Loop I of U1 small nuclear RNA is the only essential RNA sequence for binding of specific U1 small nuclear ribonucleoprotein particle proteins

Sequence-specific interaction of U1 snRNA with the SMN complex

The survival of motor neurons (SMN) protein complex functions in the biogenesis of spliceosomal small nuclear ribonucleoprotein particles (snRNPs) and prob ably other RNPs. All spliceosomal snRNPs have a common core of seven Sm proteins. To mediate the assembly of snRNPs, the SMN complex must be able to bring together Sm proteins with U snRNAs. We showed previously that SMN and other components of the SMN complex interact directly with several Sm proteins. Here, we show that the SMN complex also interacts specifically with U1 snRNA. The stem--loop 1 domain of U1 (SL1) is necessary and sufficient for SMN complex binding in vivo and in vitro. Substitution of three nucleotides in the SL1 loop (SL1A3) abolishes SMN interaction, and the corresponding U1 snRNA (U1A3) is impaired in U1 snRNP biogenesis. Microinjection of excess SL1 but not SL1A3 into Xenopus oocytes inhibits SMN complex binding to U1 snRNA and U1 snRNP assembly. These findings indicate that SMN complex interaction with SL1 is sequence-specific and critical for U1 snRNP biogenesis, further supporting the direct role of the SMN complex in RNP biogenesis.

Identification of Prp40, a novel essential yeast splicing factor associated with the U1 small nuclear ribonucleoprotein particle

We have used suppressor genetics to identify factors that interact with Saccharomyces cerevisiae U1 small nuclear RNA (snRNA). In this way, we isolated PRP40-1, a suppressor that restores growth at 18 degrees C to a strain bearing a cold-sensitive mutation in U1 RNA. A gene disruption experiment shows that PRP40 is an essential gene. To study the role of PRP40 in splicing, we created a pool of temperature-sensitive prp40 strains. Primer extension analysis of intron-containing transcripts in prp40 temperature-sensitive strains reveals a splicing defect, indicating that Prp40 plays a direct role in pre-mRNA splicing. In addition, U1 RNA coimmunoprecipitates with Pro40, indicating that Prp40 is bound to the U1 small nuclear ribonucleoprotein particle in vivo. Therefore, we conclude that PRP40 encodes a novel, essential splicing component that associates with the yeast U1 small nuclear ribonucleoprotein particle.

The amino-terminal domain of yeast U1-70K is necessary and sufficient for function

The Saccharomyces cerevisiae SNP1 gene encodes a protein that shares 30% amino acid identity with the mammalian U1 small nuclear ribonucleoprotein particle protein 70K (U1-70K). We have demonstrated that yeast strains in which the SNP1 gene was disrupted are viable but exhibit greatly increased doubling times and severe temperature sensitivity. Furthermore, snp1-null strains are defective in pre-mRNA splicing. We have tested deletion alleles of SNP1 for their ability to complement these phenotypes. We found that the highly conserved RNA recognition motif consensus domain of Snp1 is not required for complementation of the snp1-null growth or splicing defects nor for the in vivo association with the U1 small nuclear ribonucleoprotein particle. However, the amino-terminal domain of Snp1, less strongly conserved, is necessary and sufficient for complementation.

Commitment of yeast pre-mRNA to the splicing pathway requires a novel U1 small nuclear ribonucleoprotein polypeptide, Prp39p

The binding of a U1 small nuclear ribonucleoprotein (snRNP) particle to the 5' splice site region of a pre-mRNA is a primary step of intron recognition. In this report, we identify a novel 75-kDa polypeptide of Saccharomyces cerevisiae, Prp39p, necessary for the stable interaction of mRNA precursors with the snRNP components of the pre-mRNA splicing machinery. In vivo, temperature inactivation or metabolic depletion of Prp39p blocks pre-mRNA splicing and causes growth arrest. Analyses of cell extracts reveal a specific and dramatic increase in the electrophoretic mobility of the U1 snRNP particle upon Prp39p depletion and demonstrate that extracts deficient in Prp39p activity are unable to form either the CC1 or CC2 commitment complex band characteristic of productive U1 snRNP/pre-mRNA association. Immunological studies establish that Prp39p is uniquely associated with the U1 snRNP and is recruited with the U1 snRNP into splicing complexes. On the basis of these and related observations, we propose that Prp39p functions, at least in part, prior to stable branch point recognition by the U1 snRNP particle to facilitate or stabilize the U1 snRNP/5' splice site interaction.

