Expression and Purification of Human U1-70K (snRNP70) and its BAD Domains Using an E. coli Expression System.

U1-70K (snRNP70) serves as an indispensable protein component within the U1 complex, assuming a pivotal role in both constitutive and alternative RNA splicing processes. Notably, U1-70K engages in interactions with SR proteins, instigating the assembly of the spliceosome. This protein undergoes regulation through phosphorylation at multiple sites. Of significant interest, U1-70K has been implicated in Alzheimer's disease, in which it tends to form detergent-insoluble aggregates. Even though it was identified more than three decades ago, our understanding of U1-70K remains notably constrained, primarily due to challenges such as low levels of recombinant expression, susceptibility to protein degradation, and insolubility. In endeavoring to address these limitations, we devised a multifaceted approach encompassing codon optimization, strategic purification, and a solubilization protocol. This methodology has enabled us to achieve a high yield of full-length, soluble U1-70K, paving the way for its comprehensive biophysical and biochemical characterization. Furthermore, we provide a detailed protocol for the preparation of phosphorylated U1-70K. This set of protocols promises to be a valuable resource for scientists exploring the intricate web of U1-70K-related mechanisms in the context of RNA splicing and its implications in neurodegenerative disorders and other disorders and biological processes. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Expression and purification of full-length U1-70K from E. coli Support Protocol 1: Making chemically competent BL21 Star pRARE/pBB535 cells Basic Protocol 2: Phosphorylation of full-length U1-70K using SRPK1 Support Protocol 2: Purification of SRPK1 Basic Protocol 3: Expression and purification of U1-70K BAD1 from E. coli Basic Protocol 4: Phosphorylation of U1-70K BAD1 using SRPK1 Basic Protocol 5: Expression and purification of U1-70K BAD2 from E. coli.

Molecular principles underlying dual RNA specificity in the Drosophila SNF protein.

The first RNA recognition motif of the Drosophila SNF protein is an example of an RNA binding protein with multi-specificity. It binds different RNA hairpin loops in spliceosomal U1 or U2 small nuclear RNAs, and only in the latter case requires the auxiliary U2A' protein. Here we investigate its functions by crystal structures of SNF alone and bound to U1 stem-loop II, U2A' or U2 stem-loop IV and U2A', SNF dynamics from NMR spectroscopy, and structure-guided mutagenesis in binding studies. We find that different loop-closing base pairs and a nucleotide exchange at the tips of the loops contribute to differential SNF affinity for the RNAs. U2A' immobilizes SNF and RNA residues to restore U2 stem-loop IV binding affinity, while U1 stem-loop II binding does not require such adjustments. Our findings show how U2A' can modulate RNA specificity of SNF without changing SNF conformation or relying on direct RNA contacts.

Soluble full-length expression and characterization of snRNP protein U1-68/70K.

The autoantigen U1-68/70K is the dominant diagnostic marker in Mixed Connective Tissue Disease (MCTD) that until recently could not be expressed in its full-length form (Northemann et al., 1995, [16]). Using cell-free expression screening, we successfully produced the snRNP protein U1-68/70K in a soluble full-length form in Escherichia coli cells. The protein length and identity was determined by Western Blot and MS/MS analysis. Additionally, its reactivity in the autoimmune diagnostic was confirmed. Establishment of a cell-free expression system for this protein was important for further elucidation of protein expression properties such as the cDNA construct, expression temperature and folding properties; these parameters can now be determined in a fast and resource-conserving manner.

Composition of yeast snRNPs and snoRNPs in the absence of trimethylguanosine caps reveals nuclear cap binding protein as a gained U1 component implicated in the cold-sensitivity of tgs1Δ cells.

