A U5 small nuclear ribonucleoprotein particle protein involved only in the second step of pre-mRNA splicing in Saccharomyces cerevisiae

The PRP18 gene, which had been identified in a screen for pre-mRNA splicing mutants in Saccharomyces cerevisiae, has been cloned and sequenced. Yeast strains bearing only a disrupted copy of PRP18 are temperature sensitive for growth; even at a low temperature, they grow extremely slowly and do not splice pre-mRNA efficiently. This unusual temperature sensitivity can be reproduced in vitro; extracts immunodepleted of PRP18 are temperature sensitive for the second step of splicing. The PRP18 protein has been overexpressed in active form in Escherichia coli and has been purified to near homogeneity. Antibodies directed against PRP18 precipitate the U4/U5/U6 small nuclear ribonucleoprotein particle (snRNP) from yeast extracts. From extracts depleted of the U6 small nuclear RNA (snRNA), the U4 and U5 snRNAs can be immunoprecipitated, while no snRNAs can be precipitated from extracts depleted of the U5 snRNA. PRP18 therefore appears to be primarily associated with the U5 snRNP. The antibodies against PRP18 inhibit the second step of pre-mRNA splicing in vitro. Together, these results imply that the U5 snRNP plays a role in the second step of splicing and suggest a model for the action of PRP18.

PRP19: a novel spliceosomal component

We have isolated the gene of a splicing factor, PRP19, by complementation of the temperature-sensitive growth defect of the prp19 mutant of Saccharomyces cerevisiae. The gene encodes a protein of 502 amino acid residues of molecular weight 56,500, with no homology to sequences in the data base. Unlike other PRP proteins or mammalian splicing factors, the sequence of PRP19 has no discernible motif. Immunoprecipitation studies showed that PRP19 is associated with the spliceosome during the splicing reaction. Although the exact function of PRP19 remains unknown, PRP19 appears to be distinct from the other PRP proteins or other spliceosomal components.

Novel activity of a yeast ligase deletion polypeptide. Evidence for GTP-dependent tRNA splicing

Yeast tRNA ligase possesses multiple activities which are required for the joining of tRNA halves during the tRNA splicing process: cyclic phosphodiesterase, kinase, adenylylate synthetase, and ligase. A deletion polypeptide of a dihydrofolate reductase-ligase fusion protein, designated DAC, was previously shown to join tRNA halves although ATP-dependent kinase activity was not measurable in the assay used. We describe here a characterization of the mechanism of joining used by DAC and the structure of the tRNA product. DAC produces a joined tRNA and a splice junction with a structure identical to that produced by DAKC, the full-length dihydrofolate reductase-ligase fusion. Furthermore, DAC can use GTP as the sole cofactor in the joining reaction, in contrast to DAKC, which can only complete splicing in the presence of ATP. Both enzymes exhibit GTP-dependent kinase activity at 100-fold greater efficiency than with ATP. These results suggest that a potential function for the center domain of tRNA ligase (missing in DAC) is to provide structural integrity and aid in substrate interactions and specificity. They also support the hypothesis that ligase may prefer to use two different cofactors during tRNA splicing.

Multiple nucleotide cofactor use by yeast ligase in tRNA splicing. Evidence for independent ATP- and GTP-binding sites

We have examined multiple cofactor usage by yeast tRNA ligase in splicing in vitro. The ligase mechanism of action requires expenditure of two molar equivalents of nucleotide cofactor per mole of tRNA product. Recent evidence (Westaway, S.K., Belford, H.G., Apostol, B.L., Abelson, J., and Greer, C.L. (1993) J. Biol. Chem. 268, 2435-2443) demonstrated that the ligase-associated kinase activity is more efficient with GTP as cofactor than with ATP. Employing a ligase fusion construct with dihydrofolate reductase (Apostol, B.L., Westaway, S.K., Abelson, J., and Greer, C.L. (1991) J. Biol. Chem. 266, 7445-7455) for purposes of enzyme purification, we performed joining assays demonstrating that ATP and GTP are the most effective combination of cofactors. ATP was essential to the joining reaction, while UTP, CTP, or ATP replaced GTP inefficiently. Specific and functionally independent binding sites were confirmed for ATP and GTP by direct binding measurement. A third site was implicated in UTP- and CTP-ligase interactions. Comparison of binding constants with Kapp values determined for nucleotide-dependent joining suggested both that nucleotide triphosphate binding may be limiting in tRNA joining and that tRNA ligation occurs most efficiently using GTP for the kinase reaction and ATP as the adenylylate synthetase cofactor.

