Yeast ortholog of the Drosophila crooked neck protein promotes spliceosome assembly through stable U4/U6.U5 snRNP addition

The Prp19C/NTC subunit Syf2 and the Prp19C/NTC-associated protein Cwc15 function in TREX occupancy and transcription elongation

The Prp19 complex (Prp19C), also named NineTeen Complex (NTC), is conserved from yeast to human and functions in many different processes such as genome stability, splicing, and transcription elongation. In the latter, Prp19C ensures TREX occupancy at transcribed genes. TREX, in turn, couples transcription to nuclear mRNA export by recruiting the mRNA exporter to transcribed genes and consequently to nascent mRNAs. Here, we assess the function of the nonessential Prp19C subunit Syf2 and the nonessential Prp19C-associated protein Cwc15 in the interaction of Prp19C and TREX with the transcription machinery, Prp19C and TREX occupancy, and transcription elongation. Whereas both proteins are important for Prp19C-TREX interaction, Syf2 is needed for full Prp19C occupancy, and Cwc15 is important for the interaction of Prp19C with RNA polymerase II and TREX occupancy. These partially overlapping functions are corroborated by a genetic interaction between Δcwc15 and Δsyf2 Finally, Cwc15 also interacts genetically with the transcription elongation factor Dst1 and functions in transcription elongation. In summary, we uncover novel roles of the Prp19C component Syf2 and the Prp19C-associated protein Cwc15 in Prp19C's function in transcription elongation.

Binding by calmodulin is coupled to transient unfolding of the third FF domain of Prp40A

Human pre-mRNA processing protein 40 homolog A (hPrp40A) is a splicing factor that interacts with the Huntington's disease protein huntingtin (Htt). Evidence has accumulated that both Htt and hPrp40A are modulated by the intracellular Ca2+ sensor calmodulin (CaM). Here we report characterization of the interaction of human CM with the third FF domain (FF3 ) of hPrp40A using calorimetric, fluorescence and structural approaches. Homology modeling, differential scanning calorimetry and small angle X-ray scattering (SAXS) data show FF3 forms a folded globular domain. CaM was found to bind FF3 in a Ca2+ -dependent manner with a 1:1 stoichiometry and a dissociation constant (Kd ) of 25 ± 3 μM at 25°C. NMR studies showed that both domains of CaM are engaged in binding and SAXS analysis of the FF3 -CaM complex revealed CaM occupies an extended configuration. Analysis of the FF3 sequence showed that the anchors for CaM binding must be buried in its hydrophobic core, suggesting that binding to CaM requires unfolding of FF3 . Trp anchors were proposed based on sequence analysis and confirmed by intrinsic Trp fluorescence of FF3 upon binding of CaM and substantial reductions in affinity for Trp-Ala FF3 mutants. The consensus model of the complex showed that binding to CaM binding occurs to an extended, non-globular state of the FF3 , consistent with coupling to transient unfolding of the domain. The implications of these results are discussed in the context of the complex interplay of Ca2+ signaling and Ca2+ sensor proteins in modulating Prp40A-Htt function.

A termination-independent role of Rat1 in cotranscriptional splicing

Rat1 is a 5′→3′ exoribonuclease in budding yeast. It is a highly conserved protein with homologs being present in fission yeast, flies, worms, mice and humans. Rat1 and its human homolog Xrn2 have been implicated in multiple nuclear processes. Here we report a novel role of Rat1 in mRNA splicing. We observed an increase in the level of unspliced transcripts in mutants of Rat1. Accumulation of unspliced transcripts was not due to the surveillance role of Rat1 in degrading unspliced mRNA, or an indirect effect of Rat1 function in termination of transcription or on the level of splicing factors in the cell, or due to an increased elongation rate in Rat1 mutants. ChIP-Seq analysis revealed Rat1 crosslinking to the introns of a subset of yeast genes. Mass spectrometry and coimmunoprecipitation revealed an interaction of Rat1 with the Clf1, Isy1, Yju2, Prp43 and Sub2 splicing factors. Furthermore, recruitment of splicing factors on the intron was compromised in the Rat1 mutant. Based on these findings we propose that Rat1 has a novel role in splicing of mRNA in budding yeast. Rat1, however, is not a general splicing factor as it crosslinks to only long introns with an average length of 400 nucleotides.

The splicing-factor Prp40 affects dynein-dynactin function in Aspergillus nidulans

The multi-component cytoplasmic dynein transports cellular cargoes with the help of another multi-component complex dynactin, but we do not know enough about factors that may affect the assembly and functions of these proteins. From a genetic screen for mutations affecting early-endosome distribution in Aspergillus nidulans, we identified the prp40A^L438* mutation in Prp40A, a homologue of Prp40, an essential RNA-splicing factor in the budding yeast. Prp40A is not essential for splicing, although it associates with the nuclear splicing machinery. The prp40A^L438* mutant is much healthier than the ∆prp40A mutant, but both mutants exhibit similar defects in dynein-mediated early-endosome transport and nuclear distribution. In the prp40A^L438* mutant, the frequency but not the speed of dynein-mediated early-endosome transport is decreased, which correlates with a decrease in the microtubule plus-end accumulations of dynein and dynactin. Within the dynactin complex, the actin-related protein Arp1 forms a mini-filament. In a pull-down assay, the amount of Arp1 pulled down with its pointed-end protein Arp11 is lowered in the prp40A^L438* mutant. In addition, we found from published interactome data that a mammalian Prp40 homologue PRPF40A interacts with Arp1. Thus, Prp40 homologues may regulate the assembly or function of dynein–dynactin and their mechanisms deserve to be further studied.

Mud2 functions in transcription by recruiting the Prp19 and TREX complexes to transcribed genes

The different steps of gene expression are intimately linked to coordinate and regulate this complex process. During transcription, numerous RNA-binding proteins are already loaded onto the nascent mRNA and package the mRNA into a messenger ribonucleoprotein particle (mRNP). These RNA-binding proteins are often also involved in other steps of gene expression than mRNA packaging. For example, TREX functions in transcription, mRNP packaging and nuclear mRNA export. Previously, we showed that the Prp19 splicing complex (Prp19C) is needed for efficient transcription as well as TREX occupancy at transcribed genes. Here, we show that the splicing factor Mud2 interacts with Prp19C and is needed for Prp19C occupancy at transcribed genes in Saccharomyces cerevisiae. Interestingly, Mud2 is not only recruited to intron-containing but also to intronless genes indicating a role in transcription. Indeed, we show for the first time that Mud2 functions in transcription. Furthermore, these functions of Mud2 are likely evolutionarily conserved as Mud2 is also recruited to an intronless gene and interacts with Prp19C in Drosophila melanogaster. Taken together, we classify Mud2 as a novel transcription factor that is necessary for the recruitment of mRNA-binding proteins to the transcription machinery. Thus, Mud2 is a multifunctional protein important for transcription, splicing and most likely also mRNP packaging.

