The 20kD protein of human [U4/U6.U5] tri-snRNPs is a novel cyclophilin that forms a complex with the U4/U6-specific 60kD and 90kD proteins.

Cyclophilins (Cyps) catalyze the cis/trans isomerization of peptidyl-prolyl bonds, a rate-limiting step in protein folding. In some cases, cyclophilins have also been shown to form stable complexes with specific proteins in vivo and may thus also act as chaperone-like molecules. We have characterized the 20kD protein of the spliceosomal 25S [U4/U6.U5] tri-snRNP complex from HeLa cells and show that it is a novel human cyclophilin (denoted SnuCyp-20). Purified [U4/U6.U5] tri-snRNPs, but not U1, U2, or U5 snRNPs, exhibit peptidyl-prolyl cis/trans isomerase activity in vitro, which is cyclosporin A-sensitive, suggesting that SnuCyp-20 is an active isomerase. Consistent with its specific association with tri-snRNPs in vitro, immunofluorescence microscopy studies showed that SnuCyp-20 is predominantly located in the nucleus, where it colocalizes in situ with typical snRNP-containing structures referred to as nuclear speckles. As a first step toward the identification of possible targets of SnuCyp-20, we have investigated the interaction of SnuCyp-20 with other proteins of the tri-snRNP. Fractionation of RNA-free protein complexes dissociated from isolated tri-snRNPs by treatment with high salt revealed that SnuCyp-20 is part of a biochemically stable heteromer containing additionally the U4/U6-specific 60kD and 90kD proteins. By coimmunoprecipitation experiments performed with in vitro-translated proteins, we could further demonstrate a direct interaction between SnuCyp-20 and the 60kD protein, but failed to detect a protein complex containing the 90kD protein. The formation of a stable SnuCyp-20/60kD/90kD heteromer may thus require additional factors not present in our in vitro reconstitution system. We discuss possible roles of SnuCyp-20 in the assembly of [U4/U6.U5] tri-snRNPs and/or in conformational changes occurring during the splicing process.     

The yeast Prp3 protein is a U4/U6 snRNP protein necessary for integrity of the U4/U6 snRNP and the U4/U6.U5 tri-snRNP.

Previously, yeast prp3 mutants were found to be blocked prior to the first catalytic step of pre-mRNA splicing. No splicing intermediates or products are formed from pre-mRNA in heat-inactivated prp3 mutants or prp3 mutant extracts. Here we show that Prp3p is a component of the U4/U6 snRNP and is also present in the U4/U6.U5 tri-snRNP. Heat inactivation of prp3 extracts results in depletion of free U6 snRNPs and U4/U6.U5 tri-snRNPs, but not U4/U6 snRNPs or U5 snRNPs. Free U4 snRNP, normally not present in wild-type extracts, accumulates under these conditions. Assays of in vivo levels of snRNAs in a prp3 mutant revealed that amounts of free U6 snRNA decreased, free U4 snRNA increased, and U4/U6 hybrids decreased slightly. These results suggest that Prp3p is required for formation of stable U4/U6 snRNPs and for assembly of the U4/U6.U5 tri-snRNP from its component snRNPs. Upon inactivation of Prp3p, spliceosomes cannot assemble from prespliceosomes due to the absence of intact U4/U6.U5 tri-snRNPs. Prp3p is homologous to a human protein that is a component of U4/U6 snRNPs, exemplifying the conservation of splicing factors between yeast and metazoans.     

Structural basis for the dual U4 and U4atac snRNA-binding specificity of spliceosomal protein hPrp31

Human proteins 15.5K and hPrp31 are components of the major spliceosomal U4 snRNP and of the minor spliceosomal U4atac snRNP. The two proteins bind to related 5'-stem loops (5'SLs) of the U4 and U4atac snRNAs in a strictly sequential fashion. The primary binding 15.5K protein binds at K-turns that exhibit identical sequences in the two snRNAs. However, RNA sequences contacted by the secondary binding hPrp31 differ in U4 and U4atac snRNAs, and the mechanism by which hPrp31 achieves its dual specificity is presently unknown. We show by crystal structure analysis that the capping pentaloops of the U4 and U4atac 5'SLs adopt different structures in the ternary hPrp31-15.5K-snRNA complexes. In U4atac snRNA, a noncanonical base pair forms across the pentaloop, based on which the RNA establishes more intimate interactions with hPrp31 compared with U4 snRNA. Stacking of hPrp31-His270 on the noncanonical base pair at the base of the U4atac pentaloop recapitulates intramolecular stabilizing principles known from the UUCG and GNRA families of RNA tetraloops. Rational mutagenesis corroborated the importance of the noncanonical base pair and the U4atac-specific hPrp31-RNA interactions for complex stability. The more extensive hPrp31-U4atac snRNA interactions are in line with a higher stability of the U4atac compared with the U4-based ternary complex seen in gel-shift assays, which may explain how U4atac snRNA can compete with the more abundant U4 snRNA for the same protein partners in vivo.     

