Modifications of U2 snRNA are required for snRNP assembly and pre-mRNA splicing.

Among the spliceosomal snRNAs, U2 has the most extensive modifications, including a 5' trimethyl guanosine (TMG) cap, ten 2'-O-methylated residues and 13 pseudouridines. At short times after injection, cellularly derived (modified) U2 but not synthetic (unmodified) U2 rescues splicing in Xenopus oocytes depleted of endogenous U2 by RNase H targeting. After prolonged reconstitution, synthetic U2 regenerates splicing activity; a correlation between the extent of U2 modification and U2 function in splicing is observed. Moreover, 5-fluorouridine-containing U2 RNA, a potent inhibitor of U2 pseudouridylation, specifically abolishes rescue by synthetic U2, while rescue by cellularly derived U2 is not affected. By creating chimeric U2 molecules in which some sequences are from cellularly derived U2 and others are from in vitro transcribed U2, we demonstrate that the functionally important modifications reside within the 27 nucleotides at the 5' end of U2. We further show that 2'-O-methylation and pseudouridylation activities reside in the nucleus and that the 5' TMG cap is not necessary for internal modification but is crucial for splicing activity. Native gel analysis reveals that unmodified U2 is not incorporated into the spliceosome. Examination of the U2 protein profile and glycerol-gradient analysis argue that U2 modifications directly contribute to conversion of the 12S to the 17S U2 snRNP particle, which is essential for spliceosome assembly.     

Antisense probes targeted to an internal domain in U2 snRNP specifically inhibit the second step of pre-mRNA splicing.

Functional domains within the mammalian U2 snRNP particle that are required for pre-mRNA splicing have been analysed using antisense oligonucleotides. A comparison of the melting temperatures of duplexes formed between RNA and different types of antisense oligonucleotides has demonstrated that the most stable hybrids are formed with probes made of 2'-O-allyl RNA incorporating the modified base 2-aminoadenine. We have therefore used these 2'-O-allyl probes to target sequences within the central domain of U2 snRNA. Overlapping biotinylated 2'-O-allyloligoribonucleotides complementary to the stem loop Ila region of U2 snRNA (nucleotides 54-72) specifically affinity selected U2 snRNA from HeLa nuclear extracts. These probes inhibited mRNA production in an in vitro splicing assay and caused a concomitant accumulation of splicing intermediates. Little or no inhibition of spliceosome assembly and 5' splice site cleavage was observed for all pre-mRNAs tested, indicating that the oligonucleotides were specifically inhibiting exon ligation. This effect was most striking with a 2'-O-allyloligoribonucleotide complementary to U2 snRNA nucleotides 57-68. These results provide evidence for a functional requirement for U2 snRNP in the splicing mechanism occurring after spliceosome assembly.     

The Pre-mRNA 5′ Cap Determines Whether U6 Small Nuclear RNA Succeeds U1 Small Nuclear Ribonucleoprotein Particle at 5′ Splice Sites

Efficient splicing of the 5′-most intron of pre-mRNA requires a 5′ m⁷G(5′)ppp(5′)N cap, which has been implicated in U1 snRNP binding to 5′ splice sites. We demonstrate that the cap alters the kinetic profile of U1 snRNP binding, but its major effect is on U6 snRNA binding. With two alternative wild-type splice sites in an adenovirus pre-mRNA, the cap selectively alters U1 snRNA binding at the site to which cap-independent U1 snRNP binding is stronger and that is used predominantly in splicing; with two consensus sites, the cap acts on both, even though one is substantially preferred for splicing. However, the most striking quantitative effect of the 5′ cap is neither on U1 snRNP binding nor on the assembly of large complexes but on the replacement of U1 snRNP by U6 snRNA at the 5′ splice site. Inhibition of splicing by a cap analogue is correlated with the loss of U6 interactions at the 5′ splice site and not with any loss of U1 snRNP binding.     

Roles of U4 and U6 snRNAs in the assembly of splicing complexes.

A series of U4 and U6 snRNA mutants was analysed in Xenopus oocytes to determine whether they block splicing complex assembly or splicing itself. All the U4 and U6 mutants found to be inactive in splicing complementation resulted in defects in assembly of either U4/U6 snRNP or of splicing complexes. No mutants were found to separate the entry of U5 and U6 snRNAs into splicing complexes and neither of these RNAs was able to associate with the pre-mRNA in the absence of U4. In the absence of U6 snRNA, however, U4 entered a complex containing pre-mRNA as well as the U1 and U2 snRNAs. U6 nucleotides whose mutation resulted in specific blockage of the second step of splicing in Saccharomyces cerevisiae are shown not to be essential for splicing in the oocyte assay. The results are discussed in terms of the roles of U4 and U6 in the assembly and catalytic steps of the splicing process.     

