Evidence for a role of Sky1p-mediated phosphorylation in 3' splice site recognition involving both Prp8 and Prp17/Slu4

Functions of SRPK, CLK and DYRK kinases in stem cells, development, and human developmental disorders

Human developmental disorders encompass a wide range of debilitating physical conditions and intellectual disabilities. Perturbation of protein kinase signalling underlies the development of some of these disorders. For example, disrupted SRPK signalling is associated with intellectual disabilities, and the gene dosage of DYRKs can dictate the pathology of disorders including Down's syndrome. Here, we review the emerging roles of the CMGC kinase families SRPK, CLK, DYRK, and sub-family HIPK during embryonic development and in developmental disorders. In particular, SRPK, CLK, and DYRK kinase families have key roles in developmental signalling and stem cell regulation, and can co-ordinate neuronal development and function. Genetic studies in model organisms reveal critical phenotypes including embryonic lethality, sterility, musculoskeletal errors, and most notably, altered neurological behaviours arising from defects of the neuroectoderm and altered neuronal signalling. Further unpicking the mechanisms of specific kinases using human stem cell models of neuronal differentiation and function will improve our understanding of human developmental disorders and may provide avenues for therapeutic strategies.

Serine-arginine protein kinase-like protein, SrpkF, stimulates both cellobiose-responsive and D-xylose-responsive signaling pathways in Aspergillus aculeatus

Aspergillus aculeatus produces cellulolytic enzymes in the presence of their substrates. We screened a library of 12,000 A. aculeatus T-DNA-inserted mutants to identify a regulatory factor involved in the expression of their enzyme genes in response to inducers. We found one mutant that reduced the expression of FIII-avicelase (chbI) in response to cellulose. T-DNA was inserted into a putative protein kinase gene similar to AN10082 in A. nidulans, serine-arginine protein kinase F, SrpkF. Fold increases in srpkF gene expression in response to various carbon sources were 2.3 (D-xylose), 44 (Avicel^®), 59 (Bacto^™ Tryptone), and 98 (no carbon) compared with D-glucose. Deletion of srpkF in A. aculeatus resulted in a significant reduction in cellulose-responsive expression of chbI, hydrocellulase (cel7b), and FIb-xylanase (xynIb) genes at an early induction phase. Further, the srpkF-overexpressing strain showed upregulation of the srpkF gene from four- to nine-fold higher than in the control strain. srpkF overexpression upregulated cbhI and cel7b in response to cellobiose and the FI-carboxymethyl cellulase gene (cmc1) and xynIb in response to D-xylose. However, the srpkF deletion did not affect the expression of xynIb in response to D-xylose due to the less expression of srpkF under the D-xylose condition. Our data demonstrate that SrpkF is primarily involved in cellulose-responsive expression, though it has a potential to stimulate gene expression in response to both cellobiose and D-xylose in A. aculeatus.

Functional Diversification of SRSF Protein Kinase to Control Ubiquitin-Dependent Neurodevelopmental Signaling

Conserved protein kinases with core cellular functions have been frequently redeployed during metazoan evolution to regulate specialized developmental processes. The Ser/Arg (SR)-rich splicing factor (SRSF) protein kinase (SRPK), which is implicated in splicing regulation, is one such conserved eukaryotic kinase. Surprisingly, we show that SRPK has acquired the capacity to control a neurodevelopmental ubiquitin signaling pathway. In mammalian embryonic stem cells and cultured neurons, SRPK phosphorylates Ser-Arg motifs in RNF12/RLIM, a key developmental E3 ubiquitin ligase that is mutated in an intellectual disability syndrome. Processive phosphorylation by SRPK stimulates RNF12-dependent ubiquitylation of nuclear transcription factor substrates, thereby acting to restrain a neural gene expression program that is aberrantly expressed in intellectual disability. SRPK family genes are also mutated in intellectual disability disorders, and patient-derived SRPK point mutations impair RNF12 phosphorylation. Our data reveal unappreciated functional diversification of SRPK to regulate ubiquitin signaling that ensures correct regulation of neurodevelopmental gene expression.