Commitment of yeast pre-mRNA to the splicing pathway requires a novel U1 small nuclear ribonucleoprotein polypeptide, Prp39p

The binding of a U1 small nuclear ribonucleoprotein (snRNP) particle to the 5' splice site region of a pre-mRNA is a primary step of intron recognition. In this report, we identify a novel 75-kDa polypeptide of Saccharomyces cerevisiae, Prp39p, necessary for the stable interaction of mRNA precursors with the snRNP components of the pre-mRNA splicing machinery. In vivo, temperature inactivation or metabolic depletion of Prp39p blocks pre-mRNA splicing and causes growth arrest. Analyses of cell extracts reveal a specific and dramatic increase in the electrophoretic mobility of the U1 snRNP particle upon Prp39p depletion and demonstrate that extracts deficient in Prp39p activity are unable to form either the CC1 or CC2 commitment complex band characteristic of productive U1 snRNP/pre-mRNA association. Immunological studies establish that Prp39p is uniquely associated with the U1 snRNP and is recruited with the U1 snRNP into splicing complexes. On the basis of these and related observations, we propose that Prp39p functions, at least in part, prior to stable branch point recognition by the U1 snRNP particle to facilitate or stabilize the U1 snRNP/5' splice site interaction.

Epitope regions on U1 small nuclear RNA recognized by anti-U1RNA-specific autoantibodies

Autoantibodies specifically directed to U1RNA were found in patients suffering from systemic lupus erythematosus (SLE) overlap syndromes. To obtain more insight in the mechanism responsible for this U1RNA-specific antibody formation and to use the antibodies eventually as a tool to study U1RNA-protein (U1RNP) interactions, the B cell epitopes on U1RNA were mapped. Using in vitro synthesized domains of U1RNA, the main epitope regions were found in stemloops II and IV. Furthermore, 3'-end or 5'-end truncation of both stemloop II and stemloop IV showed that the conformation of the stemloops is critical for antibody recognition. Mutant studies on both stemloops indicated that in the case of stemloop II the stem is the main antigenic region, whereas in stemloop IV, the loop (E-loop) is a main target. The results of this study support the idea that the anti-U1RNA autoantibody could be the result of a process driven by the human U1RNP complex itself (antigen-driven process).

The yeast homolog of the U1 snRNP protein 70K is encoded by the SNP1 gene

The product of the yeast SNP1 gene has high homology to two domains of the metazoan U1 snRNP protein 70K, which binds to stem/loop I of the U1 RNA. However, the absence of other domains conserved in metazoan 70K and the minimal effect of yeast U1 RNA stem/loop I deletion make the assignment of SNP1 as yeast 70K less clear. To address this question, we have expressed the SNP1 gene as a fusion protein in E. coli and developed a gel shift assay for U1 RNA binding. We show here that the product of the yeast SNP1 gene binds directly and specifically to the first 47 nucleotides of yeast U1 RNA, which include the stem/loop 1 structure. We therefore conclude that the SNP1 gene product is the yeast 70K homolog. This is the first yeast protein to be identified as a homolog of a metazoan snRNP protein.

Structure of the small nuclear RNP particle U1: identification of the two structural protuberances with RNP-antigens A and 70K

We have investigated the structure of the small nuclear RNP (snRNP) U1 by combining EM of complete and partially protein-deficient particles with immunoelectron microscopy employing mAbs against known components of the U1 snRNP. It was found that the two main protuberances of this particle can be identified with the U1-specific proteins A and 70K. The 70K protuberance is the one lying closer to the 5' terminus of the snRNA, as identified by its 5'-terminal m3G cap. The round-shaped main body of U1 snRNP represents its core RNP domain containing the common snRNP proteins. Functional implications of these results are discussed. Our results may also point to the physical basis for the production of autoantibodies directed against specific groups of snRNP proteins. The physical grouping of the common proteins (Sm epitopes) and the specific proteins (RNP epitopes) could result in one or the other being presented to the immune system as is the case in patients suffering from SLE or MCTD, respectively.