Stages in the second reaction of pre-mRNA splicing: the final step is ATP independent

We have analyzed pre-mRNA splicing in yeast extracts immunodepleted of the PRP18 protein. We find that while the first step of splicing (cleavage at the 5' splice site, and generation of the exon 1 and lariat intermediates) is unaffected by the absence of PRP18, the second step of splicing (excision of the lariat intron and formation of mRNA) is substantially slower in the absence of PRP18. The splicing intermediates that are formed in the absence of PRP18 can be rapidly chased into products by the addition of purified PRP18 protein. This chasing is not dependent on ATP, implying that ATP is not required during the second cleavage-and-ligation reaction. This result suggests that there are ordered stages within the second step of splicing and that PRP18 acts late in the second step, perhaps during the catalytic step. The ATP independence also supports the idea that this reaction proceeds by a transesterification mechanism.

Yeast tRNA-splicing endonuclease cleaves precursor tRNA in a random pathway

Introns interrupt many of the tRNA genes of Saccharomyces cerevisiae at a constant position in the anticodon loop. Pre-tRNA transcripts must be accurately cleaved at 3' and 5' splice sites by tRNA endonuclease to release these introns. In order to study splice site cleavage order, substrates were prepared in which the ribose 2'-OH at each of the splice sites was phosphorylated. This modification blocked cleavage by the endonuclease. We found that whichever splice site was blocked the endonuclease can cleave the other site, indicating that the two splice sites were cleaved independently. The endonuclease also cleaved both 3'- and 5'-nicked pre-tRNA(Phe). In addition, both kinds of "2/3 molecules" (exon+intron) were observed in kinetic studies, indicating that they were true biochemical intermediates. The rates of cleavage at the 3' and 5' splice sites of pre-tRNA were compared in several ways. The results showed that the endonuclease cleaves 3' and 5' sites at almost the same rate in the first cleavage, whereas in the second cleavage the 3' site was cleaved faster, indicating that the rates of the two routes for cleavage were unequal. These results demonstrated that the endonuclease cleaved pre-tRNA in a random order, creating two routes for removal of introns from pre-tRNA.

Evidence for a base-pairing interaction between U6 small nuclear RNA and 5' splice site during the splicing reaction in yeast

U6 small nuclear RNA (snRNA) is an essential factor in mRNA splicing. On the basis of the high conservation of its sequence, it has been proposed that U6 snRNA may function catalytically during the splicing reaction. If this is the case, it is likely that U6 snRNA interacts with the splice sites in the spliceosome to catalyze the reaction. We have used UV crosslinking to analyze the interactions of U6 snRNA with the splicing substrates during the yeast splicing reaction. Crosslinked products in which the central region of U6 snRNA was joined to the 5' splice site region of mRNA precursor and lariat intermediate were identified. The crosslinking sites were precisely located in one of these products. The results suggest a possible base-pairing interaction between U6 snRNA and the 5' splice site of the mRNA precursor.

Mutational analysis of the yeast U2 snRNA suggests a structural similarity to the catalytic core of group I introns

We have used an in vitro reconstitution system to determine the effects of a large number of mutations in the highly conserved 5' terminal domain of the yeast U2 snRNA on pre-mRNA splicing. Whereas many mutations have little or no functional consequence, base substitutions in two regions were found to have drastic effects on pre-mRNA splicing. A previously unrecognized function for the U2 snRNA in the second step of splicing was found by alteration of the absolutely conserved sequence AGA upstream of the branch point recognition sequence. The effects of these mutations suggest the formation of a structure involving the U2 snRNA similar to the guanosine-binding site found in the catalytic core of group I introns.