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.

Diverse regulation of 3' splice site usage

The regulation of splice site (SS) usage is important for alternative pre-mRNA splicing and thus proper expression of protein isoforms in cells; its disruption causes diseases. In recent years, an increasing number of novel regulatory elements have been found within or nearby the 3'SS in mammalian genes. The diverse elements recruit a repertoire of trans-acting factors or form secondary structures to regulate 3'SS usage, mostly at the early steps of spliceosome assembly. Their mechanisms of action mainly include: (1) competition between the factors for RNA elements, (2) steric hindrance between the factors, (3) direct interaction between the factors, (4) competition between two splice sites, or (5) local RNA secondary structures or longer range loops, according to the mode of protein/RNA interactions. Beyond the 3'SS, chromatin remodeling/transcription, posttranslational modifications of trans-acting factors and upstream signaling provide further layers of regulation. Evolutionarily, some of the 3'SS elements seem to have emerged in mammalian ancestors. Moreover, other possibilities of regulation such as that by non-coding RNA remain to be explored. It is thus likely that there are more diverse elements/factors and mechanisms that influence the choice of an intron end. The diverse regulation likely contributes to a more complex but refined transcriptome and proteome in mammals.

The architecture of the spliceosomal U4/U6.U5 tri-snRNP

U4/U6.U5 tri-snRNP is a 1.5-megadalton pre-assembled spliceosomal complex comprising U5 small nuclear RNA (snRNA), extensively base-paired U4/U6 snRNAs and more than 30 proteins, including the key components Prp8, Brr2 and Snu114. The tri-snRNP combines with a precursor messenger RNA substrate bound to U1 and U2 small nuclear ribonucleoprotein particles (snRNPs), and transforms into a catalytically active spliceosome after extensive compositional and conformational changes triggered by unwinding of the U4 and U6 (U4/U6) snRNAs. Here we use cryo-electron microscopy single-particle reconstruction of Saccharomyces cerevisiae tri-snRNP at 5.9 Å resolution to reveal the essentially complete organization of its RNA and protein components. The single-stranded region of U4 snRNA between its 3' stem-loop and the U4/U6 snRNA stem I is loaded into the Brr2 helicase active site ready for unwinding. Snu114 and the amino-terminal domain of Prp8 position U5 snRNA to insert its loop I, which aligns the exons for splicing, into the Prp8 active site cavity. The structure provides crucial insights into the activation process and the active site of the spliceosome.

Sky1 regulates the expression of sulfur metabolism genes in response to cisplatin

Cisplatin is commonly used in cancer therapy and yeast cells are also sensitive to this compound. We present a transcriptome analysis discriminating between RNA changes induced by cisplatin treatment, which are dependent on or independent of SKY1 function – a gene whose deletion increases resistance to the drug. Gene expression changes produced by addition of cisplatin to W303 and W303-Δsky1 cells were recorded using DNA microarrays. The data, validated by quantitative PCR, revealed 122 differentially expressed genes: 69 upregulated and 53 downregulated. Among the upregulated genes, those related to sulfur metabolism were over-represented and partially dependent on Sky1. Deletions of MET4 or other genes encoding co-regulators of the expression of sulfur-metabolism-related genes, with the exception of MET28, did not modify the cisplatin sensitivity of yeast cells. One of the genes with the highest cisplatin-induced upregulation was SEO1, encoding a putative permease of sulfur compounds. We also measured the platinum, sulfur and glutathione content in W303, W303-Δsky1 and W303-Δseo1 cells after cisplatin treatment, and integration of the data suggested that these transcriptional changes might represent a cellular response that allowed chelation of cisplatin with sulfur-containing amino acids and also helped DNA repair by stimulating purine biosynthesis. The transcription pattern of stimulation of sulfur-containing amino acids and purine synthesis decreased, or even disappeared, in the W303-Δsky1 strain.