Nineteen complex-related factor Prp45 is required for the early stages of cotranscriptional spliceosome assembly

Splicing in S. cerevisiae has been shown to proceed cotranscriptionally, but the nature of the coupling remains a subject of debate. Here, we examine the effect of nineteen complex-related splicing factor Prp45 (a homolog of SNW1/SKIP) on cotranscriptional splicing. RNA-sequencing and RT-qPCR showed elevated pre-mRNA levels but only limited reduction of spliced mRNAs in cells expressing C-terminally truncated Prp45, Prp45(1-169). Assays with a series of reporters containing the AMA1 intron with regulatable splicing confirmed decreased splicing efficiency and showed the leakage of unspliced RNAs in prp45(1-169) cells. We also measured pre-mRNA accumulation of the meiotic MER2 gene, which depends on the expression of Mer1 factor for splicing. prp45(1-169) cells accumulated approximately threefold higher levels of MER2 pre-mRNA than WT cells only when splicing was induced. To monitor cotranscriptional splicing, we determined the presence of early spliceosome assembly factors and snRNP complexes along the ECM33 and ACT1 genes. We found that prp45(1-169) hampered the cotranscriptional recruitment of U2 and, to a larger extent, U5 and NTC, while the U1 profile was unaffected. The recruitment of Prp45(1-169) was impaired similarly to U5 snRNP and NTC. Our results imply that Prp45 is required for timely formation of complex A, prior to stable physical association of U5/NTC with the emerging pre-mRNA substrate. We suggest that Prp45 facilitates conformational rearrangements and/or contacts that couple U1 snRNP-recognition to downstream assembly events.

Prp40 and early events in splice site definition

The alternative splicing (AS) of precursor messenger RNA (pre-mRNA) is a tightly regulated process through which introns are removed to leave the resulting exons in the mRNA appropriately aligned and ligated. The AS of pre-mRNA is a key mechanism for increasing the complexity of proteins encoded in the genome. In humans, more than 90% of genes undergo AS, underscoring the importance of this process in RNA biogenesis. As such, AS misregulation underlies multiple human diseases. The splicing reaction is catalyzed by the spliceosome, a highly dynamic complex that assembles at or near the intron/exon boundaries and undergoes sequential conformational and compositional changes during splicing. The initial recognition of splice sites defines the exons that are going to be removed, which is a critical step in the highly regulated splicing process. Although the available lines of evidence are increasing, the molecular mechanisms governing AS, including the initial interactions occurring at intron/exon boundaries, and the factors that modulate these critical connections by functioning as a scaffold for active-site RNAs or proteins, remain poorly understood. In this review, we summarize the major hallmarks of the initial steps in the splicing process and the role of auxiliary factors that contribute to the assembly of the spliceosomal complex. We also discuss the role of the essential yeast Prp40 protein and its mammalian homologs in the specificity of this pre-mRNA processing event. In addition, we provide the first exhaustive phylogenetic analysis of the molecular evolution of Prp40 family members. WIREs RNA 2016, 7:17-32. doi: 10.1002/wrna.1312 For further resources related to this article, please visit the WIREs website.

The multifaceted roles of metabolic enzymes in the Paracoccidioides species complex

Paracoccidioides species are dimorphic fungi and are the etiologic agents of paracoccidioidomycosis, which is a serious disease that involves multiple organs. The many tissues colonized by this fungus suggest a variety of surface molecules involved in adhesion. A surprising finding is that most enzymes in the glycolytic pathway, tricarboxylic acid (TCA) cycle and glyoxylate cycle in Paracoccidioides spp. have adhesive properties that aid in interacting with the host extracellular matrix and thus act as ‘moonlighting’ proteins. Moonlighting proteins have multiple functions, which adds a dimension to cellular complexity and benefit cells in several ways. This phenomenon occurs in both eukaryotes and prokaryotes. For example, moonlighting proteins from the glycolytic pathway or TCA cycle can play a role in bacterial pathogenesis by either acting as proteins secreted in a conventional pathway and/or as cell surface components that facilitate adhesion or adherence. This review outlines the multifunctionality exhibited by many Paracoccidioides spp. enzymes, including aconitase, aldolase, glyceraldehyde-3-phosphate dehydrogenase, isocitrate lyase, malate synthase, triose phosphate isomerase, fumarase, and enolase. We discuss the roles that moonlighting activities play in the virulence characteristics of this fungus and several other human pathogens during their interactions with the host.

Structural requirement of Ntc77 for spliceosome activation and first catalytic step

The Prp19-associated complex is required for spliceosome activation by stabilizing the binding of U5 and U6 on the spliceosome after the release of U4. The complex comprises at least eight proteins, among which Ntc90 and Ntc77 contain multiple tetratricopeptide repeat (TPR) elements. We have previously shown that Ntc90 is not involved in spliceosome activation, but is required for the recruitment of essential first-step factor Yju2 to the spliceosome. We demonstrate here that Ntc77 has dual functions in both spliceosome activation and the first catalytic step in recruiting Yju2. We have identified an amino-terminal region of Ntc77, which encompasses the N-terminal domain and the first three TPR motifs, dispensable for spliceosome activation but required for stable interaction of Yju2 with the spliceosome. Deletion of this region had no severe effect on the integrity of the NTC, binding of NTC to the spliceosome or spliceosome activation, but impaired splicing and exhibited a dominant-negative growth phenotype. Our data reveal functional roles of Ntc77 in both spliceosome activation and the first catalytic step, and distinct structural domains of Ntc77 required for these two steps.

Alternative splicing in Drosophila neuronal development

Post-transcriptional pre-mRNA splicing has emerged as a critical step in the gene expression cascade greatly influencing diversification and spatiotemporal control of the proteome in many developmental processes. The percentage of genes targeted by alternative splicing (AS) is shown to be over 95% in humans and 60% in Drosophila. Therefore, it is evident that deregulation of this process underlies many genetic diseases. Among all tissues, the brain shows the highest transcriptome diversity, which is not surprising in view of the complex inter- and intracellular networks underlying the development of this organ. Reports of isoforms known to function at different steps during Drosophila nervous system development are rapidly increasing as well as knowledge on their regulation and function, highlighting the role of AS during neuronal development in Drosophila.

Splicing and beyond: the many faces of the Prp19 complex

The conserved Prp19 complex (Prp19C) - also known as NineTeen Complex (NTC) - functions in several processes of paramount importance for cellular homeostasis. NTC/Prp19C was discovered as a complex that functions in splicing and more specifically during the catalytic activation of the spliceosome. More recent work revealed that NTC/Prp19C plays a role in transcription elongation in Saccharomyces cerevisiae and in genome maintenance in higher eukaryotes. In addition, mouse PRP19 might ubiquity late proteins targeted for degradation and guide them to the proteasome. Furthermore, NTC/Prp19C has been implicated in lipid droplet biogenesis. In the future, the molecular function of NTC/Prp19C in all of these processes needs to be refined or elucidated. Most of NTC/Prp19C's functions have been shown in only one or few organisms. However, since this complex is highly conserved it is likely that it has the same functions across all species. Moreover, one NTC/Prp19C or different subcomplexes could function in the above-mentioned processes. Intriguingly, NTC/Prp19C might link these different processes to ensure an optimal coordination of cellular processes. Thus, many important questions about the functions of this interesting complex remain to be investigated. In this review we discuss the different functions of NTC/Prp19C focusing on the novel and emerging ones as well as open questions.

