spp42, identified as a classical suppressor of prp4-73, which encodes a kinase involved in pre-mRNA splicing in fission yeast, is a homologue of the splicing factor Prp8p

The tri-snRNP specific protein FgSnu66 is functionally related to FgPrp4 kinase in Fusarium graminearum

Deletion of Prp4, the only kinase among spliceosome components, is not lethal in Fusarium graminearum but Fgprp4 mutants have severe growth defects and produced spontaneous suppressors. To identify novel suppressor mutations of Fgprp4, we sequenced the genome of suppressor S37 that was normal in growth but only partially recovered for intron splicing and identified a tandem duplication of 9-aa in the tri-snRNP component FgSNU66. Among the 19 additional suppressor strains found to have mutations in FgSNU66 (out of 260 screened), five had the same 9-aa duplication event with S37 and another five had the R477H/C mutation. The rest had nonsense or G-to-D mutations in the C-terminal 27-aa (CT27) region of FgSnu66, which is absent in its yeast ortholog. Truncation of this C-terminal region reduced the interaction of FgSnu66 with FgHub1 but increased its interaction with FgPrp8 and FgPrp6. Five phosphorylation sites were identified in FgSnu66 by phosphoproteomic analysis and the T418A-S420A-S422A mutation was shown to reduce virulence. Overall, our results showed that mutations in FgSNU66 can suppress deletion of Fgprp4, which has not been reported in other organisms, and the C-terminal tail of FgSnu66 plays a role in its interaction with key tri-snRNP components during spliceosome activation.

FgPrp4 Kinase Is Important for Spliceosome B-Complex Activation and Splicing Efficiency in Fusarium graminearum

PRP4 encodes the only kinase among the spliceosome components. Although it is an essential gene in the fission yeast and other eukaryotic organisms, the Fgprp4 mutant was viable in the wheat scab fungus Fusarium graminearum. Deletion of FgPRP4 did not block intron splicing but affected intron splicing efficiency in over 60% of the F. graminearum genes. The Fgprp4 mutant had severe growth defects and produced spontaneous suppressors that were recovered in growth rate. Suppressor mutations were identified in the PRP6, PRP31, BRR2, and PRP8 orthologs in nine suppressor strains by sequencing analysis with candidate tri-snRNP component genes. The Q86K mutation in FgMSL1 was identified by whole genome sequencing in suppressor mutant S3. Whereas two of the suppressor mutations in FgBrr2 and FgPrp8 were similar to those characterized in their orthologs in yeasts, suppressor mutations in Prp6 and Prp31 orthologs or FgMSL1 have not been reported. Interestingly, four and two suppressor mutations identified in FgPrp6 and FgPrp31, respectively, all are near the conserved Prp4-phosphorylation sites, suggesting that these mutations may have similar effects with phosphorylation by Prp4 kinase. In FgPrp31, the non-sense mutation at R464 resulted in the truncation of the C-terminal 130 aa region that contains all the conserved Prp4-phosphorylation sites. Deletion analysis showed that the N-terminal 310-aa rich in SR residues plays a critical role in the localization and functions of FgPrp4. We also conducted phosphoproteomics analysis with FgPrp4 and identified S289 as the phosphorylation site that is essential for its functions. These results indicated that FgPrp4 is critical for splicing efficiency but not essential for intron splicing, and FgPrp4 may regulate pre-mRNA splicing by phosphorylation of other components of the tri-snRNP although itself may be activated by phosphorylation at S289.

Structural and functional characterization of the N terminus of Schizosaccharomyces pombe Cwf10

The spliceosome is a dynamic macromolecular machine that catalyzes the removal of introns from pre-mRNA, yielding mature message. Schizosaccharomyces pombe Cwf10 (homolog of Saccharomyces cerevisiae Snu114 and human U5-116K), an integral member of the U5 snRNP, is a GTPase that has multiple roles within the splicing cycle. Cwf10/Snu114 family members are highly homologous to eukaryotic translation elongation factor EF2, and they contain a conserved N-terminal extension (NTE) to the EF2-like portion, predicted to be an intrinsically unfolded domain. Using S. pombe as a model system, we show that the NTE is not essential, but cells lacking this domain are defective in pre-mRNA splicing. Genetic interactions between cwf10-ΔNTE and other pre-mRNA splicing mutants are consistent with a role for the NTE in spliceosome activation and second-step catalysis. Characterization of Cwf10-NTE by various biophysical techniques shows that in solution the NTE contains regions of both structure and disorder. The first 23 highly conserved amino acids of the NTE are essential for its role in splicing but when overexpressed are not sufficient to restore pre-mRNA splicing to wild-type levels in cwf10-ΔNTE cells. When the entire NTE is overexpressed in the cwf10-ΔNTE background, it can complement the truncated Cwf10 protein in trans, and it immunoprecipitates a complex similar in composition to the late-stage U5.U2/U6 spliceosome. These data show that the structurally flexible NTE is capable of independently incorporating into the spliceosome and improving splicing function, possibly indicating a role for the NTE in stabilizing conformational rearrangements during a splice cycle.