Interactions between highly conserved U2 small nuclear RNA structures and Prp5p, Prp9p, Prp11p, and Prp21p proteins are required to ensure integrity of the U2 small nuclear ribonucleoprotein in Saccharomyces cerevisiae.

Binding of U2 small nuclear ribonucleoprotein (snRNP) to the pre-mRNA is an early and important step in spliceosome assembly. We searched for evidence of cooperative function between yeast U2 small nuclear RNA (snRNA) and several genetically identified splicing (Prp) proteins required for the first chemical step of splicing, using the phenotype of synthetic lethality. We constructed yeast strains with pairwise combinations of 28 different U2 alleles with 10 prp mutations and found lethal double-mutant combinations with prp5, -9, -11, and -21 but not with prp3, -4, -8, or -19. Many U2 mutations in highly conserved or invariant RNA structures show no phenotype in a wild-type PRP background but render mutant prp strains inviable, suggesting that the conserved but dispensable U2 elements are essential for efficient cooperative function with specific Prp proteins. Mutant U2 snRNA fails to accumulate in synthetic lethal strains, demonstrating that interaction between U2 RNA and these four Prp proteins contributes to U2 snRNP assembly or stability. Three of the proteins (Prp9p, Prp11p, and Prp21p) are associated with each other and pre-mRNA in U2-dependent splicing complexes in vitro and bind specifically to synthetic U2 snRNA added to crude splicing extracts depleted of endogenous U2 snRNPs. Taken together, the results suggest that Prp9p, -11p, and -21p are U2 snRNP proteins that interact with a structured region including U2 stem loop IIa and mediate the association of the U2 snRNP with pre-mRNA.     

Probing the structure and function of U2 snRNP with antisense oligonucleotides made of 2'-OMe RNA

We have used oligonucleotides made of 2'-OMe RNA to analyze the role of separate domains of U2 snRNA in the splicing process. We show that antisense 2'-OMe RNA oligonucleotides bind efficiently and specifically to U2 snRNP and demonstrate that masking of two separate regions of U2 snRNA can inhibit splicing by affecting different steps in the spliceosome assembly pathway. Masking the 5' terminus of U2 snRNA does not prevent U2 snRNP binding to pre-mRNA but blocks subsequent assembly of a functional spliceosome. By contrast, masking of U2 sequences complementary to the pre-mRNA branch site completely inhibits binding of pre-mRNA. Hybrid formation at the branch site complementary region also triggers a specific change which affects the 5' terminus of U2 snRNA.     

Structure and assembly of the SF3a splicing factor complex of U2 snRNP

SF3a is an evolutionarily conserved heterotrimeric complex essential for pre-mRNA splicing. It functions in spliceosome assembly within the mature U2 snRNP (small nuclear ribonucleoprotein particle), and its displacement from the spliceosome initiates the first step of the splicing reaction. We have identified a core domain of the yeast SF3a complex required for complex assembly and determined its crystal structure. The structure shows a bifurcated assembly of three subunits, Prp9, Prp11 and Prp21, with Prp9 interacting with Prp21 via a bidentate-binding mode, and Prp21 wrapping around Prp11. Structure-guided biochemical analysis also shows that Prp9 harbours a major binding site for stem-loop IIa of U2 snRNA. These findings provide mechanistic insights into the assembly of U2 snRNP.     

U2 snRNA-protein contacts in purified human 17S U2 snRNPs and in spliceosomal A and B complexes

The 17S U2 snRNP plays an essential role in branch point selection and catalysis during pre-mRNA splicing. Much remains to be learned about the molecular architecture of the U2 snRNP, including which proteins contact the functionally important 5' end of the U2 snRNA. Here, RNA-protein interactions within immunoaffinity-purified human 17S U2 snRNPs were analyzed by lead(II)-induced RNA cleavage and UV cross-linking. Contacts between the U2 snRNA and SF3a60, SF3b49, SF3b14a/p14 and SmG and SmB were detected. SF3b49 appears to make multiple contacts, interacting with the 5' end of U2 and nucleotides in loops I and IIb. SF3a60 also contacted different regions of the U2 snRNA, including the base of stem-loop I and a bulge in stem-loop III. Consistent with it contacting the pre-mRNA branch point adenosine, SF3b14a/p14 interacted with the U2 snRNA near the region that base pairs with the branch point sequence. A comparison of U2 cross-linking patterns obtained with 17S U2 snRNP versus purified spliceosomal A and B complexes revealed that RNA-protein interactions with stem-loop I and the branch site-interacting region of U2 are dynamic. These studies provide important insights into the molecular architecture of 17S U2 snRNPs and reveal U2 snRNP remodeling events during spliceosome assembly.     