Functional roles of protein splicing factors

RNA splicing is one of the fundamental processes in gene expression in eukaryotes. Splicing of pre-mRNA is catalysed by a large ribonucleoprotein complex called the spliceosome, which consists of five small nuclear RNAs and numerous protein factors. The spliceosome is a highly dynamic structure, assembled by sequential binding and release of the small nuclear RNAs and protein factors. DExD/H-box RNA helicases are required to mediate structural changes in the spliceosome at various steps in the assembly pathway and have also been implicated in the fidelity control of the splicing reaction. Other proteins also play key roles in mediating the progression of the spliceosome pathway. In this review, we discuss the functional roles of the protein factors involved in the spliceosome pathway primarily from studies in the yeast system.

The regulation of glial-specific splicing of Neurexin IV requires HOW and Cdk12 activity

The differentiation of the blood-brain barrier (BBB) is an essential process in the development of a complex nervous system and depends on alternative splicing. In the fly BBB, glial cells establish intensive septate junctions that require the cell-adhesion molecule Neurexin IV. Alternative splicing generates two different Neurexin IV isoforms: Neurexin IV(exon3), which is found in cells that form septate junctions, and Neurexin IV(exon4), which is found in neurons that form no septate junctions. Here, we show that the formation of the BBB depends on the RNA-binding protein HOW (Held out wings), which triggers glial specific splicing of Neurexin IV(exon3). Using a set of splice reporters, we show that one HOW-binding site is needed to include one of the two mutually exclusive exons 3 and 4, whereas binding at the three further motifs is needed to exclude exon 4. The differential splicing is controlled by nuclear access of HOW and can be induced in neurons following expression of nuclear HOW. Using a novel in vivo two-color splicing detector, we then screened for genes required for full HOW activity. This approach identified Cyclin-dependent kinase 12 (Cdk12) and the splicesosomal component Prp40 as major determinants in regulating HOW-dependent splicing of Neurexin IV. Thus, in addition to the control of nuclear localization of HOW, the phosphorylation of the C-terminal domain of the RNA polymerase II by Cdk12 provides an elegant mechanism in regulating timed splicing of newly synthesized mRNA molecules.

RNA binding and RNA remodeling activities of the half-a-tetratricopeptide (HAT) protein HCF107 underlie its effects on gene expression

The half-a-tetratricopeptide repeat (HAT) motif is a helical repeat motif found in proteins that influence various aspects of RNA metabolism, including rRNA biogenesis, RNA splicing, and polyadenylation. This functional association with RNA suggested that HAT repeat tracts might bind RNA. However, RNA binding activity has not been reported for any HAT repeat tract, and recent literature has emphasized a protein binding role. In this study, we show that a chloroplast-localized HAT protein, HCF107, is a sequence-specific RNA binding protein. HCF107 consists of 11 tandem HAT repeats and short flanking regions that are also predicted to form helical hairpins. The minimal HCF107 binding site spans ∼11 nt, consistent with the possibility that HAT repeats bind RNA through a modular one repeat-1 nt mechanism. Binding of HCF107 to its native RNA ligand in the psbH 5' UTR remodels local RNA structure and protects the adjacent RNA from exonucleases in vitro. These activities can account for the RNA stabilizing, RNA processing, and translational activation functions attributed to HCF107 based on genetic data. We suggest that analogous activities contribute to the functions of HAT domains found in ribonucleoprotein complexes in the nuclear-cytosolic compartment.

Saccharomyces cerevisiae NineTeen complex (NTC)-associated factor Bud31/Ycr063w assembles on precatalytic spliceosomes and improves first and second step pre-mRNA splicing efficiency

Pre-mRNA splicing occurs in spliceosomes whose assembly and activation are critical for splice site selection and catalysis. The highly conserved NineTeen complex protein complex stabilizes various snRNA and protein interactions early in the spliceosome assembly pathway. Among several NineTeen complex-associated proteins is the nonessential protein Bud31/Ycr063w, which is also a component of the Cef1p subcomplex. A role for Bud31 in pre-mRNA splicing is implicated by virtue of its association with splicing factors, but its specific functions and spliceosome interactions are uncharacterized. Here, using in vitro splicing assays with extracts from a strain lacking Bud31, we illustrate its role in efficient progression to the first catalytic step and its requirement for the second catalytic step in reactions at higher temperatures. Immunoprecipitation of functional epitope-tagged Bud31 from in vitro reactions showed that its earliest association is with precatalytic B complex and that the interaction continues in catalytically active complexes with stably bound U2, U5, and U6 small nuclear ribonucleoproteins. In complementary experiments, wherein precatalytic spliceosomes are selected from splicing reactions, we detect the occurrence of Bud31. Cross-linking of proteins to pre-mRNAs with a site-specific 4-thio uridine residue at the -3 position of exon 1 was tested in reactions with WT and bud31 null extracts. The data suggest an altered interaction between a ∼25-kDa protein and this exonic residue of pre-mRNAs in the arrested bud31 null spliceosomes. These results demonstrate the early spliceosomal association of Bud31 and provide plausible functions for this factor in stabilizing protein interactions with the pre-mRNA.

The spliceosome-activating complex: molecular mechanisms underlying the function of a pleiotropic regulator

Correct interpretation of the coding capacity of RNA polymerase II transcribed eukaryotic genes is determined by the recognition and removal of intronic sequences of pre-mRNAs by the spliceosome. Our current knowledge on dynamic assembly and subunit interactions of the spliceosome mostly derived from the characterization of yeast, Drosophila, and human spliceosomal complexes formed on model pre-mRNA templates in cell extracts. In addition to sequential structural rearrangements catalyzed by ATP-dependent DExH/D-box RNA helicases, catalytic activation of the spliceosome is critically dependent on its association with the NineTeen Complex (NTC) named after its core E3 ubiquitin ligase subunit PRP19. NTC, isolated recently from Arabidopsis, occurs in a complex with the essential RNA helicase and GTPase subunits of the U5 small nuclear RNA particle that are required for both transesterification reactions of splicing. A compilation of mass spectrometry data available on the composition of NTC and spliceosome complexes purified from different organisms indicates that about half of their conserved homologs are encoded by duplicated genes in Arabidopsis. Thus, while mutations of single genes encoding essential spliceosome and NTC components lead to cell death in other organisms, differential regulation of some of their functionally redundant Arabidopsis homologs permits the isolation of partial loss of function mutations. Non-lethal pleiotropic defects of these mutations provide a unique means for studying the roles of NTC in co-transcriptional assembly of the spliceosome and its crosstalk with DNA repair and cell death signaling pathways.