Small nuclear and nucleolar RNAs that program pre-mRNA splicing and rRNA processing have a signature 5'-trimethylguanosine (TMG) cap. Whereas the mechanism of TMG synthesis by Tgs1 methyltransferase has been elucidated, we know little about whether or how RNP biogenesis, structure and function are perturbed when TMG caps are missing. Here, we analyzed RNPs isolated by tandem-affinity purification from TGS1 and tgs1Δ yeast strains. The protein and U-RNA contents of total SmB-containing RNPs were similar. Finer analysis revealed stoichiometric association of the nuclear cap-binding protein (CBP) subunits Sto1 and Cbc2 with otherwise intact Mud1- and Nam8-containing U1 snRNPs from tgs1Δ cells. CBP was not comparably enriched in Lea1-containing U2 snRNPs from tgs1Δ cells. Moreover, CBP was not associated with mature Nop58-containing C/D snoRNPs or mature Cbf5- and Gar1-containing H/ACA snoRNPs from tgs1Δ cells. The protein composition and association of C/D snoRNPs with the small subunit (SSU) processosome were not grossly affected by absence of TMG caps, nor was the composition of H/ACA snoRNPs. The cold-sensitive (cs) growth defect of tgs1Δ yeast cells could be suppressed by mutating the cap-binding pocket of Cbc2, suggesting that ectopic CBP binding to the exposed U1 m(7)G cap in tgs1Δ cells (not lack of TMG caps per se) underlies the cs phenotype.

The U1 snRNP-associated factor Luc7p affects 5' splice site selection in yeast and human.

yLuc7p is an essential subunit of the yeast U1 snRNP and contains two putative zinc fingers. Using RNA-protein cross-linking and directed site-specific proteolysis (DSSP), we have established that the N-terminal zinc finger of yLuc7p contacts the pre-mRNA in the 5' exon in a region close to the cap. Modifying the pre-mRNA sequence in the region contacted by yLuc7p affects splicing in a yLuc7p-dependent manner indicating that yLuc7p stabilizes U1 snRNP-pre-mRNA interaction, thus reminding of the mode of action of another U1 snRNP component, Nam8p. Database searches identified three putative human yLuc7p homologs (hLuc7A, hLuc7B1 and hLuc7B2). These proteins have an extended C-terminal tail rich in RS and RE residues, a feature characteristic of splicing factors. Consistent with a role in pre-mRNA splicing, hLuc7A localizes in the nucleus and antibodies raised against hLuc7A specifically co-precipitate U1 snRNA from human cell extracts. Interestingly, hLuc7A overexpression affects splicing of a reporter in vivo. Taken together, our data suggest that the formation of a wide network of protein-RNA interactions around the 5' splice site by U1 snRNP-associated factors contributes to alternative splicing regulation.

Proteomic analysis of the U1 snRNP of Schizosaccharomyces pombe reveals three essential organism-specific proteins.

Characterization of spliceosomal complexes in the fission yeast Schizosaccharomyces pombe revealed particles sedimenting in the range of 30-60S, exclusively containing U1 snRNA. Here, we report the tandem affinity purification (TAP) of U1-specific protein complexes. The components of the complexes were identified using (LC-MS/MS) mass spectrometry. The fission yeast U1 snRNP contains 16 proteins, including the 7 Sm snRNP core proteins. In both fission and budding yeast, the U1 snRNP contains 9 and 10 U1 specific proteins, respectively, whereas the U1 particle found in mammalian cells contains only 3. Among the U1-specific proteins in S. pombe, three are homolog to the mammalian and six to the budding yeast Saccharomyces cerevisiae U1-specific proteins, whereas three, called U1H, U1J and U1L, are proteins specific to S. pombe. Furthermore, we demonstrate that the homolog of U1-70K and the three proteins specific to S. pombe are essential for growth. We will discuss the differences between the U1 snRNPs with respect to the organism-specific proteins found in the two yeasts and the resulting effect it has on pre-mRNA splicing.

Association of polyadenylation cleavage factor I with U1 snRNP.

Splicing and polyadenylation factors interact for the control of polyadenylation and the coupling of splicing and polyadenylation. We document an interaction between the U1 snRNP and mammalian polyadenylation cleavage factor I (CF Im), one of several polyadenylation factors needed for the cleavage of the pre-mRNA at the polyadenylation site. Sucrose density gradient centrifugation demonstrated that CF Im separated into two fractions, a light fraction which contained the known CF Im subunits (72, 68, 59, and 25 kD), and a heavy fraction, rich in snRNPs, which contained predominately the 68- and 25-kD CF Im subunits. Using specific antibodies we found that the heavy fraction contains U1 snRNP/CF Im coprecipitable complexes. These complexes were insensitive to RNase treatment, suggesting that the coprecipitation is not due to RNA tethering. In vitro binding experiments show that both the 68- and 25-kD subunits bind to and comigrate with U1 snRNP. In addition, the 25-kD CF Im subunit binds specifically to the 70K protein of U1 snRNP (U1 70K). This binding may account for the CF Im/U1 snRNP interaction. During these studies we found that mAb 2.73 (mAb 2.73), an established U1 70K antibody, efficiently precipitates the bulk of the CF Im from cellular extracts. Because mAb 2.73 has been used in a number of previous studies related to the U1 snRNP and the U1 70K protein, the precipitation of CF Im must be considered in evaluating past and future data based on the use of mAb 2.73.