Thiophosphates in yeast U6 snRNA specifically affect pre-mRNA splicing in vitro

A thorough mutational analysis of U6 RNA in combination with a functional reconstitution assay, revealed that three domains are important for U6 function in pre-mRNA splicing. In order to further analyze why these regions are so critical for splicing, we make use of phosphorothioate substituted U6 RNAs. Wild-type U6 RNA was transcribed in vitro with T7 RNA polymerase in the presence of either phosphorothiate (alpha-S) ATP, GTP, UTP or CTP. The functionality of the transcripts was monitored by in vitro reconstitution. While substitution with alpha-S ATP, GTP or UTP blocked splicing, substitution with alpha-S CTP had little or no effect on splicing. We made use of this alpha-S CTP effect in an attempt to elucidate which phosphates in the U6 RNA molecule play a role in the first or in the second step of splicing. U6 mutants in which a change of an A, G or U to C does not have any significant effect on splicing were transcribed in the presence of alpha-S CTP. Observed effects on splicing thus have to be attributed to the presence of the thio-substituted phosphate group rather than the nucleotide change. The results of in vitro reconstitution give a clear answer for at least three phosphates; two of them play a role in the first step, while one of them is involved in the second step of splicing.

Eight base changes are sufficient to convert a leucine-inserting tRNA into a serine-inserting tRNA

Each aminoacyl-tRNA synthetase must functionally distinguish its cognate tRNAs from all others. We have determined the minimum number of changes required to transform a leucine amber suppressor tRNA to serine identity. Eight changes are required. These are located in the acceptor stem and in the D stem.

Yeast tRNA ligase mutants are nonviable and accumulate tRNA splicing intermediates

We show here that yeast tRNA ligase protein is essential in the cell and participates in joining together tRNA half-molecules resulting from excision of the intron by the splicing endonuclease. A haploid yeast strain carrying a chromosomal deletion of the ligase gene is viable only if ligase protein can be supplied from a plasmid copy of the gene. When synthesis of the plasmid-borne ligase gene is repressed, cells eventually die and accumulate endonuclease cut but unligated half-molecules and intervening sequences. Half-molecules that accumulate appear to be fully end-processed. Two temperature-sensitive ligase mutant strains have been isolated; these strains accumulate a similar set of unligated half-molecules at the nonpermissive temperature.

An essential 45 kDa yeast transmembrane protein reacts with anti-nuclear pore antibodies: purification of the protein, immunolocalization and cloning of the gene

A yeast membrane protein was isolated by its binding to tRNA Sepharose column. The 45 kDa protein shares characteristics with rat liver nuclear pore proteins in having reactivity with a monoclonal antibody (RL1) raised against rat liver nuclear pore proteins and by the binding of wheat germ agglutinin (WGA), indicating the presence of N-acetylglucosamine (GlcNAc) moieties. Immunofluorescence microscopy and cell fractionation experiments indicate that the protein is located in the nuclear envelope and the endoplasmic reticulum of the cell. The gene for the 45 kDa protein was cloned using degenerate oligonucleotides derived from the N-terminal protein sequence and confirmed by internal peptide sequences. The gene was named WBP1. The protein coding sequence of the WBP1 gene reveals an ER entry signal peptide and a C-terminal membrane spanning domain. Topological studies indicate that the C-terminus of the protein is located in the cytoplasm. The cytoplasmic tail of the protein contains the K-K-X-X signal known to be sufficient for retention of transmembrane proteins in higher eukaryotic cells. Gene disruption experiments show that the 45 kDa protein is essential for the vegetative life cycle of the yeast cell.

Deletion analysis of a multifunctional yeast tRNA ligase polypeptide. Identification of essential and dispensable functional domains

Splicing of tRNA precursors in extracts of Saccharomyces cerevisiae requires the action of two enzymes: a site specific endonuclease and a tRNA ligase. The tRNA ligase contains three distinct enzymatic activities: a polynucleotide kinase, a cyclic phosphodiesterase, and an RNA ligase. The polypeptide also has a high affinity pre-tRNA binding site based on its ability to form stable complexes with pre-tRNA substrates. To investigate the organization of functional enzymatic and binding elements within the polypeptide a series of defined tRNA ligase gene deletions were constructed and corresponding proteins were expressed in Escherichia coli as fusions with bacterial dihydrofolate reductase (DHFR). The DHFR/ligase derivative proteins were then efficiently purified by affinity chromatography. The complete ligase fusion protein retained enzymatic and binding activities which were unaffected by the presence of the DHFR segment. Examination of tRNA ligase deletion derivatives revealed that the amino-terminal region was required for adenylylation, while the carboxyl-terminal region was sufficient for cyclic phosphodiesterase activity. Deletions within the central region affected kinase activity. Pre-tRNA binding activity was not strictly correlated with a distinct enzymatic domain. A DHFR/ligase-derived protein lacking kinase activity efficiently joined tRNA halves. We postulate that this variant utilizes a novel RNA ligation mechanism.

