Human U4/U6.U5 and U4atac/U6atac.U5 tri-snRNPs exhibit similar protein compositions

In the U12-dependent spliceosome, the U4atac/U6atac snRNP represents the functional analogue of the major U4/U6 snRNP. Little information is available presently regarding the protein composition of the former snRNP and its association with other snRNPs. In this report we show that human U4atac/U6atac di-snRNPs associate with U5 snRNPs to form a 25S U4atac/U6atac.U5 trimeric particle. Comparative analysis of minor and major tri-snRNPs by using immunoprecipitation experiments revealed that their protein compositions are very similar, if not identical. Not only U5-specific proteins but, surprisingly, all tested U4/U6- and major tri-snRNP-specific proteins were detected in the minor tri-snRNP complex. Significantly, the major tri-snRNP-specific proteins 65K and 110K, which are required for integration of the major tri-snRNP into the U2-dependent spliceosome, were among those proteins detected in the minor tri-snRNP, raising an interesting question as to how the specificity of addition of tri-snRNP to the corresponding spliceosome is maintained. Moreover, immunodepletion studies demonstrated that the U4/U6-specific 61K protein, which is involved in the formation of major tri-snRNPs, is essential for the association of the U4atac/U6atac di-snRNP with U5 to form the U4atac/U6atac.U5 tri-snRNP. Subsequent immunoprecipitation studies demonstrated that those proteins detected in the minor tri-snRNP complex are also incorporated into U12-dependent spliceosomes. This remarkable conservation of polypeptides between minor and major spliceosomes, coupled with the absence of significant sequence similarity between the functionally analogous snRNAs, supports an evolutionary model in which most major and minor spliceosomal proteins, but not snRNAs, are derived from a common ancestor.     

p110, a novel human U6 snRNP protein and U4/U6 snRNP recycling factor

During each spliceosome cycle, the U6 snRNA undergoes extensive structural rearrangements, alternating between singular, U4-U6 and U6-U2 base-paired forms. In Saccharomyces cerevisiae, Prp24 functions as an snRNP recycling factor, reannealing U4 and U6 snRNAs. By database searching, we have identified a Prp24-related human protein previously described as p110(nrb) or SART3. p110 contains in its C-terminal region two RNA recognition motifs (RRMs). The N-terminal two-thirds of p110, for which there is no counterpart in the S.cerevisiae Prp24, carries seven tetratricopeptide repeat (TPR) domains. p110 homologs sharing the same domain structure also exist in several other eukaryotes. p110 is associated with the mammalian U6 and U4/U6 snRNPs, but not with U4/U5/U6 tri-snRNPs nor with spliceosomes. Recom binant p110 binds in vitro specifically to human U6 snRNA, requiring an internal U6 region. Using an in vitro recycling assay, we demonstrate that p110 functions in the reassembly of the U4/U6 snRNP. In summary, p110 represents the human ortholog of Prp24, and associates only transiently with U6 and U4/U6 snRNPs during the recycling phase of the spliceosome cycle.     

Evolutionary conservation of minor U12-type spliceosome between plants and humans

Splicing of rare, U12-type or AT-AC introns is mediated by a distinct spliceosome that assembles from U11, U12, U4atac, U6atac, and U5 snRNPs. Although in human cells the protein composition of minor and major snRNPs is similar, differences, particularly in U11 and U12 snRNPs, have been recently described. We have identified an Arabidopsis U11 snRNP-specific 35K protein as an interacting partner of an RS-domain-containing cyclophilin. By using a transient expression system in Arabidopsis protoplasts, we show that the 35K protein incorporates into snRNP. Oligo affinity selection and glycerol gradient centrifugation revealed that the Arabidopsis 35K protein is present in monomeric U11 snRNP and in U11/U12-di snRNP. The interaction of the 35K protein with Arabidopsis SR proteins together with its strong sequence similarity to U1-70K suggests that its function in splicing of minor introns is analogous to that of U1-70K. Analysis of Arabidopsis and Oryza sativa genome sequences revealed that all U11/U12-di-snRNP-specific proteins are conserved in dicot and monocot plants. In addition, we have identified an Arabidopsis gene encoding the homolog of U4atac snRNA and a second Arabidopsis gene encoding U6atac snRNA. Secondary structure predictions indicate that the Arabidopsis U4atac is able to form dimeric complexes with both Arabidopsis U6atac snRNAs. As revealed by RNaseA/T1 protection assay, the U4atac snRNA gene is expressed as an ~160-nt RNA, whereas the second U6atac snRNA gene seems to be a pseudogene. Taken together, our data indicate that recognition and splicing of minor, AT-AC introns in plants is highly similar to that in humans.     