Regulation of splicing by SR proteins and SR protein-specific kinases

Genomic sequencing reveals similar but limited numbers of protein-coding genes in different genomes, which begs the question of how organismal diversities are generated. Alternative pre-mRNA splicing, a widespread phenomenon in higher eukaryotic genomes, is thought to provide a mechanism to increase the complexity of the proteome and introduce additional layers for regulating gene expression in different cell types and during development. Among a large number of factors implicated in the splicing regulation are the SR protein family of splicing factors and SR protein-specific kinases. Here, we summarize the rules for SR proteins to function as splicing regulators, which depend on where they bind in exons versus intronic regions, on alternative exons versus flanking competing exons, and on cooperative as well as competitive binding between different SR protein family members on many of those locations. We review the importance of cycles of SR protein phosphorylation/dephosphorylation in the splicing reaction with emphasis on the recent molecular insight into the role of SR protein phosphorylation in early steps of spliceosome assembly. Finally, we highlight recent discoveries of SR protein-specific kinases in transducing growth signals to regulate alternative splicing in the nucleus and the connection of both SR proteins and SR protein kinases to human diseases, particularly cancer.

The spliceosome-activating complex: molecular mechanisms underlying the function of a pleiotropic regulator

Correct interpretation of the coding capacity of RNA polymerase II transcribed eukaryotic genes is determined by the recognition and removal of intronic sequences of pre-mRNAs by the spliceosome. Our current knowledge on dynamic assembly and subunit interactions of the spliceosome mostly derived from the characterization of yeast, Drosophila, and human spliceosomal complexes formed on model pre-mRNA templates in cell extracts. In addition to sequential structural rearrangements catalyzed by ATP-dependent DExH/D-box RNA helicases, catalytic activation of the spliceosome is critically dependent on its association with the NineTeen Complex (NTC) named after its core E3 ubiquitin ligase subunit PRP19. NTC, isolated recently from Arabidopsis, occurs in a complex with the essential RNA helicase and GTPase subunits of the U5 small nuclear RNA particle that are required for both transesterification reactions of splicing. A compilation of mass spectrometry data available on the composition of NTC and spliceosome complexes purified from different organisms indicates that about half of their conserved homologs are encoded by duplicated genes in Arabidopsis. Thus, while mutations of single genes encoding essential spliceosome and NTC components lead to cell death in other organisms, differential regulation of some of their functionally redundant Arabidopsis homologs permits the isolation of partial loss of function mutations. Non-lethal pleiotropic defects of these mutations provide a unique means for studying the roles of NTC in co-transcriptional assembly of the spliceosome and its crosstalk with DNA repair and cell death signaling pathways.

A bird's-eye view of post-translational modifications in the spliceosome and their roles in spliceosome dynamics

Pre-mRNA splicing, the removal of noncoding intron sequences from the pre-mRNA, is a critical reaction in eukaryotic gene expression. Pre-mRNA splicing is carried out by a remarkable macromolecular machine, the spliceosome, which undergoes dynamic rearrangements of its RNA and protein components to assemble its catalytic center. While significant progress has been made in describing the "moving parts" of this machine, the mechanisms by which spliceosomal proteins mediate the ordered rearrangements within the spliceosome remain elusive. Here we explore recent evidence from proteomics studies revealing extensive post-translational modification of splicing factors. While the functional significance of most of these modifications remains to be characterized, we describe recent studies in which the roles of specific post-translational modifications of splicing factors have been characterized. These examples illustrate the importance of post-translational modifications in spliceosome dynamics.