Discrete domains of human U6 snRNA required for the assembly of U4/U6 snRNP and splicing complexes.

U6 snRNA sequences required for assembly of U4/U6 snRNP and splicing complexes were determined by in vitro reconstitution of snRNPs. Both mutagenesis and chemical modification/interference assays identify a U6 snRNA domain required for U4/U6 snRNP formation. The results support the existence of a U4/U6 snRNA interaction domain previously proposed on the basis of phylogenetic evidence. In addition, two short U6 snRNA regions flanking the U4/U6 interaction domain are essential to assemble the U4/U6 snRNP into splicing complexes. These two regions may represent binding sites for splicing factors or may facilitate the formation of an alternative U6 snRNA secondary structure during spliceosome assembly.     

The conserved central domain of yeast U6 snRNA: importance of U2-U6 helix Ia in spliceosome assembly

In the pre-mRNA processing machinery of eukaryotic cells, U6 snRNA is located at or near the active site for pre-mRNA splicing catalysis, and U6 is involved in catalyzing the first chemical step of splicing. We have further defined the roles of key features of yeast U6 snRNA in the splicing process. By assaying spliceosome assembly and splicing in yeast extracts, we found that mutations of yeast U6 nt 56 and 57 are similar to previously reported deletions of U2 nt 27 or 28, all within yeast U2-U6 helix Ia. These mutations lead to the accumulation of yeast A1 spliceosomes, which form just prior to the Prp2 ATPase step and the first chemical step of splicing. These results strongly suggest that, at a late stage of spliceosome assembly, the presence of U2-U6 helix Ia is important for promoting the first chemical step of splicing, presumably by bringing together the 5' splice site region of pre-mRNA, which is base paired to U6 snRNA, and the branchsite region of the intron, which is base paired to U2 snRNA, for activation of the first chemical step of splicing, as previously proposed by Madhani and Guthrie [Cell, 1992, 71: 803-817]. In the 3' intramolecular stem-loop of U6, mutation G81C causes an allele-specific accumulation of U6 snRNP. Base pairing of the U6 3' stem-loop in yeast spliceosomes does not extend as far as to include the U6 sequence of U2-U6 helix Ib, in contrast to the human U6 3' stem-loop structure.     

Mapping U2 snRNP--pre-mRNA interactions using biotinylated oligonucleotides made of 2''-OMe RNA.

Biotinylated 2'-OMe RNA oligonucleotides complementary to two separate regions of human U2 snRNA have been used as affinity probes to study U2 snRNP--pre-mRNA interactions. Both oligonucleotides bind specifically and allow highly selective removal of U2 snRNP from HeLa cell nuclear extracts. Pre-mRNA substrates can also be specifically affinity selected through oligonucleotides binding to U2 snRNP particles in splicing complexes. Stable binding of U2 snRNP to pre-mRNA is blocked by the pre-binding of an oligonucleotide to the branch site complementary region of U2 snRNA, but not by an oligonucleotide binding to the 5' terminus of U2. Both oligonucleotides affinity select the intron product, but not the intron intermediate, when added after spliceosome assembly has taken place. The effect of 2'-OMe RNA oligonucleotides on splicing complex formation has been used to demonstrate that complexes containing U2 snRNP and unspliced pre-mRNA are precursors to functional spliceosomes.     

Progression through the spliceosome cycle requires Prp38p function for U4/U6 snRNA dissociation.

The elaborate and energy-intensive spliceosome assembly pathway belies the seemingly simple chemistry of pre-mRNA splicing. Prp38p was previously identified as a protein required in vivo and in vitro for the first pre-mRNA cleavage reaction catalyzed by the spliceosome. Here we show that Prp38p is a unique component of the U4/U6.U5 tri-small nuclear ribonucleoprotein (snRNP) particle and is necessary for an essential step late in spliceosome maturation. Without Prp38p activity spliceosomes form, but arrest in a catalytically impaired state. Functional spliceosomes shed U4 snRNA before 5' splice-site cleavage. In contrast, Prp38p-defective spliceosomes retain U4 snRNA bound to its U6 snRNA base-pairing partner. Prp38p is the first tri-snRNP-specific protein shown to be dispensable for assembly, but required for conformational changes which lead to catalytic activation of the spliceosome.     

In vitro reconstitution of mammalian U2 and U5 snRNPs active in splicing: Sm proteins are functionally interchangeable and are essential for the formation of functional U2 and U5 snRNPs.

