Multiple splicing factors are released from endogenous complexes during in vitro pre-mRNA splicing.

Pre-mRNA splicing occurs in a macromolecular complex called the spliceosome. Efforts to isolate spliceosomes from in vitro splicing reactions have been hampered by the presence of endogenous complexes that copurify with de novo spliceosomes formed on added pre-mRNA. We have found that removal of these large complexes from nuclear extracts prevents the splicing of exogenously added pre-mRNA. We therefore examined these complexes for the presence of splicing factors and proteins known or thought to be involved in RNA splicing. These fast-sedimenting structures were found to contain multiple small nuclear ribonucleoproteins (snRNPs) and a fragmented heterogeneous nuclear ribonucleoprotein complex. At least two splicing factors other than the snRNPs were also associated with these large structures. Upon incubation with ATP, these splicing factors as well as U1 and U2 snRNPs were released from these complexes. The presence of multiple splicing factors suggests that these complexes may be endogenous spliceosomes released from nuclei during preparation of splicing extracts. The removal of these structures from extracts that had been preincubated with ATP yielded a splicing extract devoid of large structures. This extract should prove useful in the fractionation of splicing factors and the isolation of native spliceosomes formed on exogenously added pre-mRNA.     

Genetic and physical interactions involving the yeast nuclear cap-binding complex.

Yeast strains lacking the yeast nuclear cap-binding complex (yCBC) are viable, although impaired in growth. We have taken advantage of this observation to carry out a genetic screen for components that show synthetic lethality (SL) with a cbp20-Delta cbp80-Delta double mutation. One set of SL interactions was due to mutations that were complemented by components of U1 small nuclear RNP (snRNP) and the yeast splicing commitment complex. These interactions confirm the role of yCBC in commitment complex formation. Physical interaction of yCBC with the commitment complex components Mud10p and Mud2p, which may directly mediate yCBC function, was demonstrated. Unexpectedly, we identified multiple SL mutations that were complemented by Cbf5p and Nop58p. These are components of the two major classes of yeast small nucleolar RNPs, which function in the maturation of rRNA precursors. Mutants lacking yCBC were found to be defective in rRNA processing. Analysis of the yCBC deletion phenotype suggests that this is likely to be due to a defect in the splicing of a subset of ribosomal protein mRNA precursors.     

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.     

Autonomous splicing and complementation of in vivo-assembled spliceosomes

We have used an in vivo system generating assayable amounts of a specific pre-mRNA to study the relationship between splicing and an operationally defined nuclear matrix preparation (NM). When NM is prepared by extraction of DNase I-treated nuclei with an approximately physiological concentration of KCl (0.1 M), a portion of NM-associated precursor can be spliced in vitro in the presence of ATP and Mg2+ and in the absence of splicing extract ("autonomous splicing"). We propose that the autonomous reaction, which does not exhibit a temporal lag and is half-complete in 5 min, occurs in fully assembled, matrix-bound ribonucleoprotein complexes (in vivo spliceosomes). Extraction of the NM with concentrations of KCl greater than 0.4 M eliminates autonomous splicing but leaves behind preassembled complexes that can be complemented for splicing with HeLa cell nuclear extract. The splicing complementing factor, representing one or more activities present in the nuclear extract and also in the cytoplasmic S100 fraction, is relatively heat resistant, devoid of an RNA component, and does not bind to DEAE-Sepharose in 0.1 M KCl. It exists in the nucleus in two forms; bound to autonomous spliceosomes and free in the nucleoplasm. Biochemical features of the complementation reaction, and conditions for reversible uncoupling of the two splicing steps are described and discussed.     

Spliceostatin A inhibits spliceosome assembly subsequent to prespliceosome formation

Pre-mRNA splicing is catalyzed by the large ribonucleoprotein spliceosome. Spliceosome assembly is a highly dynamic process in which the complex transitions through a number of intermediates. Recently, the potent anti-tumor compound Spliceostatin A (SSA) was shown to inhibit splicing and to interact with an essential component of the spliceosome, SF3b. However, it was unclear whether SSA directly impacts the spliceosome and, if so, by what mechanism, which limits interpretation of the drugs influence on splicing. Here, we report that SSA inhibits pre-mRNA splicing by interfering with the spliceosome subsequent to U2 snRNP addition. We demonstrate that SSA inhibition of spliceosome assembly requires ATP, key pre-mRNA splicing sequences and intact U1 and U2 snRNAs. Furthermore all five U snRNAs in addition to the SSA molecule associate with pre-mRNA during SSA inhibition. Kinetic analyses reveal that SSA impedes the A to B complex transition. Remarkably, our data imply that, in addition to its established function in early U2 snRNP recruitment, SF3b plays a role in later maturation of spliceosomes. This work establishes SSA as a powerful tool for dissecting the dynamics of spliceosomes in cells. In addition our data will inform the design of synthetic splicing modulator compounds for targeted anti-tumor treatment.     