Structure of a multipartite protein-protein interaction domain in splicing factor prp8 and its link to retinitis pigmentosa

Protein Prp8 interacts with several other spliceosomal proteins, snRNAs, and the pre-mRNA and thereby organizes the active site(s) of the spliceosome. The DEAD-box protein Brr2 and the GTPase Snu114 bind to the Prp8 C terminus, a region where mutations in human Prp8 are linked to the RP13 form of Retinitis pigmentosa. We show crystallographically that the C-terminal domain of yeast Prp8p exhibits a Jab1/MPN-like core known from deubiquitinating enzymes. Insertions and terminal appendices are grafted onto this core, covering a putative isopeptidase center whose metal binding site is additionally impaired. Targeted yeast-two-hybrid analyses show that the RP13-linked region in the C-terminal appendix of human Prp8 is essential for binding of human Brr2 and Snu114, and that RP13 point mutations in this fragment weaken these interactions. We conclude that the expanded Prp8 Jab1/MPN domain represents a pseudoenzyme converted into a protein-protein interaction platform and that dysfunction of this platform underlies Retinitis pigmentosa.

Activation of AP-1-dependent transcription by a truncated translation initiation factor

Int6/eIF3e is a highly conserved subunit of eukaryotic translation initiation factor 3 (eIF3) that has also been reported to interact with subunits of the proteasome and the COP9 signalosome. Overexpression of full-length Int6 or a 13-kDa C-terminal fragment, Int6CT, in the fission yeast Schizosaccharomyces pombe causes multidrug resistance that requires the otherwise inessential AP-1 transcription factor Pap1. Here we show for the first time that Int6CT acts to increase the transcriptional activity of Pap1. Microarray hybridization data indicate that Int6CT overexpression resulted in the up-regulation of 67 genes; this expression profile closely matched that of cells overexpressing Pap1. Analysis of the upstream regulatory sequences of these genes showed that the majority contained AP-1 consensus binding sites. Partial defects in ubiquitin-dependent proteolysis have been suggested to confer Pap1-dependent multidrug resistance, but no such defect was seen on Int6CT overexpression. Indeed, none of the previously identified interactions of endogenous Int6 was required for the activation of Pap1 transcription described here. Moreover, Int6CT-induced activation of Pap1-responsive gene expression was independent of the ability of Pap1 to undergo a redox-regulated conformational change which mediates its relocalization to the nucleus and expression of oxidative stress response genes. Int6CT therefore activates Pap1-dependent transcription by a novel mechanism.

Multiple genetic and biochemical interactions of Brr2, Prp8, Prp31, Prp1 and Prp4 kinase suggest a function in the control of the activation of spliceosomes in Schizosaccharomyces pombe

The spliceosomal component Prp1 (U5-102 kD) is found in Schizosaccharomyces pombe, a physiological substrate of Prp4 kinase. Here, we identify, spp41-1, a previously isolated extragenic suppressor of Prp4 kinase. The gene encodes an ATP-dependent RNA helicase homologous to the splicing factor Brr2 of Saccharomyces cerevisiae and U5-200 kD of mammalia. The suppressor allele, spp41-1, interacts genetically with alleles of prp1. We show that Prp1 and Brr2 are complexed in vivo with spliceosomal particles containing the five snRNAs U1, U2, U5, and base-paired U4/U6. Prp1 was found exclusively in small ribonucleoprotein particle (snRNP) complexes sedimenting in the range of 30S-60S, whereas Brr2 was also found sedimenting lower than 30S and free of snRNAs. Moreover, we find that the splicing factor Prp31 is complexed with Prp1 in the same spliceosomal particles containing the five snRNAs. These data indicate that in fission yeast spliceosomal particles larger than 30S exist, which can be considered as pre-catalytic spliceosomes. In addition, we show that S. pombe cells lacking Prp1 still contain these large pre-catalytic spliceosomal particles associated with Prp31. These data are consistent with the notion that in fission yeast phosphorylation of Prp1 by Prp4 kinase is involved in the activation of pre-catalytic spliceosomes.