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.     

Organization of core spliceosomal components U5 snRNA loop I and U4/U6 Di-snRNP within U4/U6.U5 Tri-snRNP as revealed by electron cryomicroscopy

In eukaryotes, pre-mRNA exons are interrupted by large noncoding introns. Alternative selection of exons and nucleotide-exact removal of introns are performed by the spliceosome, a highly dynamic macromolecular machine. U4/U6.U5 tri-snRNP is the largest and most conserved building block of the spliceosome. By 3D electron cryomicroscopy and labeling, the exon-aligning U5 snRNA loop I is localized at the center of the tetrahedrally shaped tri-snRNP reconstructed to approximately 2.1 nm resolution in vitrified ice. Independent 3D reconstructions of its subunits, U4/U6 and U5 snRNPs, show how U4/U6 and U5 combine to form tri-snRNP and, together with labeling experiments, indicate a close proximity of the spliceosomal core components U5 snRNA loop I and U4/U6 at the center of tri-snRNP. We suggest that this central tri-snRNP region may be the site to which the prespliceosomal U2 snRNA has to approach closely during formation of the catalytic core of the spliceosome.     

SR proteins escort the U4/U6.U5 tri-snRNP to the spliceosome.

Pre-spliceosomes, formed in HeLa nuclear extracts and isolated by sedimentation on glycerol gradients, were chased into spliceosomes, the macromolecular enzyme that catalyzes intron removal. We demonstrate that the pre-spliceosome to spliceosome transition was dependent on ATP hydrolysis and required both a U-rich small nuclear ribonucleoprotein (U snRNP)-containing fraction and a fraction of non-snRNP factors. The active components in the non-snRNP fraction were identified as SR proteins and were purified to apparent homogeneity. Recombinant SR proteins (ASF, SC35, SRp55), as well as gel-purified SR proteins, with the exception of SRp20, were able to restore efficient spliceosome formation. We also demonstrate that the pre-spliceosome to spliceosome transition requires phosphorylated SR proteins. This is the first evidence that SR proteins are required for the pre-spliceosome to spliceosome transition, the step at which the U4/U6.U5 tri-snRNP assembles on the pre-mRNA. The results shown here, together with previous data, suggest U snRNPs require SR proteins as escorts to enter the assembling spliceosome.     

Biochemical and genetic analyses of the U5, U6, and U4/U6 x U5 small nuclear ribonucleoproteins from Saccharomyces cerevisiae.

We have purified the yeast U5 and U6 pre-mRNA splicing small nuclear ribonucleoproteins (snRNPs) by affinity chromatography and analyzed the associated polypeptides by mass spectrometry. The yeast U5 snRNP is composed of the two variants of U5 snRNA, six U5-specific proteins and the 7 proteins of the canonical Sm core. The U6 snRNP is composed of the U6 snRNA, Prp24, and the 7 Sm-Like (LSM) proteins. Surprisingly, the yeast DEAD-box helicase-like protein Prp28 is stably associated with the U5 snRNP, yet is absent from the purified U4/U6 x U5 snRNP. A novel yeast U5 and four novel yeast U4/U6 x U5 snRNP polypeptides were characterized by genetic and biochemical means to demonstrate their involvement in the pre-mRNA splicing reaction. We also show that, unlike the human tri-snRNP, the yeast tri-snRNP dissociated upon addition of ATP or dATP.     