Two spliceosomes can form simultaneously and independently on synthetic double-intron messenger RNA precursors.

We have investigated the formation of splicing complexes in vitro on mRNA precursors (pre-mRNAs) containing two introns. Sucrose gradient sedimentation analysis revealed that the double-intron substrate becomes associated with 60S structures, which are larger than the 50S splicing complexes we previously observed with single-intron pre-mRNA precursors. We have demonstrated that the 60S complex represents the assembly of two single splicing complexes on the individual introns by conversion of the 60S double splicing complexes into single 50S spliceosomes by oligodeoxynucleotide directed RNase H cleavage of the double-intron pre-mRNAs within the middle exon. In addition, we have observed by native gel electrophoresis a transient double 'pre-splicing' complex analogous to the 35S 'pre-splicing' complex previously found with single-intron pre-mRNAs. Our results indicate that splicing complexes can form independently and simultaneously on the individual introns of multi-intron pre-mRNAs and that the assembly of these multiple spliceosomes proceeds with the same stepwise pathway observed for single-intron RNAs.     

SLU7 and a novel activity, SSF1, act during the PRP16-dependent step of yeast pre-mRNA splicing.

Understanding the mechanism of pre-mRNA splicing requires the characterization of all components involved. In the present study, we used the genetically and biochemically defined yeast PRP16 protein as a point of departure for the identification of additional factors required for the second catalytic step in vitro. We isolated by glycerol gradient sedimentation spliceosomes that were formed in yeast extracts depleted of PRP16. This procedure separated the spliceosomal complexes containing lariat intermediate and exon 1 from free proteins present in the whole-cell yeast extract. We then supplemented these spliceosomes with purified proteins or yeast extract fractions as a functional assay for second-step splicing factors. We show that SLU7 protein and a novel activity that we named SSF1 (second-step factor 1) were required in concert with PRP16 to promote progression through the second catalytic step of splicing. Taking advantage of a differential ATP requirement for PRP16 and SLU7 function, we show that SLU7 can act after PRP16 in the splicing pathway.     

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 degrees 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 prp17Delta 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 prp17Delta 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 human Prp8 protein is a component of both U2- and U12-dependent spliceosomes.

This study reports the cloning, sequencing, and development of antisera against the human U5 snRNP 220-kDa protein or hPrp8p. Prp8p is the most highly conserved large nuclear protein known to date, but it is not related to any other protein. Southern, Northern, and expressed sequence tag analyses indicate that hPrp8p is encoded by a single gene. Prp8p is a core component of U5 snRNP and the U4/U6.U5 tri-snRNP, and antibodies raised against it immunoprecipitate both the major, U2-dependent and minor, U12-dependent spliceosomes. These spliceosomes, which excise different classes of introns, contain distinct sets of snRNAs overlapping only with U5 snRNA. Other than the core Sm proteins, hPrp8p is the first splicing factor shown to be common to both spliceosomes.     

SMNrp is an essential pre-mRNA splicing factor required for the formation of the mature spliceosome

SMNrp, also termed SPF30, has recently been identified in spliceosomes assembled in vitro. We have functionally characterized this protein and show that it is an essential splicing factor. We show that SMNrp is a 17S U2 snRNP-associated protein that appears in the pre-spliceosome (complex A) and the mature spliceosome (complex B) during splicing. Immunodepletion of SMNrp from nuclear extract inhibits the first step of pre-mRNA splicing by preventing the formation of complex B. Re-addition of recombinant SMNrp to immunodepleted extract reconstitutes both spliceosome formation and splicing. Mutations in two domains of SMNrp, although similarly deleterious for splicing, differed in their consequences on U2 snRNP binding, suggesting that SMNrp may also engage in interactions with splicing factors other than the U2 snRNP. In agreement with this, we present evidence for an additional interaction between SMNrp and the [U4/U6.U5] tri-snRNP. A candidate that may mediate this interaction, namely the U4/U6-90 kDa protein, has been identified. We suggest that SMNrp, as a U2 snRNP-associated protein, facilitates the recruitment of the [U4/U6.U5] tri-snRNP to the pre-spliceosome.     

