sedimentation equilibrium analytical ultracentrifugation. Global analysis of the combined results showed that truncation of C-terminus from 306 to 300 had no appreciable effect on the quaternary structure, and the enzyme remained catalytically active. However, further deletion of Gln-299 or Arg-298 drastically decreased the enzyme activity to 1–2% of wild type (WT), and the major form was a monomeric one. Detailed analysis of the point mutants of these two amino acid residues and their nearby hydrogen bond partner Ser-123 and Ser-139 revealed a strong correlation between the enzyme activity loss and dimer dissociation.

The structure of the CstF-77 homodimer provides insights into CstF assembly

The cleavage stimulation factor (CstF) is essential for the first step of poly(A) tail formation at the 3' ends of mRNAs. This heterotrimeric complex is built around the 77-kDa protein bridging both CstF-64 and CstF-50 subunits. We have solved the crystal structure of the 77-kDa protein from Encephalitozoon cuniculi at a resolution of 2 Å. The structure folds around 11 Half-a-TPR repeats defining two domains. The crystal structure reveals a tight homodimer exposing phylogenetically conserved areas for interaction with protein partners. Mapping experiments identify the C-terminal region of Rna14p, the yeast counterpart of CstF-77, as the docking domain for Rna15p, the yeast CstF-64 homologue.