Simultaneous identification of various antinuclear antibodies using an automated multiparameter line immunoassay system.

The objective was to determine the sensitivity and specificity of an automated multiparameter line immunoassay system compared with other techniques for the identification of autoantibodies in rheumatic diseases. We studied sera from 90 patients. Anti-U1RNP, anti-Sm, anti-Ro/SS-A, anti-La/SS-B, anti-Jo 1 and anti-Scl 70 antibodies were identified by counterimmunoelectrophoresis (CIE) techniques, enzyme-linked immunosorbent assay (ELISA), immunoblotting (IB) using extracts of rabbit thymus and human placenta, and an automated multiparameter line immunoassay system (INNO-LIA ANA UPDATE K-1090) that detects nine different antibodies simultaneously (anti-U1RNP, anti-Sm, anti-Ro/SS-A, anti-La/SS-B, anti-Scl 70, anti-Jo 1, anticentromere, antihistone, and antiribosomal P protein). The line immunoassay system equaled or surpassed the other techniques in the identification of anti-Sm, anti-La/SS-B, anti-Jo 1 and anti-Scl 70 antibodies (sensitivity 100%, specificity 94-100%) and was similarly effective in the case of anti-U1RNP (sensitivity 87.5%, specificity 93.9%) and anti-Ro/SS-A (sensitivity 91.4%, specificity 87.2%) antibodies. In addition, this technique detected more 52 and 60 kD anti-Ro/SS-A sera than IB. Nine antibodies can be detected with this method at a cost of 25.38 Euros per serum sample. In five hours, 19 sera can be studied. The approximate cost of detecting these nine antibodies with an automated ELISA system would be 28.93 Euros, which allows 10 sera to be studied in four hours. In conclusion, the automated multiparameter line immunoassay system is a valid method for the detection of autoantibodies in rheumatic diseases. Its most notable advantages are automated simultaneous detection of several autoantibodies in the same serum and its lower cost compared with ELISA techniques.

The yeast U5 snRNP coisolated with the U1 snRNP has an unexpected protein composition and includes the splicing factor Aar2p.

We describe the purification and characterization of a 16S U5 snRNP from the yeast Saccharomyces cerevisiae and the identification of its proteins. In contrast to the human 20S U5 snRNP, it has a comparatively simple protein composition. In addition to the Sm core proteins, it contains only two of the U5 snRNP specific proteins, Prp8p and Snu114p. Interestingly, the 16S U5 snRNP contains also Aar2p, a protein that was previously implicated in splicing of the two introns of the MATa1 pre-mRNA. Here, we demonstrate that Aar2p is essential and required for in vivo splicing of U3 precursors. However, it is not required for splicing in vitro. Aar2p is associated exclusively with this simple form of the U5 snRNP (Aar2-U5), but not with the [U4/U6.U5] tri-snRNP or spliceosomal complexes. Consistent with this, we show that depletion of Aar2p interferes with later rounds of splicing, suggesting that it has an effect when splicing depends on snRNP recycling. Remarkably, the Aar2-U5 snRNP is invariably coisolated with the U1 snRNP regardless of the purification protocol used. This is consistent with the previously suggested cooperation between the U1 and U5 snRNPs prior to the catalytic steps of splicing. Electron microscopy of the Aar2-U5 snRNP revealed that, despite the comparatively simple protein composition, the yeast Aar2-U5 snRNP appears structurally similar to the human 20S U5 snRNP. Thus, the basic structural scaffold of the Aar2-U5 snRNP seems to be essentially determined by Prp8p, Snu114p, and the Sm proteins.

Purification of Drosophila snRNPs and characterization of two populations of functional U1 particles.