A novel U2 and U11/U12 snRNP protein that associates with the pre-mRNA branch site

Previous UV cross-linking studies demonstrated that, upon integration of the U2 snRNP into the spliceosome, a 14 kDa protein (p14) interacts directly with the branch adenosine, the nucleophile for the first transesterification step of splicing. We have identified the cDNA encoding this protein by microsequencing a 14 kDa protein isolated from U2-type spliceosomes. This protein contains an RNA recognition motif and is highly conserved across species. Antibodies raised against this cDNA-encoded protein precipitated the 14 kDa protein cross-linked to the branch adenosine, confirming the identity of the p14 cDNA. A combination of immunoblotting, protein microsequencing and immunoprecipitation revealed that p14 is a component of both 17S U2 and 18S U11/U12 snRNPs, suggesting that it contributes to the interaction of these snRNPs with the branch sites of U2- and U12-type pre-mRNAs, respectively. p14 was also shown to be a subunit of the heteromeric splicing factor SF3b and to interact directly with SF3b155. Immuno precipitations indicated that p14 is present in U12-type spliceosomes, consistent with the idea that branch point selection is similar in the major and minor spliceosomes.     

Purification and characterization of native spliceosomes suitable for three-dimensional structural analysis

We describe characterization of spliceosomes affinity purified under native conditions. These spliceosomes consist largely of C complex containing splicing intermediates. After C complex assembly on an MS2 affinity-tagged pre-mRNA substrate containing a 3' splice site mutation, followed by RNase H digestion of earlier complexes, spliceosomes were purified by size exclusion and affinity selection. This protocol yielded 40S C complexes in sufficient quantities to visualize in negative stain by electron microscopy. Complexes purified in this way contain U2, U5, and U6 snRNAs, but very little U1 or U4 snRNA. Analysis by tandem mass spectrometry confirmed the presence of core snRNP proteins (SM and LSM), U2 and U5 snRNP-specific proteins, and the second step factors Prp16, Prp17, Slu7, and Prp22. In contrast, proteins specific to earlier splicing complexes, such as U2AF and U1 snRNP components, were not detected in C complex, but were present in similarly purified H complex. Images of these spliceosomes revealed single particles with dimensions of approximately 270 x 240 A that assort into well-defined classes. These images represent an important first step toward attaining a comprehensive three-dimensional understanding of pre-mRNA splicing.     

PRP18, a protein required for the second reaction in pre-mRNA splicing.

We have investigated the role of a novel temperature-sensitive splicing mutation, prp18. We had previously demonstrated that an accumulation of the lariat intermediate of splicing occurred at the restrictive temperature in vivo. We have now used the yeast in vitro splicing system to show that extracts from this mutant strain are heat labile for the second reaction of splicing. The heat inactivation of prp18 extracts results from loss of activity of an exchangeable component. Inactivated prp18 extracts are complemented by heat-inactivated extracts from other mutants or by fractions from wild-type extracts. In heat-inactivated prp18 extracts, 40S splicing complexes containing lariat intermediate and exon 1 can assemble. The intermediates in this 40S complex can be chased to products by complementing extracts in the presence of ATP. Both complementation of extracts and chasing of the isolated prp18 spliceosomes takes place with micrococcal nuclease-treated extracts. Furthermore, the complementation profile with fractions of wild-type extracts indicates that the splicing defect results from a mutation in a previously designated factor required for the second step of splicing. The isolation of this mutant as temperature-sensitive lethal has also facilitated cloning of the wild-type allele by complementation.     

Interactions of PRP2 protein with pre-mRNA splicing complexes in Saccharomyces cerevisiae.

PRP2 protein of Saccharomyces cerevisiae is required for the pre-mRNA splicing reaction but not for the early stages of spliceosome assembly. Using anti-PRP2 antibodies we demonstrate that PRP2 protein is associated with spliceosomes prior to, and throughout step 1 of the splicing reaction. Heat-inactivated prp2 protein, by contrast, does not seem to associate with spliceosomes. By elution of electrophoretically distinct spliceosomal complexes from non-denaturing gels we identify the specific complex with which PRP2 initially interacts in the pathway of spliceosome assembly.     

Three-dimensional structure of C complex spliceosomes by electron microscopy

The spliceosome is a multimegadalton RNA-protein machine that removes noncoding sequences from nascent pre-mRNAs. Recruitment of the spliceosome to splice sites and subsequent splicing require a series of dynamic interactions among the spliceosome's component U snRNPs and many additional protein factors. These dynamics present several challenges for structural analyses, including purification of stable complexes to compositional homogeneity and assessment of conformational heterogeneity. We have isolated spliceosomes arrested before the second chemical step of splicing (C complex) in which U2, U5 and U6 snRNAs are stably associated. Using electron microscopy, we obtained images of C complex spliceosomes under cryogenic conditions and determined a three-dimensional structure of a core complex to a resolution of 30 A. The structure reveals a particle of dimensions 27 x 22 x 24 nm with a relatively open arrangement of three primary domains.     