Cotranscriptional spliceosome assembly and splicing are independent of the Prp40p WW domain

Complex cellular functions involve large networks of interactions. Pre-mRNA splicing and transcription are thought to be coupled by the C-terminal domain (CTD) of the large subunit of RNA polymerase II (Pol II). In yeast, the U1 snRNP subunit Prp40 was proposed to mediate cotranscriptional recruitment of early splicing factors through binding of its WW domains to the Pol II CTD. Here we investigate the role of Prp40 in splicing with an emphasis on the role of the WW domains, which might confer protein-protein interactions among the splicing and transcriptional machineries. Affinity purification revealed that Prp40 and Snu71 form a stable heterodimer that stably associates with the U1 snRNP only in the presence of Nam8, a known regulator of 5' splice site recognition. However, the Prp40 WW domains were dispensable for yeast viability. In their absence, no defect in splicing in vivo, U1 or U2 snRNP recruitment in vivo, or early splicing complex assembly in vitro was detected. We conclude that the WW domains of Prp40 do not mediate essential coupling between U1 snRNP and Pol II. Instead, delays in cotranscriptional U5 snRNP and Prp19 recruitment and altered spliceosome formation in vitro suggest that Prp40 WW domains assist in late steps of spliceosome assembly.

The RNA polymerase II C-terminal domain promotes splicing activation through recruitment of a U2AF65-Prp19 complex

Pre-mRNA splicing is frequently coupled to transcription by RNA polymerase II (RNAPII). This coupling requires the C-terminal domain of the RNAPII largest subunit (CTD), although the underlying mechanism is poorly understood. Using a biochemical complementation assay, we previously identified an activity that stimulates CTD-dependent splicing in vitro. We purified this activity and found that it consists of a complex of two well-known splicing factors: U2AF65 and the Prp19 complex (PRP19C). We provide evidence that both U2AF65 and PRP19C are required for CTD-dependent splicing activation, that U2AF65 and PRP19C interact both in vitro and in vivo, and that this interaction is required for activation of splicing. Providing the link to the CTD, we show that U2AF65 binds directly to the phosphorylated CTD, and that this interaction results in increased recruitment of U2AF65 and PRP19C to the pre-mRNA. Our results not only provide a mechanism by which the CTD enhances splicing, but also describe unexpected interactions important for splicing and its coupling to transcription.

Comparing peripheral glial cell differentiation in Drosophila and vertebrates

In all complex organisms, the peripheral nerves ensure the portage of information from the periphery to central computing and back again. Axons are in part amazingly long and are accompanied by several different glial cell types. These peripheral glial cells ensure electrical conductance, most likely nature the long axon, and establish and maintain a barrier towards extracellular body fluids. Recent work has revealed a surprisingly similar organization of peripheral nerves of vertebrates and Drosophila. Thus, the genetic dissection of glial differentiation in Drosophila may also advance our understanding of basic principles underlying the development of peripheral nerves in vertebrates.

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.

Control of alternative splicing by signal-dependent degradation of splicing-regulatory proteins

Alternative pre-mRNA splicing is a major gene expression regulatory mechanism in metazoan organisms. Proteins that bind pre-mRNA elements and control assembly of splicing complexes regulate utilization of pre-mRNA alternative splice sites. To understand how signaling pathways impact this mechanism, an RNA interference screen in Drosophila S2 cells was used to identify proteins that regulate TAF1 (TBP-associated factor 1) alternative splicing in response to activation of the ATR (ATM-RAD3-related) signaling pathway by the chemotherapeutic drug camptothecin (CPT). The screen identified 15 proteins that, when knocked down, caused the same change in TAF1 alternative splicing as CPT treatment. However, combined RNA interference and CPT treatment experiments indicated that only a subset of the identified proteins are targets of the CPT-induced signal, suggesting that multiple independent pathways regulate TAF1 alternative splicing. To understand how signals modulate the function of splicing factors, we characterized one of the CPT targets, Tra2 (Transformer-2). CPT was found to down-regulate Tra2 protein levels. CPT-induced Tra2 down-regulation was ATR-dependent and temporally paralleled the change in TAF1 alternative splicing, supporting the conclusion that Tra2 directly regulates TAF1 alternative splicing. Additionally, CPT-induced Tra2 down-regulation occurred independently of new protein synthesis, suggesting a post-translational mechanism. The proteasome inhibitor MG132 reduced CPT-induced Tra2 degradation and TAF1 alternative splicing, and mutation of evolutionarily conserved Tra2 lysine 81, a potential ubiquitin conjugation site, to arginine inhibited CPT-induced Tra2 degradation, supporting a proteasome-dependent alternative splicing mechanism. We conclude that CPT-induced TAF1 alternative splicing occurs through ATR-signaled degradation of a subset of splicing-regulatory proteins.

The splicing factor Prp17 interacts with the U2, U5 and U6 snRNPs and associates with the spliceosome pre- and post-catalysis

Saccharomyces cerevisiae PRP17-null mutants are temperature-sensitive for growth. In vitro splicing with extracts lacking Prp17 are kinetically slow for the first step of splicing and are arrested for the second step at temperatures greater than 34 °C. In the present study we show that these stalled spliceosomes are compromised for an essential conformational switch that is triggered by Prp16 helicase. These results suggest a plausible mechanistic basis for the second-step arrest in prp17Δ extracts and support a role for Prp17 in conjunction with Prp16. To understand the association of Prp17 with spliceosomes we used a functional epitope-tagged protein in co-immunoprecipitation experiments. Examination of co-precipitated snRNAs (small nuclear RNAs) show that Prp17 interacts with U2, U5 and U6 snRNPs (small nuclear ribonucleoproteins) but it is not a core component of any one snRNP. Prp17 association with in-vitro-assembled spliceosome complexes on actin pre-mRNAs was also investigated. Although the U5 snRNP proteins Prp8 and Snu114 are found in early pre-spliceosomes that contain all five snRNPs, Prp17 is not detectable at this step; however, Prp17 is present in the subsequent pre-catalytic A1 complex, containing unspliced pre-mRNA, formed after the dissociation of U4 snRNP. Thus Prp17 joins the spliceosome prior to both catalytic reactions. Our results indicate continued interactions in catalytic spliceosomes that contain reaction intermediates and in post-splicing complexes containing the lariat intron. These Prp17–spliceosome association analyses provide a biochemical basis for the delayed first step in prp17Δ and explain the previously known multiple genetic interactions between Prp17, factors of the Prp19-complex [NTC (nineteen complex)], functional elements in U2 and U5 snRNAs and other second-step splicing factors.

The FF domains of yeast U1 snRNP protein Prp40 mediate interactions with Luc7 and Snu71

Background

The FF domain is conserved across all eukaryotes and usually acts as an adaptor module in RNA metabolism and transcription. Saccharomyces cerevisiae encodes two FF domain proteins, Prp40, a component of the U1 snRNP, and Ypr152c, a protein of unknown function. The structure of Prp40, its relationship to other proteins within the U1 snRNP, and its precise function remain little understood.