More than half of yeast U1 snRNA is dispensable for growth

Yeast U1 snRNA (568 nucleotides) is 3.5-fold larger than its mammalian counterpart (164 nucleotides) and contains apparent sequence homology only at the 5' and 3' ends. We have used deletion analysis to determine whether the yeast-specific U1 sequences play essential roles in vivo. Yeast cells carrying a deletion of more than 60% (355 nucleotides) of the single-copy U1 gene are viable, though slow-growing, while a deletion of 316 nucleotides allows essentially wild-type growth. The boundaries of the viable deletions define a dispensable internal domain which comprises sequences unique to yeast. In contrast, the essential 5' and 3' terminal domains correspond to phylogenetically conserved sequences and/or structures previously implicated in RNA:RNA and RNA:protein interactions. The minimal essential sequences of yeast U1 can be drawn in a secondary structure which resembles metazoan U1 in four of seven structural domains.

Developmental expression of plant snRNAs

Although the number of plant U1, U2, U4 and U5 small nuclear RNA (snRNA) variants sequenced has steadily increased over the past few years, the function of these variants in plant splicing is still not understood. In an effort to elucidate the function of plant snRNA variants, we have examined the expression of U1-U6 snRNA variants during pea seedling development. In contrast to mammalian nuclei which express a single, abundant form of each snRNA, pea nuclei express several equally abundant variants of the same snRNA. Comparison of the snRNAs in pea seeds and seedlings has revealed that four (U1, U2, U4, U5) of the five snRNAs required for pre-mRNA splicing have differentially- and developmentally-regulated forms detectable on Northerns. Only U6 snRNA, which fractionates as a single species on Northerns, appears to be constitutively expressed. Switches in the expression of the pea U1, U2 and U4 snRNAs occur at three distinct stages in development: seed maturation, seed germination and seedling maturation. Surprisingly, the snRNA profiles of mature desiccated seeds and mature leaf tissues are nearly identical and different from developing seeds and seedlings suggesting that switches in the snRNA population occur at transitions between active and inactive transcription. Sequence analysis and differential hybridization of the U1 snRNA variants has demonstrated that some of the developmentally-regulated forms represent sequence variants. We conclude that select subsets of pea snRNAs accumulate at particular stages during plant development.

cDNA cloning of U1, U2, U4 and U5 snRNA families expressed in pea nuclei

Differences observed between plant and animal pre-mRNA splicing may be the result of primary or secondary structure differences in small nuclear RNAs (snRNAs). A cDNA library of pea snRNAs was constructed from anti-trimethylguanosine (m3(2,2,7)G immunoprecipitated pea nuclear RNA. The cDNA library was screened using oligo-deoxyribonucleotide probes specific for the U1, U2, U4 and U5 snRNAs. cDNA clones representing U1, U2, U4 and U5 snRNAs expressed in seedling tissue have been isolated and sequenced. Comparison of the pea snRNA variants with other organisms suggest that functionally important primary sequences are conserved phylogenetically even though the overall sequences have diverged substantially. Structural variations in U1 snRNA occur in regions required for U1-specific protein binding. In light of this sequence analysis, it is clear that the dicot snRNA variants do not differ in sequences implicated in RNA:RNA interactions with pre-mRNA. Instead, sequence differences occur in regions implicated in the binding of small ribonucleoproteins (snRNPs) to snRNAs and may result in the formation of unique snRNP particles.

Leucine periodicity of U2 small nuclear ribonucleoprotein particle (snRNP) A' protein is implicated in snRNP assembly via protein-protein interactions

Recombinant A' protein could be reconstituted into U2 small nuclear ribonucleoprotein particles (snRNPs) upon addition to HeLa cell extracts as determined by coimmunoprecipitation and particle density; however, direct binding to U2 RNA could not be demonstrated except in the presence of the U2 snRNP B" protein. Mutational analysis indicated that a central core region of A' was required for particle reconstitution. This region consists of five tandem repeats of approximately 24 amino acids each that exhibit a periodicity of leucine and asparagine residues that is distinct from the leucine zipper. Similar leucine-rich (Leu-Leu motif) repeats are characteristic of a diverse array of soluble and membrane-associated proteins from yeasts to humans but have not been reported previously to reside in nuclear proteins. Several of these proteins, including Toll, chaoptin, RNase/angiogenin inhibitors, lutropin-choriogonadotropin receptor, carboxypeptidase N, adenylyl cyclase, CD14, and human immunodeficiency virus type 1 Rev, may be involved in protein-protein interactions. Our findings suggest that in cell extracts the Leu-Leu motif of A' is required for reconstitution with U2 snRNPs and perhaps with other components involved in splicing through protein-protein interactions.