Pre-mRNA splicing in yeast

Splicing of introns from nuclear precursor messenger RNAs (pre-mRNAs) occurs in all eukaryotes. Two aspects of the splicing mechanism need to be understood: how intron sequences are recognized and aligned and how splicing is catalysed. Recent genetic and biochemical studies in the simple eukaryote Saccharomyces cerevisiae are revealing some of the features of the splicing mechanism.

SSB-1 of the yeast Saccharomyces cerevisiae is a nucleolar-specific, silver-binding protein that is associated with the snR10 and snR11 small nuclear RNAs

SSB-1, the yeast single-strand RNA-binding protein, is demonstrated to be a yeast nucleolar-specific, silver-binding protein. In double-label immunofluorescence microscopy experiments antibodies to two other nucleolar proteins, RNA Pol I 190-kD and fibrillarin, were used to reveal the site of rRNA transcription; i.e., the fibrillar region of the nucleolus. SSB-1 colocalized with fibrillarin in a double-label immunofluorescence mapping experiment to the yeast nucleolus. SSB-1 is located, though, over a wider region of the nucleolus than the transcription site marker. Immunoprecipitations of yeast cell extracts with the SSB-1 antibody reveal that in 150 mM NaCl SSB-1 is bound to two small nuclear RNAs (snRNAs). These yeast snRNAs are snR10 and snR11, with snR10 being predominant. Since snR10 has been implicated in pre-rRNA processing, the association of SSB-1 and snR10 into a nucleolar snRNP particle indicates SSB-1 involvement in rRNA processing as well. Also, another yeast protein, SSB-36-kD, isolated by single-strand DNA chromatography, is shown to bind silver under the conditions used for nucleolar-specific staining. It is, most likely, another yeast nucleolar protein.

Yeast tRNA-splicing endonuclease is a heterotrimeric enzyme

tRNA-splicing endonuclease from the yeast Saccharomyces cerevisiae was purified to homogeneity greater than 5000-fold over a crude Triton X-100 extract of yeast total membranes, with 5% overall yield. This nuclear enzyme has the unusual heterotrimeric subunit structure alpha beta gamma (alpha = 31 kDa, beta = 42 kDa, and gamma = 51 kDa), as determined by sodium dodecyl sulfate gel electrophoresis, and has a molecular mass close to the sum of the three subunits, as determined by gel filtration of the native enzyme. From the purification, we estimate that there are approximately 100 molecules of endonuclease/cell.

Two domains of yeast U6 small nuclear RNA required for both steps of nuclear precursor messenger RNA splicing

U6 is one of the five small nuclear RNA's (snRNA's) that are required for splicing of nuclear precursor messenger RNA (pre-mRNA). The size and sequence of U6 RNA are conserved among organisms as diverse as yeast and man, and so it has been proposed that U6 RNA functions as a catalytic element in splicing. A procedure for in vitro reconstitution of functional yeast U6 small nuclear ribonucleoproteins (snRNP's) with synthetic U6 RNA was applied in an attempt to elucidate the function of yeast U6 RNA. Two domains in U6 RNA were identified, each of which is required for in vitro splicing. Single nucleotide substitutions in these two domains block splicing either at the first or the second step. Invariably, U6 RNA mutants that block the first step of splicing do not enter the spliceosome. On the other hand, those that block the second step of splicing form a spliceosome but block cleavage at the 3' splice site of the intron. In both domains, the positions of base changes that block the second step of splicing correspond exactly to the site of insertion of pre-mRNA-type introns into the U6 gene of two yeast species, providing a possible explanation for the mechanism of how these introns originated and adding further evidence for the proposed catalytic role of U6 RNA.