U1 snRNP is required at an early stage during assembly of the spliceosome, the dynamic ribonucleoprotein (RNP) complex that performs nuclear pre-mRNA splicing. Here, we report the purification of U1 snRNP particles from Drosophila nuclear extracts and the characterization of their biochemical properties, polypeptide contents, and splicing activities. On the basis of their antigenicity, apparent molecular weight, and by peptide sequencing, the Drosophila 70K, SNF, B, U1-C, D1, D2, D3, E, F, and G proteins are shown to be integral components of these particles. Sequence database searches revealed that both the U1-specific and the Sm proteins are extensively conserved between human and Drosophila snRNPs. Furthermore, both species possess a conserved intrinsic U1-associated kinase activity with identical substrate specificity in vitro. Finally, our results demonstrate that a second type of functional U1 particle, completely lacking the U1/U2-specific protein SNF and the associated protein kinase activity, can be isolated from cultured Kc cell or Canton S embryonic nuclear extracts. This work describes the first characterization of a purified Drosophila snRNP particle and reinforces the view that their activity and composition, with the exception of the atypical bifunctional U1-A/U2-B" SNF protein, are highly conserved in metazoans.

Nuclear eukaryotic initiation factor 4E (eIF4E) colocalizes with splicing factors in speckles.

The eukaryotic initiation factor 4E (eIF4E) plays a pivotal role in the control of protein synthesis. eIF4E binds to the mRNA 5' cap structure, m(7)GpppN (where N is any nucleotide) and promotes ribosome binding to the mRNA. It was previously shown that a fraction of eIF4E localizes to the nucleus (Lejbkowicz, F., C. Goyer, A. Darveau, S. Neron, R. Lemieux, and N. Sonenberg. 1992. Proc. Natl. Acad. Sci. USA. 89:9612-9616). Here, we show that the nuclear eIF4E is present throughout the nucleoplasm, but is concentrated in speckled regions. Double label immunofluorescence confocal microscopy shows that eIF4E colocalizes with Sm and U1snRNP. We also demonstrate that eIF4E is specifically released from the speckles by the cap analogue m(7)GpppG in a cell permeabilization assay. However, eIF4E is not released from the speckles by RNase A treatment, suggesting that retention of eIF4E in the speckles is not RNA-mediated. 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole (DRB) treatment of cells causes the condensation of eIF4E nuclear speckles. In addition, overexpression of the dual specificity kinase, Clk/Sty, but not of the catalytically inactive form, results in the dispersion of eIF4E nuclear speckles.

Identification of both shared and distinct proteins in the major and minor spliceosomes.

In metazoans, two distinct spliceosomes catalyzing pre-messenger RNA splicing have been identified. Here, the human U11/U12 small nuclear ribonucleoprotein (snRNP), a subunit of the minor (U12-dependent) spliceosome, was isolated. Twenty U11/U12 proteins were identified, including subsets unique to the minor spliceosome or common to both spliceosomes. Common proteins include four U2 snRNP polypeptides that constitute the essential splicing factor SF3b. A 35-kilodalton U11-associated protein homologous to the U1 snRNP 70K protein was also identified. These data provide fundamental information about proteins of the minor spliceosome and shed light on its evolutionary relationship to the major spliceosome.

An inhibitor of nuclear export activates the p53 response and induces the localization of HDM2 and p53 to U1A-positive nuclear bodies associated with the PODs.

Leptomycin B is a cytotoxin which directly interacts with and inhibits the action of CRM1, an essential mediator of the nuclear exit of proteins containing nuclear export signals (NES) of the HIV1 REV type. We show that addition of leptomycin B to human primary fibroblasts increased the levels of the p53 tumor suppressor protein. This was accompanied by the induction of p53-dependent transcriptional activity in cultured cells and an increase in the levels of the products of two p53-responsive genes, the p21(CIP1/WAF1) and HDM2 proteins. Leptomycin B induced the accumulation of p53 and HDM2 in the nucleus and the appearance of discrete nuclear aggregates containing both proteins. It has been reported that the transcriptional activity of p53 is modulated by its interaction with the HDM2 protein which also targets p53 for rapid degradation. Using a model cell line conditionally expressing MDM2, the murine analogue of HDM2, we present evidence indicating that leptomycin B abrogates MDM2's role in p53 degradation and that the accumulation of p53 in distinct nuclear bodies is mediated by MDM2. Since HDM2 has recently been shown to contain a functional NES of the REV type, the most likely explanation for our results is that the effect of leptomycin B on HDM2 and p53 is due to the inhibition of nuclear export. The ability to visualize sites where p53 and HDM2 colocalize provides a new approach to study the association between the two proteins in vivo. These p53/HDM2-positive nuclear foci were found to also contain the U1A snRNP A and to be juxtaposed to the PML oncogenic domains.