Human U4/U6 snRNP recycling factor p110: mutational analysis reveals the function of the tetratricopeptide repeat domain in recycling

After each spliceosome cycle, the U4 and U6 snRNAs are released separately and are recycled to the functional U4/U6 snRNP, requiring in the mammalian system the U6-specific RNA binding protein p110 (SART3). Its domain structure is made up of an extensive N-terminal domain with at least seven tetratricopeptide repeat (TPR) motifs, followed by two RNA recognition motifs (RRMs) and a highly conserved C-terminal sequence of 10 amino acids. Here we demonstrate under in vitro recycling conditions that U6-p110 is an essential splicing factor. Recycling activity requires both the RRMs and the TPR domain but not the highly conserved C-terminal sequence. For U6-specific RNA binding, the two RRMs with some flanking regions are sufficient. Yeast two-hybrid assays reveal that p110 interacts through its TPR domain with the U4/U6-specific 90K protein, indicating a specific role of the TPR domain in spliceosome recycling. On the 90K protein, a short internal region (amino acids 416 to 550) suffices for the interaction with p110. Together, these data suggest a model whereby p110 brings together U4 and U6 snRNAs through both RNA-protein and protein-protein interactions.     

3'-cyclic phosphorylation of U6 snRNA leads to recruitment of recycling factor p110 through LSm proteins

Pre-mRNA splicing proceeds through assembly of the spliceosome complex, catalysis, and recycling. During each cycle the U4/U6.U5 tri-snRNP is disrupted and U4/U6 snRNA base-pairing unwound, releasing separate post-spliceosomal U4, U5, and U6 snRNPs, which have to be recycled to the splicing-competent tri-snRNP. Previous work implicated p110--the human ortholog of the yeast Prp24 protein--and the LSm2-8 proteins of the U6 snRNP in U4/U6 recycling. Here we show in vitro that these proteins bind synergistically to U6 snRNA: Both purified and recombinant LSm2-8 proteins are able to recruit p110 protein to U6 snRNA via interaction with the highly conserved C-terminal region of p110. Furthermore, the presence of a 2',3'-cyclic phosphate enhances the affinity of U6 snRNA for the LSm2-8 proteins and inversely reduces La protein binding, suggesting a direct role of the 3'-terminal phosphorylation in RNP remodeling during U6 biogenesis.     

Both U2 snRNA and U12 snRNA are required for accurate splicing of exon 5 of the rat calcitonin/CGRP gene

Two classes of spliceosome are present in eukaryotic cells. Most introns in nuclear pre-mRNAs are removed by a spliceosome that requires U1, U2, U4, U5, and U6 small nuclear ribonucleoprotein particles (snRNPs). A minor class of introns are removed by a spliceosome containing U11, U12, U5, U4atac, and U6 atac snRNPs. We describe experiments that demonstrate that splicing of exon 5 of the rat calcitonin/CGRP gene requires both U2 snRNA and U12 snRNA. In vitro, splicing to calcitonin/ CGRP exon 5 RNA was dependent on U2 snRNA, as preincubation of nuclear extract with an oligonucleotide complementary to U2 snRNA abolished exon 5 splicing. Addition of an oligonucleotide complementary to U12 snRNA increased splicing at a cryptic splice site in exon 5 from <5% to 50% of total spliced RNA. Point mutations in a candidate U12 branch sequence in calcitonin/CGRP intron 4, predicted to decrease U12-pre-mRNA base-pairing, also significantly increased cryptic splicing in vitro. Calcitonin/CGRP genes containing base changes disrupting the U12 branch sequence expressed significantly decreased CGRP mRNA levels when expressed in cultured cells. Coexpression of U12 snRNAs containing base changes predicted to restore U12-pre-mRNA base pairing increased CGRP mRNA synthesis to the level of the wild-type gene. These observations indicate that accurate, efficient splicing of calcitonin/CGRP exon 5 is dependent upon both U2 and U12 snRNAs.     

Functional interaction of a novel 15.5kD [U4/U6.U5] tri-snRNP protein with the 5' stem-loop of U4 snRNA.