An in vitro reconstitution/splicing complementation system has been developed which has allowed the investigation of the role of mammalian U2 and U5 snRNP components in splicing. U2 or U5 snRNP cores are first reconstituted from purified native snRNP core proteins and snRNA in the absence of cellular extract and are subsequently added to splicing extracts depleted of either U2 or U5 snRNP. When snRNPs reconstituted with HeLa U2 or U5 snRNA were added to U2- or U5-depleted nuclear extract, splicing was complemented. Addition of naked snRNA, on the other hand, did not restore splicing, demonstrating that the core proteins are essential for both U2 and U5 snRNP functions in splicing. Hybrid U2 or U5 snRNPs, reconstituted with core proteins isolated from U1 or U2 snRNPs, were equally active in splicing complementation, indicating that the snRNP core proteins are functionally interchangeable. U5 snRNPs reconstituted from in vitro transcribed U5 snRNA restored splicing to a level identical to that observed with particles reconstituted from authentic HeLa U5 snRNA. In contrast, splicing could not be restored to U2-depleted extract by the addition of snRNPs reconstituted from synthetic U2 snRNA, suggesting that U2 snRNA base modifications are essential for U2 snRNP function.     

Molecular comparison of monocot and dicot U1 and U2 snRNAs

To elucidate differences between the pre-mRNA splicing components in monocots and dicots, we have cloned and characterized several U1 and U2 snRNA sequence variants expressed in wheat seedling nuclei. Primer extension sequencing on wheat and pea snRNA populations has demonstrated that two 5'-terminal nucleotides found in most other U1 snRNAs are missing/modified in many plant U1 snRNAs. Comparison of the wheat U1 and U2 snRNA variants with their counterparts expressed in pea nuclei has defined regions of structural divergence between monocot and dicot U1 and U2 snRNAs. The U1 and U2 snRNA sequences involved in RNA:RNA interaction with pre-mRNAs are absolutely conserved. Significant differences occur between wheat and pea U1 snRNAs in stem I and II structures implicated in the binding of U1-specific proteins suggesting that the monocot and dicot U1-specific snRNP proteins differ in their binding specificities. Stem III structures, which are required in mammalian systems for splicing complex formation but not for U1-specific protein binding, differ more extensively than stems I, II, or IV. In U2 snRNAs, the sequence differences between these two species are primarily localized in stem III and in stem IV which has been implicated in snRNP protein binding. These differences suggest that monocot and dicot U1 and U2 snRNPs represent distinct entities that may have monocot- and dicot-specific snRNP protein variants associated with each snRNA.     

Identification and characterization of Uss1p (Sdb23p): a novel U6 snRNA-associated protein with significant similarity to core proteins of small nuclear ribonucleoproteins.

The SDB23 gene of Saccharomyces cerevisiae was isolated in a search for high copy-number suppressors of mutations in a cell cycle gene, DBF2, SDB23 encodes a 21,276 Da protein with significant sequence similarity to characterized mammalian snRNP core proteins. Examination of multiple sequence alignments of snRNP core proteins with Sdb23p indicates that all of these proteins share a number of highly conserved residues, and identifies a novel motif for snRNP core proteins. Sdb23p is essential for cell viability and is required for nuclear pre-mRNA splicing both in vivo and in vitro. Extracts prepared from Sdb23p-depleted cells are unable to support splicing and have vastly reduced levels of U6 snRNA. The stability of U1, U2, U4 and U5 spliceosomal snRNAs is not affected by the loss of Sdb23p. Antibodies raised against Sdb23p strongly coimmunoprecipitate free U6 snRNA and U4/U6 base-paired snRNAs. These results establish that SDB23 encodes a novel U6 snRNA-associated protein that is essential for the stability of U6 snRNA. We therefore propose the more logical name USS1 (U-Six SnRNP) for this gene.     

Requirements for U2 snRNP addition to yeast pre-mRNA.

The in vitro spliceosome assembly pathway is conserved between yeast and mammals as U1 and U2 snRNPs associate with the pre-mRNA prior to U5 and U4/U6 snRNPs. In yeast, U1 snRNP-pre-mRNA complexes are the first splicing complexes visualized on native gels, and association with U1 snRNP apparently commits pre-mRNA to the spliceosome assembly pathway. The current study addresses U2 snRNP addition to commitment complexes. We show that commitment complex formation is relatively slow and does not require ATP, whereas U2 snRNP adds to the U1 snRNP complexes in a reaction that is relatively fast and requires ATP or hydrolyzable ATP analogs. In vitro spliceosome assembly was assayed in extracts derived from strains containing several U1 sRNA mutations. The results were consistent with a critical role for U1 snRNP in early complex formation. A mutation that disrupts the base-pairing between the 5' end of U1 snRNA and the 5' splice site allows some U2 snRNP addition to bypass the ATP requirement, suggesting that ATP may be used to destabilize certain U1 snRNP:pre-mRNA interactions to allow subsequent U2 snRNP addition.