Leucine periodicity of U2 small nuclear ribonucleoprotein particle (snRNP) A' protein is implicated in snRNP assembly via protein-protein interactions

Recombinant A' protein could be reconstituted into U2 small nuclear ribonucleoprotein particles (snRNPs) upon addition to HeLa cell extracts as determined by coimmunoprecipitation and particle density; however, direct binding to U2 RNA could not be demonstrated except in the presence of the U2 snRNP B" protein. Mutational analysis indicated that a central core region of A' was required for particle reconstitution. This region consists of five tandem repeats of approximately 24 amino acids each that exhibit a periodicity of leucine and asparagine residues that is distinct from the leucine zipper. Similar leucine-rich (Leu-Leu motif) repeats are characteristic of a diverse array of soluble and membrane-associated proteins from yeasts to humans but have not been reported previously to reside in nuclear proteins. Several of these proteins, including Toll, chaoptin, RNase/angiogenin inhibitors, lutropin-choriogonadotropin receptor, carboxypeptidase N, adenylyl cyclase, CD14, and human immunodeficiency virus type 1 Rev, may be involved in protein-protein interactions. Our findings suggest that in cell extracts the Leu-Leu motif of A' is required for reconstitution with U2 snRNPs and perhaps with other components involved in splicing through protein-protein interactions.

U1 small nuclear RNA from Schizosaccharomyces pombe has unique and conserved features and is encoded by an essential single-copy gene

We have cloned, sequenced, and disrupted the gene encoding U1 small nuclear RNA (snRNA) in the fission yeast Schizosaccharomyces pombe. This RNA is close in size and exhibits a high degree of secondary structure homology to human U1 RNA. There exist two regions of extended primary sequence identity between S. pombe and human U1 RNAs; the first comprises nucleotides involved in hydrogen bonding to 5' splice junctions, and the second is a single-stranded region which, in the human snRNA, forms part of the A protein binding site. S. pombe U1 lacks two nucleotides just following the 5' cap structure which are present in all other U1 homologs examined to date, and the region which corresponds to the binding site for the human 70K protein (molecular weight of 55,000) is more divergent than in other organisms. A putative upstream transcription signal is conserved in sequence and location among all loci encoding spliceosomal snRNAs in S. pombe with the exception of U6. Disruption of the single-copy U1 gene, designated snu1, reveals that this RNA is indispensable for viability.

U1 small nuclear RNA from Schizosaccharomyces pombe has unique and conserved features and is encoded by an essential single-copy gene

We have cloned, sequenced, and disrupted the gene encoding U1 small nuclear RNA (snRNA) in the fission yeast Schizosaccharomyces pombe. This RNA is close in size and exhibits a high degree of secondary structure homology to human U1 RNA. There exist two regions of extended primary sequence identity between S. pombe and human U1 RNAs; the first comprises nucleotides involved in hydrogen bonding to 5' splice junctions, and the second is a single-stranded region which, in the human snRNA, forms part of the A protein binding site. S. pombe U1 lacks two nucleotides just following the 5' cap structure which are present in all other U1 homologs examined to date, and the region which corresponds to the binding site for the human 70K protein (molecular weight of 55,000) is more divergent than in other organisms. A putative upstream transcription signal is conserved in sequence and location among all loci encoding spliceosomal snRNAs in S. pombe with the exception of U6. Disruption of the single-copy U1 gene, designated snu1, reveals that this RNA is indispensable for viability.

The PRP4 (RNA4) protein of Saccharomyces cerevisiae is associated with the 5' portion of the U4 small nuclear RNA

We have combined oligonucleotide-directed RNase H degradation and immunoprecipitation in a study of the association of the Saccharomyces cerevisiae PRP4 protein with the U4-U6 complex. We have found that three oligonucleotides were able to direct nearly to completion the RNase H-specific cleavage of the target RNA molecules as they exist in splicing extracts. Immunoprecipitation of the degradation products with PRP4 antibody showed that the 5' portion of U4 small nuclear RNA (snRNA) and the 3' portion of U6 snRNA coimmunoprecipitated with the PRP4 protein. Micrococcal nuclease protection experiments confirmed further that the 5' portion and 3' end of U4 snRNA were very resistant to nuclease digestion, whereas the 3' portion of U6 snRNA was protected to only a very small extent. We conclude that the PRP4 protein of S. cerevisiae is associated primarily with the 5' portion of U4 snRNA in the U4-U6 small nuclear ribonucleoprotein (snRNP).