Activation of the spliceosome for splicing catalysis requires the dissociation of U4 snRNA from the U4/U6 snRNA duplex prior to the first step of splicing. We characterize an evolutionarily conserved 15.5 kDa protein of the HeLa [U4/U6.U5] tri-snRNP that binds directly to the 5' stem-loop of U4 snRNA. This protein shares a novel RNA recognition motif with several RNP-associated proteins, which is essential, but not sufficient for RNA binding. The 15.5kD protein binding site on the U4 snRNA consists of an internal purine-rich loop flanked by the stem of the 5' stem-loop and a stem comprising two base pairs. Addition of an RNA oligonucleotide comprising the 5' stem-loop of U4 snRNA (U4SL) to an in vitro splicing reaction blocked the first step of pre-mRNA splicing. Interestingly, spliceosomal C complex formation was inhibited while B complexes accumulated. This indicates that the 15.5kD protein, and/or additional U4 snRNP proteins associated with it, play an important role in the late stage of spliceosome assembly, prior to step I of splicing catalysis. Our finding that the 15.5kD protein also efficiently binds to the 5' stem-loop of U4atac snRNA indicates that it may be shared by the [U4atac/U6atac.U5] tri-snRNP of the minor U12-type spliceosome.     

Dynamic exchanges of RNA interactions leading to catalytic core formation in the U12-dependent spliceosome

Important general insights into the mechanism of pre-mRNA splicing have emerged from studies of the U12-dependent spliceosome. Here, photochemical cross-linking analyses during U12-dependent spliceosome assembly have surprisingly revealed that an upstream 5' exon region is required for establishing two essential catalytic core interactions, U12/U6atac helix Ib and U6atac/5' splice site contacts, but not for U5/5' exon interactions or partial unwinding of U4atac/U6atac. A novel intermediate, representing an alternative pathway for catalytic core formation, is a ternary snRNA complex containing U4atac/U6atac stem II and U12/U6atac helix Ia that forms even without U6atac replacing U11 at the 5' splice site. A powerful oligonucleotide displacement method suggests that the blocked complexes analyzed to deduce the interdependence of these multiple RNA exchanges are authentic intermediates in U12-dependent spliceosome assembly.     

The intramolecular stem-loop structure of U6 snRNA can functionally replace the U6atac snRNA stem-loop

The U6 spliceosomal snRNA forms an intramolecular stem-loop structure during spliceosome assembly that is required for splicing and is proposed to be at or near the catalytic center of the spliceosome. U6atac snRNA, the analog of U6 snRNA used in the U12-dependent splicing of the minor class of spliceosomal introns, contains a similar stem-loop whose structure but not sequence is conserved between humans and plants. To determine if the U6 and U6atac stem-loops are functionally analogous, the stem-loops from human and budding yeast U6 snRNAs were substituted for the U6atac snRNA structure and tested in an in vivo genetic suppression assay. Both chimeric U6/U6atac snRNA constructs were active for splicing in vivo. In contrast, several mutations of the native U6atac stem-loop that either delete putatively unpaired residues or disrupt the putative stem regions were inactive for splicing. Compensatory mutations that are expected to restore base pairing within the stem regions restored splicing activity. However, other mutants that retained base pairing potential were inactive, suggesting that functional groups within the stem regions may contribute to function. These results show that the U6atac snRNA stem-loop structure is required for in vivo splicing within the U12-dependent spliceosome and that its role is likely to be similar to that of the U6 snRNA intramolecular stem-loop.     

Base pairing with U6atac snRNA is required for 5' splice site activation of U12-dependent introns in vivo.

The minor U12-dependent class of eukaryotic nuclear pre-mRNA introns is spliced by a distinct spliceosomal mechanism that requires the function of U11, U12, U5, U4atac, and U6atac snRNAs. Previous work has shown that U11 snRNA plays a role similar to U1 snRNA in the major class spliceosome by base pairing to the conserved 5'' splice site sequence. Here we show that U6atac snRNA also base pairs to the 5'' splice site in a manner analogous to that of U6 snRNA in the major class spliceosome. We show that splicing defective mutants of the 5'' splice site can be activated for splicing in vivo by the coexpression of compensatory U6atac snRNA mutants. In some cases, maximal restoration of splicing required the coexpression of compensatory U11 snRNA mutants. The allelic specificity of mutant phenotype suppression is consistent with Watson-Crick base pairing between the pre-mRNA and the snRNAs. These results provide support for a model of the RNA-RNA interactions at the core of the U12-dependent spliceosome that is strikingly similar to that of the major class U2-dependent spliceosome.     