Results

Here we have investigated the essentiality and interaction properties of the FF domains of yeast Prp40. We show that the C-terminal two FF domains of Prp40 are dispensable. Deletion of additional FF domains is lethal. The first FF domain of Prp40 binds to U1 protein Luc7 in yeast two-hybrid and GST pulldown experiments. FF domains 2 and 3 bind to Snu71, another known U1 protein. Peptide array screens identified binding sites for FF1-2 within Snu71 (NDVHY) and for FF1 within Luc7 (ϕ[FHL] × [KR] × [GHL] with ϕ being a hydrophobic amino acid).

Conclusion

Prp40, Luc7, and Snu71 appear to form a subcomplex within the yeast U1snRNP. Our data suggests that the N-terminal FF domains are critical for these interactions. Crystallization of Prp40, Luc7, and Snu71 have failed so far but co-crystallization of pairs or the whole tri-complex may facilitate crystallographic and further functional analysis.

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.

A BBP-Mud2p heterodimer mediates branchpoint recognition and influences splicing substrate abundance in budding yeast

The 3′ end of mammalian introns is marked by the branchpoint binding protein, SF1, and the U2AF65-U2AF35 heterodimer bound at an adjacent sequence. Baker's yeast has equivalent proteins, branchpoint binding protein (BBP) (SF1) and Mud2p (U2AF65), but lacks an obvious U2AF35 homolog, leaving open the question of whether another protein substitutes during spliceosome assembly. Gel filtration, affinity selection and mass spectrometry were used to show that rather than a U2AF65/U2AF35-like heterodimer, Mud2p forms a complex with BBP without a third (U2AF35-like) factor. Using mutants of MUD2 and BBP, we show that the BBP–Mud2p complex bridges partner-specific Prp39p, Mer1p, Clf1p and Smy2p two-hybrid interactions. In addition to inhibiting Mud2p association, the bbpΔ56 mutation impairs splicing, enhances pre-mRNA release from the nucleus, and similar to a mud2::KAN knockout, suppresses a lethal sub2::KAN mutation. Unexpectedly, rather than exacerbating bbpΔ56, the mud2::KAN mutation partially suppresses a pre-mRNA accumulation defect observed with bbpΔ56. We propose that a BBP–Mud2p heterodimer binds as a unit to the branchpoint in vivo and serves as a target for the Sub2p-DExD/H-box ATPase and for other splicing factors during spliceosome assembly. In addition, our results suggest the possibility that the Mud2p may enhance the turnover of pre-mRNA with impaired BBP-branchpoint association.

Du1, encoding a novel Prp1 protein, regulates starch biosynthesis through affecting the splicing of Wxb pre-mRNAs in rice (Oryza sativa L.)

Starch is the major component of cereal grains. In rice, starch properties determine the eating and cooking quality. The dull endosperm of rice grains is a classical morphological and agronomical trait that has long been exploited for breeding and genetics study. To understand the molecular mechanism that regulates the starch biosynthesis in rice grains, we characterized a classic rice mutant dull endosperm1 (du1) and isolated Du1 through a map-based cloning approach. Du1, encoding a member of pre-mRNA processing (Prp1) family, is expressed mainly in panicles. Du1 specifically affects the splicing efficiency of Wx(b) and regulates starch biosynthesis by mediating the expression of starch biosynthesis genes. Analysis of du1wx shows that Du1 acts upstream of Wx(b). These results strongly suggest that Du1 may function as a regulator of the starch biosynthesis by affecting the splicing of Wx(b) and the expression of other genes involved in the rice starch biosynthetic pathways.

Proteomic analysis of in vivo-assembled pre-mRNA splicing complexes expands the catalog of participating factors

Previous compositional studies of pre-mRNA processing complexes have been performed in vitro on synthetic pre-mRNAs containing a single intron. To provide a more comprehensive list of polypeptides associated with the pre-mRNA splicing apparatus, we have determined the composition of the bulk pre-mRNA processing machinery in living cells. We purified endogenous nuclear pre-mRNA processing complexes from human and chicken cells comprising the massive (>200S) supraspliceosomes (a.k.a. polyspliceosomes). As expected, RNA components include a heterogeneous mixture of pre-mRNAs and the five spliceosomal snRNAs. In addition to known pre-mRNA splicing factors, 5′ end binding factors, 3′ end processing factors, mRNA export factors, hnRNPs and other RNA binding proteins, the protein components identified by mass spectrometry include RNA adenosine deaminases and several novel factors. Intriguingly, our purified supraspliceosomes also contain a number of structural proteins, nucleoporins, chromatin remodeling factors and several novel proteins that were absent from splicing complexes assembled in vitro. These in vivo analyses bring the total number of factors associated with pre-mRNA to well over 300, and represent the most comprehensive analysis of the pre-mRNA processing machinery to date.

Functional anthology of intrinsic disorder. 1. Biological processes and functions of proteins with long disordered regions

Identifying relationships between function, amino acid sequence, and protein structure represents a major challenge. In this study, we propose a bioinformatics approach that identifies functional keywords in the Swiss-Prot database that correlate with intrinsic disorder. A statistical evaluation is employed to rank the significance of these correlations. Protein sequence data redundancy and the relationship between protein length and protein structure were taken into consideration to ensure the quality of the statistical inferences. Over 200,000 proteins from the Swiss-Prot database were analyzed using this approach. The predictions of intrinsic disorder were carried out using PONDR VL3E predictor of long disordered regions that achieves an accuracy of above 86%. Overall, out of the 710 Swiss-Prot functional keywords that were each associated with at least 20 proteins, 238 were found to be strongly positively correlated with predicted long intrinsically disordered regions, whereas 302 were strongly negatively correlated with such regions. The remaining 170 keywords were ambiguous without strong positive or negative correlation with the disorder predictions. These functions cover a large variety of biological activities and imply that disordered regions are characterized by a wide functional repertoire. Our results agree well with literature findings, as we were able to find at least one illustrative example of functional disorder or order shown experimentally for the vast majority of keywords showing the strongest positive or negative correlation with intrinsic disorder. This work opens a series of three papers, which enriches the current view of protein structure-function relationships, especially with regards to functionalities of intrinsically disordered proteins, and provides researchers with a novel tool that could be used to improve the understanding of the relationships between protein structure and function. The first paper of the series describes our statistical approach, outlines the major findings, and provides illustrative examples of biological processes and functions positively and negatively correlated with intrinsic disorder.

Composition and three-dimensional EM structure of double affinity-purified, human prespliceosomal A complexes

Little is known about the higher-order structure of prespliceosomal A complexes, in which pairing of the pre-mRNA's splice sites occurs. Here, human A complexes were isolated under physiological conditions by double-affinity selection. Purified complexes contained stoichiometric amounts of U1, U2 and pre-mRNA, and crosslinking studies indicated that these form concomitant base pairing interactions with one another. A complexes contained nearly all U1 and U2 proteins plus approximately 50 non-snRNP proteins. Unexpectedly, proteins of the hPrp19/CDC5 complex were also detected, even when A complexes were formed in the absence of U4/U6 snRNPs, demonstrating that they associate independent of the tri-snRNP. Double-affinity purification yielded structurally homogeneous A complexes as evidenced by electron microscopy, and allowed for the first time the generation of a three-dimensional structure. A complexes possess an asymmetric shape (approximately 260 x 200 x 195 angstroms) and contain a main body with various protruding elements, including a head-like domain and foot-like protrusions. Complexes isolated here are well suited for in vitro assembly studies to determine factor requirements for the A to B complex transition.