The PRP4 (RNA4) protein of Saccharomyces cerevisiae is associated with the 5' portion of the U4 small nuclear RNA

We have combined oligonucleotide-directed RNase H degradation and immunoprecipitation in a study of the association of the Saccharomyces cerevisiae PRP4 protein with the U4-U6 complex. We have found that three oligonucleotides were able to direct nearly to completion the RNase H-specific cleavage of the target RNA molecules as they exist in splicing extracts. Immunoprecipitation of the degradation products with PRP4 antibody showed that the 5' portion of U4 small nuclear RNA (snRNA) and the 3' portion of U6 snRNA coimmunoprecipitated with the PRP4 protein. Micrococcal nuclease protection experiments confirmed further that the 5' portion and 3' end of U4 snRNA were very resistant to nuclease digestion, whereas the 3' portion of U6 snRNA was protected to only a very small extent. We conclude that the PRP4 protein of S. cerevisiae is associated primarily with the 5' portion of U4 snRNA in the U4-U6 small nuclear ribonucleoprotein (snRNP).

Structure-probing of U1 snRNPs gradually depleted of the U1-specific proteins A, C and 70k. Evidence that A interacts differentially with developmentally regulated mouse U1 snRNA variants

The interaction of the U1-specific proteins 70k, A and C with U1 snRNP was studied by depleting gradually U1 snRNPs of the U1-specific proteins by Mono-Q chromatography at elevated temperatures (20-37 degrees C). U1 snRNP species were obtained which were selectively depleted of either protein C, A, C and A, or of all three U1-specific proteins C, A and 70k while retaining the common proteins B' to G. These various types of U1 snRNP particles were used to study the differential accessibility of defined regions of U1 RNA towards nucleases V1 and S1 dependent on the U1 snRNP protein composition. The data indicate that in the U1 snRNP protein 70k interacts with stem/loop A and protein A with stem/loop B of U1 RNA. The presence or absence of protein C did not affect the nuclease digestion patterns of U1 RNA. Our results suggest further that the binding of protein A to the U1 snRNP particle should be independent of proteins 70k and C. Mouse cells contain two U1 RNA species, U1a and U1b, which differ in the structure of stem/loop B, with U1a exhibiting the same stem/loop B sequence as U1 RNA from HeLa cells. When we used Mono Q chromatography to investigate possible structural differences in the two types of U1 snRNPs, we observed that protein A was always preferentially lost from U1b snRNP as compared to U1a snRNPs. This indicates that one consequence of the structural difference between U1a and U1b is a lowering of the strength of binding of protein A to U1b snRNP. The possible functional significance of this finding is discussed with respect to the fact that U1b RNA is preferentially expressed in embryonal cells.

A specific 31-nucleotide domain of U1 RNA directly interacts with the 70K small nuclear ribonucleoprotein component

We have defined the nucleotide sequence of a protein-binding domain within U1 RNA that specifically recognizes and binds both to a U1 small nuclear ribonucleoprotein component (the 70K protein) and to the previously defined RNA-binding domain of the 70K protein. We have investigated direct interactions between purified U1 RNA and 70K protein by reconstitution in vitro. Thirty-one nucleotides of U1 RNA, corresponding to stem-loop I, were required for this interaction. Nucleotides at the 5' end of U1 RNA that are involved in base pairing with the 5' splice site of pre-mRNA were not required for binding. In contrast to other reports, these findings demonstrate that a specific domain of U1 RNA can bind directly to the 70K protein independently of any other snRNP-associated proteins.

Direct, sequence-specific binding of the human U1-70K ribonucleoprotein antigen protein to loop I of U1 small nuclear RNA

We have studied the interaction of two of the U1 small nuclear ribonucleoprotein (snRNP)-specific proteins, U1-70K and U1-A, with U1 small nuclear RNA (snRNA). The U1-70K protein is a U1-specific RNA-binding protein. Deletion and mutation analyses of a beta-galactosidase/U1-70K partial fusion protein indicated that the central portion of the protein, including the RNP sequence domain, is both necessary and sufficient for specific U1 snRNA binding in vitro. The highly conserved eight-amino-acid RNP consensus sequence was found to be essential for binding. Deletion and mutation analyses of U1 snRNA showed that both the U1-70K fusion protein and the native HeLa U1-70K protein bound directly to loop I of U1 snRNA. Binding was sequence specific, requiring 8 of the 10 bases in the loop. The U1-A snRNP protein also interacted specifically with U1 snRNA, principally with stem-loop II.