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

Mutants in the Drosophila crooked neck (crn) gene show an embryonic lethal phenotype with severe developmental defects. The unusual crn protein consists of sixteen tandem repeats of the 34 amino acid tetratricopeptide (TPR) protein recognition domain. Crn-like TPR elements are found in several RNA processing proteins, although it is unknown how the TPR repeats or the crn protein contribute to Drosophila development. We have isolated a Saccharomyces cerevisiae gene, CLF1, that encodes a crooked neck-like factor. CLF1 is an essential gene but the lethal phenotype of a clf1::HIS3 chromosomal null mutant can be rescued by plasmid-based expression of CLF1 or the Drosophila crn open reading frame. Clf1p is required in vivo and in vitro for pre-mRNA 5' splice site cleavage. Extracts depleted of Clf1p arrest spliceosome assembly after U2 snRNP addition but prior to productive U4/U6.U5 association. Yeast two-hybrid analyses and in vitro binding studies show that Clf1p interacts specifically and differentially with the U1 snRNP-Prp40p protein and the yeast U2AF65 homolog, Mud2p. Intriguingly, Prp40p and Mud2p also bind the phylogenetically conserved branchpoint binding protein (BBP/SF1). Our results indicate that Clf1p acts as a scaffolding protein in spliceosome assembly and suggest that Clf1p may support the cross-intron bridge during the prespliceosome-to-spliceosome transition.     

The [U4/U6.U5] tri-snRNP-specific 27K protein is a novel SR protein that can be phosphorylated by the snRNP-associated protein kinase.

SR proteins play important roles in the recognition and selection of the 3' and 5' splice site of a given intron and contribute to the phosphorylation/dephosphorylation-mediated regulation of pre-mRNA splicing. Recent studies have demonstrated that the U1 snRNP is recruited to the 5' splice site by protein/protein interactions involving the SR domains of the U1-70K protein and SF2/ASF. Recently, it was suggested that SR proteins might also contribute to the binding of the [U4/U6.U5] tri-snRNP to the pre-spliceosome (Roscigno RF, Garcia-Blanco MA, 1995, RNA 1:692-706), although it remains unclear whether these SR proteins interact with proteins of the tri-snRNP complex. As a first step toward the identification of proteins that could potentially mediate the integration of the [U4/U6.U5] tri-snRNP complex into the spliceosome, we investigated whether purified [U4/U6.U5] tri-snRNP complexes contain SR proteins. Three proteins in the tri-snRNP complex with approximate molecular weights of 27, 60, and 100 kDa were phosphorylated by purified snRNP-associated protein kinase, which has been shown previously to phosphorylate the serine/ arginine-rich domains of U1-70K and SF2/ASF (Woppmann A et al., 1993, Nucleic Acids Res 21:2815-2822). These proteins are thus prime candidates for novel tri-snRNP SR proteins. Here, we describe the biochemical and molecular characterization of the 27K protein. Analysis of a cDNA encoding the 27K protein revealed an N-terminal SR domain strongly homologous (54% identity) to the SR domain of the U1 snRNP-specific 70K protein. In contrast to many other SR proteins, the 27K protein does not contain an RNA-binding domain. The 27K protein can be phosphorylated in vitro by the snRNP-associated protein kinase and exhibits several isoelectric variants upon 2D gel electrophoresis. Thus, the tri-snRNP-specific 27K protein could potentially be involved in SR protein-mediated protein/protein interactions and, additionally, its phosphorylation state could modulate pre-mRNA splicing.     

Hierarchical,clustered protein interactions with U4/U6 snRNA: a biochemical role for U4/U6 proteins

During activation of the spliceosome, the U4/U6 snRNA duplex is dissociated, releasing U6 for subsequent base pairing with U2 snRNA. Proteins that directly bind the U4/U6 interaction domain potentially could mediate these structural changes. We thus investigated binding of the human U4/U6-specific proteins, 15.5K, 61K and the 20/60/90K protein complex, to U4/U6 snRNA in vitro. We demonstrate that protein 15.5K is a nucleation factor for U4/U6 snRNP assembly, mediating the interaction of 61K and 20/60/90K with U4/U6 snRNA. A similar hierarchical assembly pathway is observed for the U4atac/U6atac snRNP. In addition, we show that protein 61K directly contacts the 5' portion of U4 snRNA via a novel RNA-binding domain. Furthermore, the 20/60/90K heteromer requires stem II but not stem I of the U4/U6 duplex for binding, and this interaction involves a direct contact between protein 90K and U6. This uneven clustering of the U4/U6 snRNP-specific proteins on U4/U6 snRNA is consistent with a sequential dissociation of the U4/U6 duplex prior to spliceosome catalysis.     