The splicing factor crooked neck associates with the RNA-binding protein HOW to control glial cell maturation in Drosophila

In both vertebrates and invertebrates, glial cells wrap axonal processes to ensure electrical conductance. Here we report that Crooked neck (Crn), the Drosophila homolog of the yeast Clf1p splicing factor, is directing peripheral glial cell maturation. We show that crooked neck is expressed and required in glial cells to control migration and axonal wrapping. Within the cytoplasm, Crn interacts with the RNA-binding protein HOW and then translocates to the nucleus where the Crn/HOW complex controls glial differentiation by facilitating splicing of specific target genes. By using a GFP-exon trap approach, we identified some of the in vivo target genes that encode proteins localized in autocellular septate junctions. In conclusion, here we show that glial cell differentiation is controlled by a cytoplasmic assembly of splicing components, which upon translocation to the nucleus promote the splicing of genes involved in the assembly of cellular junctions.

Muscle-dependent maturation of tendon cells is induced by post-transcriptional regulation of stripeA

Terminal differentiation of single cells selected from a group of equivalent precursors may be random, or may be regulated by external signals. In the Drosophila embryo, maturation of a single tendon cell from a field of competent precursors is triggered by muscle-dependent signaling. The transcription factor Stripe was reported to induce both the precursor cell phenotype, as well as the terminal differentiation of muscle-bound tendons. The mechanism by which Stripe activates these distinct differentiation programs remained unclear. Here, we demonstrate that each differentiation state is associated with a distinct Stripe isoform and that the Stripe isoforms direct different transcriptional outputs. Importantly, the transition to the mature differentiation state is triggered post-transcriptionally by enhanced production of the stripeA splice variant, which is typical of the tendon mature state. This elevation is mediated by the RNA-binding protein How(S), with levels sensitive to muscle-dependent signals. In how mutant embryos the expression of StripeA is significantly reduced, while overexpression of How(S) enhances StripeA protein as well as mRNA levels in embryos. Analysis of the expression of a stripeA minigene in S-2 cells suggests that this elevation may be due to enhanced splicing of stripeA. Consistently, stripeA mRNA is specifically reduced in embryos mutant for the splicing factor Crn, which physically interacts with How(S). Thus, we demonstrate a mechanism by which tendon cell terminal differentiation is maintained and reinforced by the approaching muscle.

Protein composition and electron microscopy structure of affinity-purified human spliceosomal B complexes isolated under physiological conditions

The spliceosomal B complex is the substrate that undergoes catalytic activation leading to catalysis of pre-mRNA splicing. Previous characterization of this complex was performed in the presence of heparin, which dissociates less stably associated components. To obtain a more comprehensive inventory of the B complex proteome, we isolated this complex under low-stringency conditions using two independent methods. MS2 affinity-selected B complexes supported splicing when incubated in nuclear extract depleted of snRNPs. Mass spectrometry identified over 110 proteins in both independently purified B complex preparations, including approximately 50 non-snRNP proteins not previously found in the spliceosomal A complex. Unexpectedly, the heteromeric hPrp19/CDC5 complex and 10 additional hPrp19/CDC5-related proteins were detected, indicating that they are recruited prior to spliceosome activation. Electron microscopy studies revealed that MS2 affinity-selected B complexes exhibit a rhombic shape with a maximum dimension of 420 A and are structurally more homogeneous than B complexes treated with heparin. These data provide novel insights into the composition and structure of the spliceosome just prior to its catalytic activation and suggest a potential role in activation for proteins recruited at this stage. Furthermore, the spliceosomal complexes isolated here are well suited for complementation studies with purified proteins to dissect factor requirements for spliceosome activation and splicing catalysis.

Interactions of the yeast SF3b splicing factor

The U2 snRNP promotes prespliceosome assembly through interactions that minimally involve the branchpoint binding protein, Mud2p, and the pre-mRNA. We previously showed that seven proteins copurify with the yeast (Saccharomyces cerevisiae) SF3b U2 subcomplex that associates with the pre-mRNA branchpoint region: Rse1p, Hsh155p, Hsh49p, Cus1p, and Rds3p and unidentified subunits p10 and p17. Here proteomic and genetic studies identify Rcp10p as p10 and show that it contributes to SF3b stability and is necessary for normal cellular Cus1p accumulation and for U2 snRNP recruitment in splicing. Remarkably, only the final 53 amino acids of Rcp10p are essential. p17 is shown to be composed of two accessory splicing factors, Bud31p and Ist3p, the latter of which independently associates with the RES complex implicated in the nuclear pre-mRNA retention. A directed two-hybrid screen reveals a network of prospective interactions that includes previously unreported intra-SF3b contacts and SF3b interactions with the RES subunit Bud13p, the Prp5p DExD/H-box protein, Mud2p, and the late-acting nineteen complex. These data establish the concordance of yeast and mammalian SF3b complexes, implicate accessory splicing factors in U2 snRNP function, and support SF3b contribution from early pre-mRNP recognition to late steps in splicing.

A genome-wide RNA interference screen in Drosophila melanogaster cells for new components of the Hh signaling pathway

Members of the Hedgehog (Hh) family of signaling proteins are powerful regulators of developmental processes in many organisms and have been implicated in many human disease states. Here we report the results of a genome-wide RNA interference screen in Drosophila melanogaster cells for new components of the Hh signaling pathway. The screen identified hundreds of potential new regulators of Hh signaling, including many large protein complexes with pleiotropic effects, such as the coat protein complex I (COPI) complex, the ribosome and the proteasome. We identified the multimeric protein phosphatase 2A (PP2A) and two new kinases, the D. melanogaster orthologs of the vertebrate PITSLRE and cyclin-dependent kinase-9 (CDK9) kinases, as Hh regulators. We also identified a large group of constitutive and alternative splicing factors, two nucleoporins involved in mRNA export and several RNA-regulatory proteins as potent regulators of Hh signal transduction, indicating that splicing regulation and mRNA transport have a previously unrecognized role in Hh signaling. Finally, we showed that several of these genes have conserved roles in mammalian Hh signaling.