Initiation and termination of human U1 RNA transcription requires the concerted action of multiple flanking elements

Sequences in the 5' flanking region of small nuclear RNA (snRNA) genes are responsible for recognition of 3' end signals. Formation of the pre-U1 3' end occurs at the downstream signal closest to the promoter, probably by transcription termination. We have analyzed promoter elements for their participation in formation of the 3' ends of pre-U1 RNA. To do this, a human U1 RNA gene with deletions in individual promoter elements was microinjected into Xenopus laevis oocytes and the resulting RNAs were analyzed by a nuclease S1 protection assay. Each of the promoter elements, except element B (the functional equivalent of a TATA box), was shown to be dispensable for recognition of the snRNA 3' end signal. This latter element was necessary, but not sufficient, for initiation of transcription; so its possible role in termination could not be assessed. Therefore, it is likely that recognition of the 3' end signal is an inherent feature of transcription complexes that initiate at an snRNA promoter.

U1 small nuclear ribonucleoprotein particle-specific proteins interact with the first and second stem-loops of U1 RNA, with the A protein binding directly to the RNA independently of the 70K and Sm proteins

The U1 small nuclear ribonucleoprotein particle (U1 snRNP), a cofactor in pre-mRNA splicing, contains three proteins, termed 70K, A, and C, that are not present in the other spliceosome-associated snRNPs. We studied the binding of the A and C proteins to U1 RNA, using a U1 snRNP reconstitution system and an antibody-induced nuclease protection technique. Antibodies that reacted with the A and C proteins induced nuclease protection of the first two stem-loops of U1 RNA in reconstituted U1 snRNP. Detailed analysis of the antibody-induced nuclease protection patterns indicated the existence of relatively long-range protein-protein interactions in the U1 snRNP, with the 5' end of U1 RNA and its associated specific proteins interacting with proteins bound to the Sm domain near the 3' end. UV cross-linking experiments in conjunction with an A-protein-specific antibody demonstrated that the A protein bound directly to the U1 RNA rather than assembling in the U1 snRNP exclusively via protein-protein interactions. This conclusion was supported by additional experiments revealing that the A protein could bind to U1 RNA in the absence of bound 70K and Sm core proteins.

A specific 31-nucleotide domain of U1 RNA directly interacts with the 70K small nuclear ribonucleoprotein component

We have defined the nucleotide sequence of a protein-binding domain within U1 RNA that specifically recognizes and binds both to a U1 small nuclear ribonucleoprotein component (the 70K protein) and to the previously defined RNA-binding domain of the 70K protein. We have investigated direct interactions between purified U1 RNA and 70K protein by reconstitution in vitro. Thirty-one nucleotides of U1 RNA, corresponding to stem-loop I, were required for this interaction. Nucleotides at the 5' end of U1 RNA that are involved in base pairing with the 5' splice site of pre-mRNA were not required for binding. In contrast to other reports, these findings demonstrate that a specific domain of U1 RNA can bind directly to the 70K protein independently of any other snRNP-associated proteins.

The U1 RNA-binding site of the U1 small nuclear ribonucleoprotein (snRNP)-associated A protein suggests a similarity with U2 snRNPs

The site of interaction between human U1 RNA and one of its uniquely associated proteins, A, was examined with in vitro binding assays. The A protein bound directly to stem-loop II of U1 RNA in a region which exhibits sequence similarity to U2 RNA. The similarity with U2 RNA was in a region that has been shown to interact with a U2 RNA-associated protein. The A protein-binding site on U1 RNA overlapped a previously described epitope for an RNA-specific human autoantibody (S. L. Deutscher and J. D. Keene, Proc. Natl. Acad. Sci. USA 85:3299-3303, 1988), supporting the hypothesis that the anti-RNA antibody originated as an anti-idiotypic response to A protein-specific autoantibodies.