RNA structural requirements for the association of the spliceosomal hPrp31 protein with the U4 and U4atac small nuclear ribonucleoproteins

The kink-turn, a stem I-internal loop-stem II structure of the 5 ' stem-loop of U4 and U4atac small nuclear (sn) RNAs bound by 15.5K protein is required for binding of human Prp31 protein (hPrp31) during U4 and U4atac snRNP assembly. In box C/D snoRNPs a similar kink-turn with bound 15.5K protein is required for selective binding of proteins NOP56 and NOP58. Here we analyzed RNA structural requirements for association of hPrp31 with U4 snRNP in vitro by hydroxyl radical footprinting. hPrp31 induced protection of the terminal penta-loop, as well as of stems I and II flanking the kink-turn. Similar protection was found with U4/U6 snRNA duplex prebound with 15.5K protein. A detailed mutational analysis of the U4 snRNA elements by electrophoretic mobility shift analysis revealed that stem I could not be shortened, although it tolerated sequence alterations. However, introduction of a third Watson-Crick base pair into stem II significantly reduced hPrp31 binding. While stem I of U4atac snRNA showed relaxed binding requirements, its stem II requirements were likewise restricted to two base pairs. In contrast, as shown previously, stem II of the kink-turn motif in box C/D snoRNAs is comprised of three base pairs, and NOP56 and NOP58 require a G-C pair at the central position. This indicates that hPrp31 binding specificity is achieved by the recognition of the two base pair long stem II of the U4 and U4atac snRNAs and suggests how discrimination is achieved by RNA structural elements during assembly of U4/U6 and U4atac/U6atac snRNPs and box C/D snoRNPs.     

The network of protein-protein interactions within the human U4/U6.U5 tri-snRNP

The human 25S U4/U6.U5 tri-snRNP is a major building block of the U2-type spliceosome and contains, in addition to the U4, U6, and U5 snRNAs, at least 30 distinct proteins. To learn more about the molecular architecture of the tri-snRNP, we have investigated interactions between tri-snRNP proteins using the yeast two-hybrid assay and in vitro binding assays, and, in addition, have identified distinct protein domains that are critical for the connectivity of this protein network in the human tri-snRNP. These studies revealed multiple interactions between distinct domains of the U5 proteins hPrp8, hBrr2 (a DExH/D-box helicase), and hSnu114 (a putative GTPase), which are key players in the catalytic activation of the spliceosome, during which the U4/U6 base-pairing interaction is disrupted and U4 is released from the spliceosome. Both the U5-specific, TPR/HAT-repeat-containing hPrp6 protein and the tri-snRNP-specific hSnu66 protein interact with several U5- and U4/U6-associated proteins, including hBrr2 and hPrp3, which contacts the U6 snRNA. Thus, both proteins are located at the interface between U5 and U4/U6 in the tri-snRNP complex, and likely play an important role in transmitting the activity of hBrr2 and hSnu114 in the U5 snRNP to the U4/U6 duplex during spliceosome activation. A more detailed analysis of these protein interactions revealed that different HAT repeats mediate interactions with specific hPrp6 partners. Taken together, data presented here provide a detailed picture of the network of protein interactions within the human tri-snRNP.     

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.     

The human U5 snRNP 52K protein (CD2BP2) interacts with U5-102K (hPrp6), a U4/U6.U5 tri-snRNP bridging protein, but dissociates upon tri-snRNP formation

The U5 snRNP plays an essential role in both U2- and U12-dependent splicing. Here, we have characterized a 52-kDa protein associated with the human U5 snRNP, designated U5-52K. Protein sequencing revealed that U5-52K is identical to the CD2BP2, which interacts with the cytoplasmic portion of the human T-cell surface protein CD2. Consistent with it associating with an snRNP, immunofluorescence studies demonstrated that the 52K protein is predominantly located in the nucleoplasm of HeLa cells, where it overlaps, at least in part, with splicing-factor compartments (or "speckles"). We further demonstrate that the 52K protein is a constituent of the 20S U5 snRNP, but is not found in U4/U6.U5 tri-snRNPs. Thus, it is the only 20S U5-specific protein that is not integrated into the tri-snRNP and resembles, in this respect, the U4/U6 di-snRNP assembly factor Prp24p/p110. Yeast two-hybrid screening and pulldown assays revealed that the 52K protein interacts with the U5-specific 102K and 15K proteins, suggesting that these interactions are responsible for its integration into the U5 particle. The N-terminal two-thirds of 52K interact with the 102K protein, whereas its C-terminal GYF-domain binds the 15K protein. As the latter lacks a proline-rich tract, our data indicate that a GYF-domain can also engage in specific protein-protein interactions in a polyproline-independent manner. Interestingly, the U5-102K protein has been shown previously to play an essential role in tri-snRNP formation, binding the U4/U6-61K protein. The interaction of 52K with a tri-snRNP bridging protein, coupled with its absence from the tri-snRNP, suggests it might function in tri-snRNP assembly.     