The structure of Prp40 FF1 domain and its interaction with the crn-TPR1 motif of Clf1 gives a new insight into the binding mode of FF domains

The yeast splicing factor Prp40 (pre-mRNA processing protein 40) consists of a pair of WW domains followed by several FF domains. The region comprising the FF domains has been shown to associate with the 5' end of U1 small nuclear RNA and to interact directly with two proteins, the Clf1 (Crooked neck-like factor 1) and the phosphorylated repeats of the C-terminal domain of RNA polymerase II (CTD-RNAPII). In this work we reported the solution structure of the first FF domain of Prp40 and the identification of a novel ligand-binding site in FF domains. By using chemical shift assays, we found a binding site for the N-terminal crooked neck tetratricopeptide repeat of Clf1 that is distinct and structurally separate from the previously identified CTD-RNAPII binding pocket of the FBP11 (formin-binding protein 11) FF1 domain. No interaction, however, was observed between the Prp40 FF1 domain and three different peptides derived from the CTD-RNAPII protein. Indeed, the equivalent CTD-RNAPII-binding site in the Prp40 FF1 domain is predominantly negatively charged and thus unfavorable for an interaction with phosphorylated peptide sequences. Sequence alignments and phylogenetic tree reconstructions using the FF domains of three functionally related proteins, Prp40, FBP11, and CA150, revealed that Prp40 and FBP11 are not orthologous proteins and supported the different ligand specificities shown by their respective FF1 domains. Our results also revealed that not all FF domains in Prp40 are functionally equivalent. We proposed that at least two different interaction surfaces exist in FF domains that have evolved to recognize distinct binding motifs.

Genome-wide Analysis of Pre-mRNA Splicing

Removal of pre-mRNA introns is an essential step in eukaryotic genome interpretation. The spliceosome, a ribonucleoprotein performs this critical function; however, precise roles for many of its proteins remain unknown. Genome-wide consequences triggered by the loss of a specific factor can elucidate its function in splicing and its impact on other cellular processes. We have employed splicing-sensitive DNA microarrays, with yeast open reading frames and intron sequences, to detect changes in splicing efficiency and global expression. Comparison of expression profiles, for intron-containing transcripts, among mutants of two second-step factors, Prp17 and Prp22, reveals their unique and shared effects on global splicing. This analysis enabled the identification of substrates dependent on Prp17. We find a significant Prp17 role in splicing of introns which are longer than 200nts and note its dispensability when introns have a < or =13-nucleotide spacing between their branch point nucleotide and 3 ' splice site. In vitro splicing of substrates with varying branch nucleotide to 3 ' splice site distances supports the differential Prp17 dependencies inferred from the in vivo analysis. Furthermore, we tested the predicted dispensability of Prp17 for splicing short introns in the evolutionarily distant yeast, Schizosaccharomyces pombe, where the genome contains predominantly short introns. SpPrp17 was non-essential at all growth temperatures implying that functional evolution of splicing factors is integrated with genome evolution. Together our studies point to a role for budding yeast Prp17 in splicing of subsets of introns and have predictive value for deciphering the functions of splicing factors in gene expression and regulation in other eukaryotes.

Interaction of U-box-type ubiquitin-protein ligases (E3s) with molecular chaperones

Members of the U-box family of proteins constitute a class of ubiquitin-protein ligases (E3s) distinct from the HECT-type and RING finger-containing E3 families. Two representative mammalian U-box proteins, UFD2a and CHIP, interact with the molecular chaperones VCP and either Hsp90 or Hsc70, respectively, and are implicated in the degradation of damaged proteins. We have now investigated the roles of mammalian U-box proteins by performing a comprehensive screen for molecules that interact with these proteins in the yeast two-hybrid system. All mammalian U-box proteins tested were found to interact with molecular chaperones or cochaperones, including Hsp90, Hsp70, DnaJc7, EKN1, CRN, and VCP. These observations suggest that the function of U box-type E3s is to mediate the degradation of unfolded or misfolded proteins in conjunction with molecular chaperones as receptors that recognize such abnormal proteins.

Rds3p is required for stable U2 snRNP recruitment to the splicing apparatus

Rds3p is a well-conserved 12-kDa protein with five CxxC zinc fingers that has been implicated in the activation of certain drug transport genes and in the pre-mRNA splicing pathway. Here we show that Rds3p resides in the yeast spliceosome and is essential for splicing in vitro. Rds3p purified from yeast stably associates with at least five U2 snRNP proteins, Cus1p, Hsh49p, Hsh155p, Rse1p, and Ist3p/Snu17p, and with the Yra1p RNA export factor. A mutation upstream of the first Rds3p zinc finger causes the conditional release of the putative branchpoint nucleotide binding protein, Ist3p/Snu17p, and weakens Rse1p interaction with the Rds3p complex. The resultant U2 snRNP particle migrates exceptionally slowly in polyacrylamide gels, suggestive of a disorganized structure. U2 snRNPs depleted of Rds3p fail to form stable prespliceosomes, although U2 snRNA stability is not affected. Metabolic depletion of Yra1p blocks cell growth but not splicing, suggesting that Yra1p association with Rds3p relates to Yra1p's role in RNA trafficking. Together these data establish Rds3p as an essential component of the U2 snRNP SF3b complex and suggest a new link between the nuclear processes of pre-mRNA splicing and RNA export.

Moonlighting proteins: old proteins learning new tricks

Recently, several laboratories identifying proteins involved in the complex processes of replication, transcription and tumor suppression found that the 'new' protein they discovered had another, previously identified, function. A single protein with multiple functions might seem surprising, but there are actually many cases of proteins that 'moonlight', or have more than one role in an organism. As well as adding to the number and types of proteins that are known to moonlight, these new examples add to our understanding of the potential importance of moonlighting proteins.

Allosteric cascade of spliceosome activation

Introns are removed from precursor messenger RNAs in the cell nucleus by a large ribonucleoprotein complex called the spliceosome. The spliceosome contains five subcomplexes called snRNPs, each with one RNA and several protein components. Interactions of the snRNPs with each other and the intron are highly dynamic, changing in an ordered progression throughout the splicing process. This allosteric cascade of interactions is programmed into the RNA and protein components of the spliceosome, and is driven by a family of DExD/H-box RNA-dependent ATPases. The dependence of cascade progression on multiple intron-recognition events likely serves to enforce the accuracy of splicing. Here, the progression of the allosteric cascade from the first recognition event to the first catalytic step of splicing is reviewed.

Genetic interactions with CLF1 identify additional pre-mRNA splicing factors and a link between activators of yeast vesicular transport and splicing

Clf1 is a conserved spliceosome assembly factor composed predominately of TPR repeats. Here we show that the TPR elements are not functionally equivalent, with the amino terminus of Clf1 being especially sensitive to change. Deletion and add-back experiments reveal that the splicing defect associated with TPR removal results from the loss of TPR-specific sequence information. Twelve mutants were found that show synthetic growth defects when combined with an allele that lacks TPR2 (i.e., clf1Delta2). The identified genes encode the Mud2, Ntc20, Prp16, Prp17, Prp19, Prp22, and Syf2 splicing factors and four proteins without established contribution to splicing (Bud13, Cet1, Cwc2, and Rds3). Each synthetic lethal with clf1Delta2 (slc) mutant is splicing defective in a wild-type CLF1 background. In addition to the splicing factors, SSD1, BTS1, and BET4 were identified as dosage suppressors of clf1Delta2 or selected slc mutants. These results support Clf1 function through multiple stages of the spliceosome cycle, identify additional genes that promote cellular mRNA maturation, and reveal a link between Rab/Ras GTPase activation and the process of pre-mRNA splicing.