Direct, sequence-specific binding of the human U1-70K ribonucleoprotein antigen protein to loop I of U1 small nuclear RNA

We have studied the interaction of two of the U1 small nuclear ribonucleoprotein (snRNP)-specific proteins, U1-70K and U1-A, with U1 small nuclear RNA (snRNA). The U1-70K protein is a U1-specific RNA-binding protein. Deletion and mutation analyses of a beta-galactosidase/U1-70K partial fusion protein indicated that the central portion of the protein, including the RNP sequence domain, is both necessary and sufficient for specific U1 snRNA binding in vitro. The highly conserved eight-amino-acid RNP consensus sequence was found to be essential for binding. Deletion and mutation analyses of U1 snRNA showed that both the U1-70K fusion protein and the native HeLa U1-70K protein bound directly to loop I of U1 snRNA. Binding was sequence specific, requiring 8 of the 10 bases in the loop. The U1-A snRNP protein also interacted specifically with U1 snRNA, principally with stem-loop II.

Molecular analysis of eight U1 RNA gene candidates from tomato that could potentially be transcribed into U1 RNA sequence variants differing from each other in similar regions of secondary structure

From a tomato genomic library we isolated and characterized eight U1 RNA gene candidates (U1.1 to U1.8) all of which possessed the canonical plant U-snRNA transcription signals in their 5' and 3' flanking regions and exhibited nucleotide sequence conservation in the 5' splice site recognition sequence, in the Sm antigen binding site and in Loops B, C, D as well as in Stems III and IV of their coding region. Deviations from the U1 RNA consensus sequence were mainly localized to Loop A and Stems I and II, suggesting that the putative transcripts of the tomato U1.1-U1.8 genes would differ from each other in their capacity of binding to the U1 RNA-specific snRNP proteins.

U1 small nuclear ribonucleoprotein particle-specific proteins interact with the first and second stem-loops of U1 RNA, with the A protein binding directly to the RNA independently of the 70K and Sm proteins

The U1 small nuclear ribonucleoprotein particle (U1 snRNP), a cofactor in pre-mRNA splicing, contains three proteins, termed 70K, A, and C, that are not present in the other spliceosome-associated snRNPs. We studied the binding of the A and C proteins to U1 RNA, using a U1 snRNP reconstitution system and an antibody-induced nuclease protection technique. Antibodies that reacted with the A and C proteins induced nuclease protection of the first two stem-loops of U1 RNA in reconstituted U1 snRNP. Detailed analysis of the antibody-induced nuclease protection patterns indicated the existence of relatively long-range protein-protein interactions in the U1 snRNP, with the 5' end of U1 RNA and its associated specific proteins interacting with proteins bound to the Sm domain near the 3' end. UV cross-linking experiments in conjunction with an A-protein-specific antibody demonstrated that the A protein bound directly to the U1 RNA rather than assembling in the U1 snRNP exclusively via protein-protein interactions. This conclusion was supported by additional experiments revealing that the A protein could bind to U1 RNA in the absence of bound 70K and Sm core proteins.

The U1 RNA-binding site of the U1 small nuclear ribonucleoprotein (snRNP)-associated A protein suggests a similarity with U2 snRNPs

The site of interaction between human U1 RNA and one of its uniquely associated proteins, A, was examined with in vitro binding assays. The A protein bound directly to stem-loop II of U1 RNA in a region which exhibits sequence similarity to U2 RNA. The similarity with U2 RNA was in a region that has been shown to interact with a U2 RNA-associated protein. The A protein-binding site on U1 RNA overlapped a previously described epitope for an RNA-specific human autoantibody (S. L. Deutscher and J. D. Keene, Proc. Natl. Acad. Sci. USA 85:3299-3303, 1988), supporting the hypothesis that the anti-RNA antibody originated as an anti-idiotypic response to A protein-specific autoantibodies.

U2 small nuclear RNP assembly in vitro

Incubation of a SP6-transcribed human U2 RNA precursor molecule in a HeLa cell S100 fraction resulted in the formation of ribonucleoprotein complexes. In the presence of ATP, the particles that assembled had several properties of native U2 snRNP, including resistance to dissociation in Cs2SO4 gradients, their buoyant density, and pattern of digestion by micrococcal nuclease. These particles also reacted with Sm monoclonal antibody and a human autoantibody with specificity for the U2 snRNP-specific proteins A' and B", but not with antibodies for U1 snRNP-specific proteins. In contrast, the particles that formed in the absence of ATP did not have these properties. ATP analogs with non-hydrolyzable beta-gamma bonds did not substitute for ATP in U2 snRNP assembly. Additional experiments with a mutant U2 RNA confirmed that nucleotides 154-167 of U2 RNA are required for binding of the U2 snRNP-specific proteins but not of the "Sm" core proteins. Pseudouridine formation, a major post-transcriptional modification of U2 RNA, was enhanced under assembly permissive conditions.