Rearrangements within human spliceosomes captured after exon ligation

In spliceosomes, dynamic RNA/RNA and RNA/protein interactions position the pre-mRNA substrate for the two chemical steps of splicing. Not all of these interactions have been characterized, in part because it has not been possible to arrest the complex at clearly defined states relative to chemistry. Previously, it was shown in yeast that the DEAD/H-box protein Prp22 requires an extended 3′ exon to promote mRNA release from the spliceosome following second-step chemistry. In line with that observation, we find that shortening the 3′ exon blocks cleaved lariat intron and mRNA release in human splicing extracts, which allowed us to stall human spliceosomes in a new post-catalytic complex (P complex). In comparison to C complex, which is blocked at a point following first-step chemistry, we detect specific differences in RNA substrate interactions near the splice sites. These differences include extended protection across the exon junction and changes in protein crosslinks to specific sites in the 5′ and 3′ exons. Using selective reaction monitoring (SRM) mass spectrometry, we quantitatively compared P and C complex proteins and observed enrichment of SF3b components and loss of the putative RNA-dependent ATPase DHX35. Electron microscopy revealed similar structural features for both complexes. Notably, additional density is present when complexes are chemically fixed, which reconciles our results with previously reported C complex structures. Our ability to compare human spliceosomes before and after second-step chemistry has opened a new window to rearrangements near the active site of spliceosomes, which may play roles in exon ligation and mRNA release.     

The protein phosphatase-1 regulator NIPP1 is also a splicing factor involved in a late step of spliceosome assembly

NIPP1 is a ubiquitous regulator of protein phosphatase-1 (PP1) and is targeted to the splicing factor storage sites (speckles) in the nucleus by its forkhead-associated domain. We show here that NIPP1 is also a component of the spliceosomes in HeLa cell-splicing extracts and that the interaction with the spliceosomes requires a functional forkhead-associated domain. The in vitro splicing of beta-globin pre-mRNA was not affected by exogenous wild type NIPP1 but was blocked by mutants that lacked residues 225-329. The inhibition by these dominant negative mutants resulted from a block in a late phase of spliceosome assembly, i.e. at the transition between the B-complex and the C-complex. Site-directed mutagenesis furthermore showed that this spliceosomal function of NIPP1 was unrelated to its ability to bind PP1 or RNA. Our data suggest that NIPP1 can function independently as a splicing factor and a phosphatase regulator.     

Activation of pre-mRNA splicing by human RNPS1 is regulated by CK2 phosphorylation

Human RNPS1 was originally characterized as a pre-mRNA splicing activator in vitro and was shown to regulate alternative splicing in vivo. RNPS1 was also identified as a protein component of the splicing-dependent mRNP complex, or exon-exon junction complex (EJC), and a role for RNPS1 in postsplicing processes has been proposed. Here we demonstrate that RNPS1 incorporates into active spliceosomes, enhances the formation of the ATP-dependent A complex, and promotes the generation of both intermediate and final spliced products. RNPS1 is phosphorylated in vivo and interacts with the CK2 (casein kinase II) protein kinase. Serine 53 (Ser-53) of RNPS1 was identified as the major phosphorylation site for CK2 in vitro, and the same site is also phosphorylated in vivo. The phosphorylation status of Ser-53 significantly affects splicing activation in vitro, but it does not perturb the nuclear localization of RNPS1. In vivo experiments indicated that the phosphorylation of RNPS1 at Ser-53 influences the efficiencies of both splicing and translation. We propose that RNPS1 is a splicing regulator whose activator function is controlled in part by CK2 phosphorylation.     

Immunoprecipitation of spliceosomal RNAs by antisera to galectin-1 and galectin-3

We have shown that galectin-1 and galectin-3 are functionally redundant splicing factors. Now we provide evidence that both galectins are directly associated with spliceosomes by analyzing RNAs and proteins of complexes immunoprecipitated by galectin-specific antisera. Both galectin antisera co-precipitated splicing substrate, splicing intermediates and products in active spliceosomes. Protein factors co-precipitated by the galectin antisera included the Sm core polypeptides of snRNPs, hnRNP C1/C2 and Slu7. Early spliceosomal complexes were also immunoprecipitated by these antisera. When splicing reactions were sequentially immunoprecipitated with galectin antisera, we found that galectin-1 containing spliceosomes did not contain galectin-3 and vice versa, providing an explanation for the functional redundancy of nuclear galectins in splicing. The association of galectins with spliceosomes was (i) not due to a direct interaction of galectins with the splicing substrate and (ii) easily disrupted by ionic conditions that had only a minimal effect on snRNP association. Finally, addition of excess amino terminal domain of galectin-3 inhibited incorporation of galectin-1 into splicing complexes, explaining the dominant-negative effect of the amino domain on splicing activity. We conclude that galectins are directly associated with splicing complexes throughout the splicing pathway in a mutually exclusive manner and they bind a common splicing partner through weak protein–protein interactions.