Detection and isolation of RNA-binding proteins by RNA-ligand screening of a cDNA expression library.

A screening assay for the detection of RNA-binding proteins was developed. It allows the rapid isolation of cDNA clones coding for proteins with sequence-specific binding affinity to a target RNA. For developing the screening protocol, constituents of the human U1 snRNP were utilized as model system. The RNA partner consisted of the U1-RNA stem-loop II and the corresponding protein consisted of the 102 amino acid N-terminal recognition motif of the U1A protein, which was fused to beta-galactosidase and expressed by the recombinant lambda phage LU1A. Following binding of the fusion protein to nitrocellulose membranes, hybridization with a 32P-labeled U1-RNA ligand was carried out to detect specific RNA-protein interaction. Parameters influencing the specificity and the detection limit of binding were systematically investigated with the aid of the model system. Processing the nitrocellulose membranes in the presence of transition metals greatly increased the signal:background ratio. A simple screening protocol involving a single-buffer system was developed. Specific RNA-protein interaction could be detected in the presence of a large excess of recombinant phages from a cDNA library. Only moderate binding affinities (Kd = 10(-8) M) were required. The suitability of the RNA-ligand screening protocol was demonstrated by the identification of new viroid-RNA binding proteins from tomato.

Analysis of genes for human snRNP Sm-D1 protein and identification of the promoter sequence which shows segmental homology to the promoters of Sm-E and U1 snRNA genes.

The Sm core proteins of U1, U2, U4/U6 and U5 snRNPs include B(B1), B'(B2), N(B3), D1, D2, D3, E, F and G polypeptides. We have isolated genomic clones encoding the Sm-D1 protein using the Sm-D1 cDNA as probe. Southern blotting and DNA sequencing analysis of these clones revealed the presence of an Sm-D1 multigene family in the human genome. Three gene members have been identified. Two of the genes are without introns and contain mutations compared to the cDNA sequence. They appear to be processed pseudogenes. The third gene, termed SNRPD1, shares 100% identity to the cDNA sequence including both 5'- and 3'-untranslated regions (UTR); it contains three introns. Analysis of the 5'-flanking region of the SNRPD1 gene revealed promoter activity, suggesting this is the functional gene that encodes the Sm-D1 protein. The promoter activity was localized in a 0.38 kb PstI fragment using CAT reporter gene fusion assays. Addition of an SV40 enhancer element did not enhance the transcription directed by that fragment. Sequence comparison of the 0.38 kb promoter sequence with the promoters of the Sm-E gene and U1 snRNA genes revealed several homologous motifs, suggesting that genes encoding the snRNP components may be coordinately regulated.

Probing the 3' UTR structure of U1A mRNA and footprinting analysis of its complex with U1A protein.

The structure of the conserved region of the U1A pre-mRNA (AgRNA) and its complex with U1A protein was investigated. The previously proposed secondary structure of Ag RNA, derived from enzymatic probing and analysis of the structure and function of mutant mRNAs, is now confirmed by chemical probing data and further refined in the regions where the enzymatic data were not conclusive. The two unpaired nucleotides in the internal loops opposite of the Box sequences as well as the tetraloop could not be cleaved by ribonucleases, but are accessible to chemical probes. Concerning the RNA-protein complex, the protection experiments showed that the Box regions are largely protected when the U1A protein is present. All stem regions in the 5' part of the structure seem protected against ribonucleases. Unexpectedly, the nucleotides of the tetraloop become accessible to ribonucleases in the RNA-protein complex. This result indicates that the tetraloop undergoes a conformational change upon U1A protein binding. The 3' part of the Ag RNA sequence, containing the polyadenylation signal in a hairpin structure, showed hardly any protection, a finding that agrees with the fact that U1A does not interfere with the binding of the cleavage polyadenylation specificity factor (CPSF) to the polyadenylation signal.

Distinctive immune response patterns of human and murine autoimmune sera to U1 small nuclear ribonucleoprotein C protein.