Proximity of the U12 snRNA with both the 5' splice site and the branch point during early stages of spliceosome assembly

U12 snRNA is required for branch point recognition in the U12-dependent spliceosome. Using site-specific cross-linking, we have captured an unexpected interaction between the 5' end of the U12 snRNA and the -2 position upstream of the 5' splice site of P120 and SCN4a splicing substrates. The U12 snRNA nucleotides that contact the 5' exon are the same ones that form the catalytically important helix Ib with U6atac snRNA in the spliceosome catalytic core. However, the U12/5' exon interaction is transient, occurring prior to the entry of the U4atac/U6atac.U5 tri-snRNP to the spliceosome. This suggests that the helix Ib region of U12 snRNA is positioned near the 5' splice site early during spliceosome assembly and only later interacts with U6atac to form helix Ib. We also provide evidence that U12 snRNA can simultaneously interact with 5' exon sequences near 5' splice site and the branch point sequence, suggesting that the 5' splice site and branch point sequences are separated by <40 to 50 A in the complex A of the U12-dependent spliceosome. Thus, no major rearrangements are subsequently needed to position these sites for the first step of catalysis.     

Minor-class splicing occurs in the nucleus of the Xenopus oocyte

A small fraction of premessenger RNA introns in certain eukaryotes is excised by the minor spliceosome, which contains low-abundance small nuclear ribonucleoproteins (snRNPs). Recently, it was suggested that minor-class snRNPs are localized to and function in the cytoplasm of vertebrate cells. To test whether U12-type splicing occurs in the cytoplasm of Xenopus oocytes, we performed microinjections of the well-characterized P120 minor-class splicing substrate into the nucleus or into the cytoplasm. Our results demonstrate that accurate splicing of this U12-dependent intron occurs exclusively in the nuclear compartment of the oocyte, where U12 and U6atac snRNPs are primarily localized. We further demonstrate that splicing of both a major-class and a minor-class intron is inhibited after nuclear envelope breakdown during meiosis.     

The abundance of the spliceosomal snRNPs is not limiting the splicing of U12-type introns

The rate of excision of U12-type introns has been reported to be slower than that of U2-type introns, suggesting a rate-limiting bottleneck that could down-regulate genes containing U12-type introns. The mechanistic reasons for this slower rate of intron excision are not known, but lower abundance of the U12-type snRNPs and slower rate of assembly or catalytic activity have been suggested. To investigate snRNP abundance we concentrated on the U4atac snRNA, which is the least abundant of the U12-type snRNAs and is limiting the formation of U4atac/U6atac complex. We identified mouse NIH-3T3 cell line isolates in which the level of both U4atac snRNA and U4atac/U6atac complexes is reduced to 10%-20% of the normal level. We used these cell lines to investigate splicing efficiency by transient transfection of a reporter gene containing a U12-type intron and by quantitative PCR analysis of endogenous genes. The splicing of the reporter U12-type intron was very inefficient, but the activity could be restored by overexpression of U4atac snRNA. Using these U4atac-deficient NIH-3T3 cells, we confirmed the results of previous studies showing that U12-type introns of endogenous genes are, indeed, excised more slowly than U2-type introns, but we found that the rate did not differ from that measured in cells displaying normal levels of U4atac snRNA. Thus our results suggest that the cellular abundance of the snRNPs does not limit U12-type intron splicing under normal conditions.     

RNAi knockdown of hPrp31 leads to an accumulation of U4/U6 di-snRNPs in Cajal bodies

Cajal bodies (CBs) are subnuclear organelles of animal and plant cells. A role of CBs in the assembly and maturation of small nuclear ribonucleoproteins (snRNP) has been proposed but is poorly understood. Here we have addressed the question where U4/U6.U5 tri-snRNP assembly occurs in the nucleus. The U4/U6.U5 tri-snRNP is a central unit of the spliceosome and must be re-formed from its components after each round of splicing. By combining RNAi and biochemical methods, we demonstrate that, after knockdown of the U4/U6-specific hPrp31 (61 K) or the U5-specific hPrp6 (102 K) protein in HeLa cells, tri-snRNP formation is inhibited and stable U5 mono-snRNPs and U4/U6 di-snRNPs containing U4/U6 proteins and the U4/U6 recycling factor p110 accumulate. Thus, hPrp31 and hPrp6 form an essential connection between the U4/U6 and U5 snRNPs in vivo. Using fluorescence microscopy, we show that, in the absence of either hPrp31 or hPrp6, U4/U6 di-snRNPs as well as p110 accumulate in Cajal bodies. In contrast, U5 snRNPs largely remain in nucleoplasmic speckles. Our data support the idea that CBs may play a role in tri-snRNP recycling.     