Prp8 protein: at the heart of the spliceosome

Pre-messenger RNA (pre-mRNA) splicing is a central step in gene expression. Lying between transcription and protein synthesis, pre-mRNA splicing removes sequences (introns) that would otherwise disrupt the coding potential of intron-containing transcripts. This process takes place in the nucleus, catalyzed by a large RNA-protein complex called the spliceosome. Prp8p, one of the largest and most highly conserved of nuclear proteins, occupies a central position in the catalytic core of the spliceosome, and has been implicated in several crucial molecular rearrangements that occur there. Recently, Prp8p has also come under the spotlight for its role in the inherited human disease, Retinitis Pigmentosa.Prp8 is unique, having no obvious homology to other proteins; however, using bioinformatical analysis we reveal the presence of a conserved RNA recognition motif (RRM), an MPN/JAB domain and a putative nuclear localization signal (NLS). Here, we review biochemical and genetical data, mostly related to the human and yeast proteins, that describe Prp8's central role within the spliceosome and its molecular interactions during spliceosome formation, as splicing proceeds, and in post-splicing complexes.

Cloning of human PRP4 reveals interaction with Clk1

Prp4 is a protein kinase of Schizosaccharomyces pombe identified through its role in pre-mRNA splicing, and belongs to a kinase family including mammalian serine/arginine-rich protein-specific kinases and Clks, whose substrates are serine/arginine-rich proteins. We cloned human PRP4 (hPRP4) full-length cDNA and the antiserum raised against a partial peptide of hPRP4 recognized 170-kDa polypeptide in HeLa S3 cell extracts. Northern blot analysis revealed that hPRP4 mRNA was ubiquitously expressed in multiple tissues. The extended NH(2)-terminal region of hPRP4 contains an arginine/serine-rich domain and putative nuclear localization signals. hPRP4 phosphorylated and interacted with SF2/ASF, one of the essential splicing factors. Indirect immunofluorescence analysis revealed that endogenous hPRP4 was distributed in a nuclear speckled pattern and colocalized with SF2/ASF in HeLa S3 cells. Furthermore, hPRP4 interacted directly with Clk1 on its COOH terminus, and the arginine/serine-rich domain of hPRP4 was phosphorylated by Clk1 in vitro. Overexpression of Clk1 caused redistribution of hPRP4, from the speckled to the diffuse pattern in nucleoplasm, whereas inactive mutant of Clk1 caused no change of hPRP4 localization. These findings suggest that the NH(2)-terminal region of hPRP4 may play regulatory roles under an unidentified signal transduction pathway through Clk1.

Fission yeast Prp4p kinase regulates pre-mRNA splicing by phosphorylating a non-SR-splicing factor

We provide evidence that Prp4p kinase activity is required for pre-mRNA splicing in vivo and show that loss of activity impairs G1-S and G2-M progression in the cell cycle. Prp4p interacts genetically with the non-SR (serine/arginine) splicing factors Prp1p and Prp5p. Bacterially produced Prp1p is phosphorylated by Prp4p in vitro. Prp4p and Prp1p also interact in the yeast two-hybrid system. In vivo labelling studies using a strain with a mutant allele of the prp4 gene in the genetic background indicate a change in phosphorylation of the Prp1p protein. These results are consistent with the notion that Prp4p kinase is involved in the control of the formation of active spliceosomes, targeting non-SR splicing factors.

Analysis of the splicing machinery in fission yeast: a comparison with budding yeast and mammals

Based on genetic and bioinformatic analysis, 80 proteins from the newly sequenced Schizosaccharomyces pombe genome appear to be splicing factors. The fission yeast splicing factors were compared to those of Homo sapiens and Saccharomyces cerevisiae in order to determine the extent of conservation or divergence that has occurred over the billion years of evolution that separate these organisms. Our results indicate that many of the factors present in all three organisms have been well conserved throughout evolution. It is calculated that 38% of the fission yeast splicing factors are more similar to the human proteins than to the budding yeast proteins (>10% more similar or similar over a greater region). Many of the factors in this category are required for recognition of the 3' splice site. Ten fission yeast splicing factors, including putative regulatory factors, have human homologs, but no apparent budding yeast homologs based on sequence data alone. Many of the budding yeast factors that are absent in fission yeast are associated with the U1 and U4/U6.U5 snRNP. Collectively the data presented in this survey indicate that of the two yeasts, S.POMBE: contains a splicing machinery more closely reflecting the archetype of a spliceosome.