The Ul small nuclear ribonucleoprotein (snRNP), a complex of nine proteins with Ul RNA, is a frequent target of autoantibodies in human and murine systemic lupus erythematosus (SLE). Anti-Sm antibodies recognizing the B'/B, D, E, F, and G proteins of Ul snRNPs are highly specific for SLE, and are nearly always accompanied by anti-nRNP antibodies recognizing the Ul snRNP-specific 70K, A, and/or C proteins. Previous studies suggest that human anti-nRNP antibodies recognize primarily the U1-70K and Ul-A proteins, whereas recognition of Ul-C is less frequent. We report here that autoantibodies to U1-C are more common in human autoimmune sera than believed previously. Using a novel immunoprecipitation technique to detect autoantibodies to native Ul-C, 75/78 human sera with anti-nRNP/ Sm antibodies were anti-Ul-C (+). In striking contrast, only 1/65 anti-nRNP/Sm (+) MRL mouse sera of various Igh allotypes was positive. Two of ten anti-nRNP/Sm (+) sera from BALB/c mice with a lupus-like syndrome induced by pristane recognized Ul-C. Thus, lupus in MRL mice was characterized by a markedly lower frequency of anti-U1-C antibodies than seen in human SLE or pristane-induced lupus. The results may indicate different pathways of intermolecular-intrastructural diversification of autoantibody responses to the components of Ul snRNPs in human and murine lupus, possibly mediated by alterations in antigen processing induced by the autoantibodies themselves.

Interaction between the U1 snRNP-A protein and the 160-kD subunit of cleavage-polyadenylation specificity factor increases polyadenylation efficiency in vitro.

We have previously shown that the U1 snRNP-A protein (U1A) interacts with elements in SV40 late polyadenylation signal and that this association increases polyadenylation efficiency. It was postulated that this interaction occurs to facilitate protein-protein association between components of the U1 snRNP and proteins of the polyadenylation complex. We have now used GST fusion protein experiments, coimmunoprecipitations and Far Western blot analyses to demonstrate direct binding between U1A and the 160-kD subunit of cleavage-polyadenylation specificity factor (CPSF). In addition, Western blot analyses of fractions from various stages of CPSF purification indicated that U1A copurified with CPSF to a point but could be separated in the highly purified fractions. These data suggest that U1A protein is not an integral component of CPSF but may be able to interact and affect its activity. In this regard, the addition of purified, recombinant U1A to polyadenylation reactions containing CPSF, poly(A) polymerase, and a precleaved RNA substrate resulted in concentration-dependent increases in both the level of polyadenylation and poly(A) tail length. In agreement with the increase in polyadenylation efficiency caused by U1A, recombinant U1A stabilized the interaction of CPSF with the AAUAAA-containing substrate RNA in electrophoretic mobility shift experiments. These findings suggest that, in addition to its function in splicing, U1A plays a more global role in RNA processing through effects on polyadenylation.

Purification of the spliceosome A-complex and its visualization by electron microscopy.

Pre-mRNA splicing occurs on spliceosomes, a family of ribonucleoprotein particles. Spliceosome assembly on exogenous adenovirus pre-mRNA was blocked at the A-complex (or pre-spliceosome) stage, either by destruction of the small nuclear ribonucleoproteins (snRNPs) that comprise the U4/U5/U6 tri-snRNP complex, or by interference in tri-snRNP assembly and interactions. The A-complex was isolated by size exclusion chromatography; homogeneity was shown by electrophoresis in nondenaturing polyacrylamide gels, gradient sedimentation, and electron microscopy. Northern hybridization showed U1 and U2 snRNAs to be present in the preparation, but not U4, U5, or U6. Antibodies specific for a component of the U1 snRNP or for a component that is common to all snRNPs (except U6) each precipitated an A-complex containing pre-mRNA, U1 and U2 snRNPs. Electron micrographs showed 230 x 270-A particles whose two components appear similar to individual U1 and U2 snRNPs. Electron micrographs of an A-complex-5'-biotinyl oligonucleotide-streptavidin-gold composite allowed identification of the U2 snRNP within the structure and the localization of the 5'-segment of U2 snRNA at a unique site in the A-complex. This region of U2 RNA is adjacent to the developing catalytic center of the spliceosome.