RNA structure and RNA-protein interactions in purified yeast U6 snRNPs

The U6 small nuclear RNA (snRNA) undergoes major conformational changes during the assembly of the spliceosome and catalysis of splicing. It associates with the specific protein Prp24p, and a set of seven LSm2p-8p proteins, to form the U6 small nuclear ribonucleoprotein (snRNP). These proteins have been proposed to act as RNA chaperones that stimulate pairing of U6 with U4 snRNA to form the intermolecular stem I and stem II of the U4/U6 duplex, whose formation is essential for spliceosomal function. However, the mechanism whereby Prp24p and the LSm complex facilitate U4/U6 base-pairing, as well as the exact binding site(s) of Prp24p in the native U6 snRNP, are not well understood. Here, we have investigated the secondary structure of the U6 snRNA in purified U6 snRNPs and compared it with its naked form. Using RNA structure-probing techniques, we demonstrate that within the U6 snRNP a large internal region of the U6 snRNA is unpaired and protected from chemical modification by bound Prp24p. Several of these U6 nucleotides are available for base-pairing interaction, as only their sugar backbone is contacted by Prp24p. Thus, Prp24p can present them to the U4 snRNA and facilitate formation of U4/U6 stem I. We show that the 3' stem-loop is not bound strongly by U6 proteins in native particles. However, when compared to the 3' stem-loop in the naked U6 snRNA, it has a more open conformation, which would facilitate formation of stem II with the U4 snRNA. Our data suggest that the combined association of Prp24p and the LSm complex confers upon U6 nucleotides a conformation favourable for U4/U6 base-pairing. Interestingly, we find that the open structure of the yeast U6 snRNA in native snRNPs can also be adopted by human U6 and U6atac snRNAs.     

The human 18S U11/U12 snRNP contains a set of novel proteins not found in the U2-dependent spliceosome

U11 and U12 snRNPs bind U12-type pre-mRNAs as a preformed di-snRNP complex, simultaneously recognizing the 5' splice site and branchpoint sequence. Thus, within the U12-type prespliceosome, U11/U12 components form a molecular bridge connecting both ends of the intron. We have affinity purified human 18S U11/U12 and 12S U11 snRNPs, and identified their protein components by using mass spectrometry. U11/U12 snRNPs lack all known U1 snRNP proteins but contain seven novel proteins (i.e., 65K, 59K, 48K, 35K, 31K, 25K, 20K) not found in the major spliceosome, four of which (59K, 48K, 35K, and 25K) are U11-associated. Thus, protein-protein and protein-RNA interactions contributing to 5' splice site recognition and/or intron bridging appear to differ significantly in the minor versus major prespliceosome. The majority of U11/U12 proteins are highly conserved in organisms known to contain U12-type introns. However, homologs of those associated with U11 were not detected in Drosophila melanogaster, consistent with the presence of a divergent U11 snRNP in flies. RNAi experiments revealed that several U11/U12 proteins are essential for cell viability, suggesting they play key roles in U12-type splicing. The presence of unique U11/U12 snRNP proteins in the U12-type spliceosome provides insight into potential evolutionary relationships between the major and minor spliceosome.     

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 3′ end domain of U6atac snRNA can direct U6 snRNA to the minor spliceosome

U6 and U6atac snRNAs play analogous critical roles in the major U2-dependent and minor U12-dependent spliceosomes, respectively. Previous results have shown that most of the functional cores of these two snRNAs are either highly similar in sequence or functionally interchangeable. Thus, a mechanism must exist to restrict each snRNA to its own spliceosome. Here we show that a chimeric U6 snRNA containing the unique and highly conserved 3′ end domain of U6atac snRNA is able to function in vivo in U12-dependent spliceosomal splicing. Function of this chimera required the coexpression of a modified U4atac snRNA; U4 snRNA could not substitute. Partial deletions of this element in vivo, as well as in vitro antisense experiments, showed that the 3′ end domain of U6atac snRNA is necessary to direct the U4atac/U6atac.U5 tri-snRNP to the forming U12-dependent spliceosome. In vitro experiments also uncovered a role for U4atac snRNA in this targeting.