Purification of the individual snRNPs U1, U2, U5 and U4/U6 from HeLa cells and characterization of their protein constituents.

A procedure is described for the purification of the individual major small nuclear ribonucleoproteins (snRNPs) U1, U2, U5 and U4/U6 from HeLa cells. The salient feature of the method is the combined usage of antibodies against 2,2,7-trimethylguanosine (m3G) and 6-methyladenosine (m6A) for differential immune affinity chromatography of the snRNPs. While anti-m3G affinity columns allow the separation of snRNPs U1, U2 and U5 from U4/U6 RNPs, anti-m6A antibodies selectively react with snRNPs U2 and U4/U6. Our technique further incorporates immune affinity chromatography of snRNPs with antibodies against snRNP proteins in addition to ion exchange chromatography. The procedure avoids the usage of denaturing agents, so as to maintain the native structure of the particles. This is mainly provided for by the possibility of eluting the anti-m3G and anti-m6A bound snRNPs with excess of the respective nucleosides. We have so far identified 12 polypeptides as constituents of the major snRNPs U1 to U6. Seven proteins of approximate mol. wts 29 kd (B'), 28 kd (B), 16 kd (D), 15.5 kd (D'), 12 kd (E), 11 kd (F) and 9 kd (G) were present in each of the individual snRNPs U1, U2, U5 and U4/U6. In addition to the common proteins, U1 RNPs contain three unique polypeptides of mol. wts 70 kd, 34 kd (A) and 22 kd (C). U2 RNPs are characterized by the presence of a 33-kd and a 28.5-kd protein, denoted A' and B". We could not detect any unique polypeptide confined to the purified snRNPs U5 or U4/U6.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

PRP4: a protein of the yeast U4/U6 small nuclear ribonucleoprotein particle.

The Saccharomyces cerevisiae prp mutants (prp2 through prp11) are known to be defective in pre-mRNA splicing at nonpermissive temperatures. We have sequenced the PRP4 gene and shown that it encodes a 52-kilodalton protein. We obtained PRP4 protein-specific antibodies and found that they inhibited in vitro pre-mRNA splicing, which confirms the essential role of PRP4 in splicing. Moreover, we found that PRP4 is required early in the spliceosome assembly pathway. Immunoprecipitation experiments with anti-PRP4 antibodies were used to demonstrate that PRP4 is a protein of the U4/U6 small nuclear ribonucleoprotein particle (snRNP). Furthermore, the U5 snRNP could be immunoprecipitated through snRNP-snRNP interactions in the large U4/U5/U6 complex.     

U5 small nuclear ribonucleoprotein: RNA structure analysis and ATP-dependent interaction with U4/U6.

To understand how the U5 small nuclear ribonucleoprotein (snRNP) interacts with other spliceosome components, its structure and binding to the U4/U6 snRNP were analyzed. The interaction of the U5 snRNP with the U4/U6 snRNP was studied by separating the snRNPs in HeLa cell nuclear extracts on glycerol gradients. A complex running at 25S and containing U4, U5, and U6 but not U1 or U2 snRNAs was identified. In contrast to results with native gel electrophoresis to separate snRNPs, this U4/U5/U6 snRNP complex requires ATP to assemble from the individual snRNPs. The structure of the U5 RNA within the U5 snRNP and the U4/5/6 snRNP complexes was then compared. Oligonucleotide-targeted RNase H digestion identified one RNA sequence in the U5 snRNP capable of base pairing to other nucleic acid sequences. Chemical modification experiments identified this sequence as well as two other U5 RNA sequences as accessible to modification within the U5 RNP. One of these regions is a large loop in the U5 RNA secondary structure whose sequence is conserved from Saccharomyces cerevisiae to humans. Interestingly, no differences in modification of free U5 snRNP as compared to U5 in the U4/U5/U6 snRNP complex were observed, suggesting that recognition of specific RNA sequences in the U5 snRNP is not required for U4/U5/U6 snRNP assembly.     

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.