Developmental roles and clinical significance of hedgehog signaling

Cell signaling plays a key role in the development of all multicellular organisms. Numerous studies have established the importance of Hedgehog signaling in a wide variety of regulatory functions during the development of vertebrate and invertebrate organisms. Several reviews have discussed the signaling components in this pathway, their various interactions, and some of the general principles that govern Hedgehog signaling mechanisms. This review focuses on the developing systems themselves, providing a comprehensive survey of the role of Hedgehog signaling in each of these. We also discuss the increasing significance of Hedgehog signaling in the clinical setting.

Cryptococcus neoformans with a mutation in the tetratricopeptide repeat-containing gene, CCN1, causes subcutaneous lesions but fails to cause systemic infection

We studied a Cryptococcus neoformans strain that caused feline chronic nasal granuloma without disseminated disease. This strain, B-4551, grows at temperatures up to 35 degrees C and fails to cause systemic infection in mice. Many cells of B-4551 formed short hyphal elements in feline nasal tissue and occasionally at 35 degrees C in vitro. A complementation and sequence analysis revealed that the temperature-sensitive (Ts) phenotype of B-4551 was due to deletion of a lysine residue in the cryptococcal CCN1 gene. B-4551 complemented with the wild type CCN1 gene grew at 37 degrees C and caused fatal systemic infection in mice. The CCN1 gene encodes a protein containing 16 copies of a tetratricopeptide repeat. CCN1 is homologous to the Saccharomyces cerevisiae CLF1 gene, which is required for pre-mRNA splicing, cell cycle progression, and DNA replication, and to the Drosophila melanogaster crn gene, which is involved in neurogenesis. CLF1 complemented the Ts phenotype of B-4551. CCN1, however, failed to rescue the clf1 mutant in S. cerevisiae. These results indicate that the Ccn1p may not be as functionally diverse as Clf1p in yeast.

The Clf1p splicing factor promotes spliceosome assembly through N-terminal tetratricopeptide repeat contacts

Spliceosome assembly follows a well conserved pathway of subunit addition that includes both small nuclear ribonucleoprotein (snRNP) particles and non-snRNP splicing factors. Clf1p is an unusual splicing factor composed almost entirely of direct repeats of the tetratricopeptide repeat (TPR) protein-binding motif. Here we show that the Clf1p protein resides in at least two multisubunit protein complexes, a small nuclear RNA-free structure similar to what was reported as the Prp19p complex (nineteen complex; NTC) and an RNP structure that contains the U2, U5, and U6 small nuclear RNAs. Thirty Ccf (Clf1p complex factor) proteins have been identified by mass spectroscopy or immune detection as known or suspected components of the yeast spliceosome. Deletion of TPR1 or TPR2 from an epitope-tagged Clf1p protein (i.e. Clf1Delta2-TAP) destabilizes Clf1p complexes assembled in vivo, causing the release of the Cef1p and Prp19p NTC factors and decreased association of the Rse1p, Snu114p, and Hsh155p snRNP proteins. In vitro, temperature inactivation of Clf1Delta2p impairs the prespliceosome to spliceosome transition and prevents Prp19p recruitment to the splicing complex. These and related data support the view that the poly-TPR Clf1p splicing factor promotes the functional integration of the U4/U6.U5 tri-snRNP particle into the U1-, U2-dependent prespliceosome.

Identification and characterization of Prp45p and Prp46p, essential pre-mRNA splicing factors

Through exhaustive two-hybrid screens using a budding yeast genomic library, and starting with the splicing factor and DEAH-box RNA helicase Prp22p as bait, we identified yeast Prp45p and Prp46p. We show that as well as interacting in two-hybrid screens, Prp45p and Prp46p interact with each other in vitro. We demonstrate that Prp45p and Prp46p are spliceosome associated throughout the splicing process and both are essential for pre-mRNA splicing. Under nonsplicing conditions they also associate in coprecipitation assays with low levels of the U2, U5, and U6 snRNAs that may indicate their presence in endogenous activated spliceosomes or in a postsplicing snRNP complex.

Drosophila crooked-neck protein co-fractionates in a multiprotein complex with splicing factors

The Drosophila crooked neck (crn) gene encodes an unusual TPR-containing protein whose function is essential for embryonic development. Homology with other TPR-proteins involved in cell cycle control, initially led to the proposal that Crn might play a critical role in regulation of embryonic cell divisions. Here, we show that Crn does not have a cell cycle function in the embryo. By using specific antibodies we also show that the Crn protein is a nuclear protein which localizes in "speckles" which could correspond to preferential localization of several other splicing factors. Fractionation of nuclear extracts on sucrose gradients revealed Crn in a 900 kDa multiproteic complex together with snRNPs, suggesting that Crn participates in the assembly of the splicing machinery in vivo.

Crooked neck is a component of the human spliceosome and implicated in the splicing process

The Drosophila crooked neck (crn) gene is essential for embryogenesis and has been implicated in cell cycle progression and in pre-mRNA splicing although a direct role in either process has not been established. Here we report isolation of the human crooked neck homolog, HCRN, and provide evidence for its function in splicing. HCRN encodes an unusual protein composed largely of tetratricopeptide repeat (TPR) elements. The crooked neck protein co-localizes with the SR and Sm protein splicing factors in discrete subnuclear domains implicated in snRNP biogenesis. In vitro assembly experiments show that an 83 kDa hcrn isoform is stably recruited to splicing complexes coincident with the addition of the U4/U6.U5 tri-snRNP particle. Crooked neck activity appears essential as extracts depleted of hcrn fail to splice pre-mRNA. These and related data support the view that crooked neck is a phylogenetically conserved pre-mRNA splicing factor.

A large nucleolar U3 ribonucleoprotein required for 18S ribosomal RNA biogenesis

Although the U3 small nucleolar RNA (snoRNA), a member of the box C/D class of snoRNAs, was identified with the spliceosomal small nuclear RNAs (snRNAs) over 30 years ago, its function and its associated protein components have remained more elusive. The U3 snoRNA is ubiquitous in eukaryotes and is required for nucleolar processing of pre-18S ribosomal RNA in all organisms where it has been tested. Biochemical and genetic analyses suggest that U3 pre-rRNA base-pairing interactions mediate endonucleolytic pre-rRNA cleavages. Here we have purified a large ribonucleoprotein (RNP) complex from Saccharomyces cerevisiae that contains the U3 snoRNA and 28 proteins. Seventeen new proteins (Utp1 17) and Rrp5 were present, as were ten known components. The Utp proteins are nucleolar and specifically associated with the U3 snoRNA. Depletion of the Utp proteins impedes production of the 18S rRNA, indicating that they are part of the active pre-rRNA processing complex. On the basis of its large size (80S; calculated relative molecular mass of at least 2,200,000) and function, this complex may correspond to the terminal knobs present at the 5' ends of nascent pre-rRNAs. We have termed this large RNP the small subunit (SSU) processome.