ICP27 phosphorylation site mutants display altered functional interactions with cellular export factors Aly/REF and TAP/NXF1 but are able to bind herpes simplex virus 1 RNA

Herpes simplex virus 1 (HSV-1) protein ICP27 is a multifunctional regulatory protein that is phosphorylated. Phosphorylation can affect protein localization, protein interactions, and protein function. The major sites of ICP27 that are phosphorylated are serine residues 16 and 18, within a CK2 site adjacent to a leucine-rich region required for ICP27 export, and serine 114, within a PKA site in the nuclear localization signal. Viral mutants bearing serine-to-alanine or glutamic acid substitutions at these sites are defective in viral replication and gene expression. To determine which interactions of ICP27 are impaired, we analyzed the subcellular localization of ICP27 and its colocalization with cellular RNA export factors Aly/REF and TAP/NXF1. In cells infected with phosphorylation site mutants, ICP27 was confined to the nucleus even at very late times after infection. ICP27 did not colocalize with Aly/REF or TAP/NXF1, and overexpression of TAP/NXF1 did not promote the export of ICP27 to the cytoplasm. However, in vitro binding experiments showed that mutant ICP27 was able to bind to the same RNA substrates as the wild type. Nuclear magnetic resonance (NMR) analysis of the N terminus of ICP27 from amino acids 1 to 160, compared to mutants with triple substitutions to alanine or glutamic acid, showed that the mutations affected the overall conformation of the N terminus, such that mutant ICP27 was more flexible and unfolded. These results indicate that these changes in the structure of ICP27 altered in vivo protein interactions that occur in the N terminus but did not prevent RNA binding.

Splice-site pairing is an intrinsically high fidelity process

The extensive alternative splicing in higher eukaryotes has initiated a debate whether alternative mRNA isoforms are generated by an inaccurate spliceosome or are the consequence of highly degenerate splice sites within the human genome. Here, we established a quantitative assay to evaluate the accuracy of splice-site pairing by determining the number of incorrect exon-skipping events made from constitutively spliced pre-mRNA transcripts. We demonstrate that the spliceosome pairs exons with an astonishingly high degree of accuracy that may be limited by the quality of pre-mRNAs generated by RNA pol II. The error rate of exon pairing is increased by the effects of the neurodegenerative disorder spinal muscular atrophy because of reduced levels of Survival of Motor Neuron, a master assembler of spliceosomal components. We conclude that all multi-intron-containing genes are alternatively spliced and that the reduction of SMN results in a general splicing defect that is mediated through alterations in the fidelity of splice-site pairing.

Spliceosome assembly and composition

Cells control alternative splicing by modulating assembly of the pre-mRNA splicing machinery at competing splice sites. Therefore, a working knowledge of spliceosome assembly is essential for understanding how alternative splice site choices are achieved. In this chapter, we review spliceosome assembly with particular emphasis on the known steps and factors subject to regulation during alternative splice site selection in mammalian cells. We also review recent advances regarding similarities and differences between the in vivo and in vitro assembly pathways, as well as proofreading mechanisms contributing to the fidelity of splice site selection.

Exon ligation is proofread by the DExD/H-box ATPase Prp22p

To produce messenger RNA, the spliceosome excises introns from precursor (pre)-mRNA and splices the flanking exons. To establish fidelity, the spliceosome discriminates against aberrant introns, but current understanding of such fidelity mechanisms is limited. Here we show that an ATP-dependent activity represses formation of mRNA from aberrant intermediates having mutations in any of the intronic consensus sequences. This proofreading activity is disabled by mutations that impair the ATPase or RNA unwindase activity of Prp22p, a conserved spliceosomal DExD/H-box ATPase. Further, cold-sensitive prp22 mutants permit aberrant mRNA formation from a mutated 3' splice-site intermediate in vivo. We conclude that Prp22p generally represses splicing of aberrant intermediates, in addition to its known ATP-dependent role in promoting release of genuine mRNA. This dual function for Prp22p validates a general model in which fidelity can be enhanced by a DExD/H-box ATPase.

Ubiquitin binding by a variant Jab1/MPN domain in the essential pre-mRNA splicing factor Prp8p

The U1, U2, U4/U6, and U5 small nuclear ribonucleoproteins (snRNPs) are components of the spliceosome, which catalyzes pre-mRNA splicing. One of the largest and the most highly conserved proteins in the spliceosome is Prp8p, a component of the U5 snRNP. Despite its size and conservation, very few motifs have been identified that suggest specific biochemical functions. A variant of the Jab1/MPN domain found in a class of deubiquitinating enzymes is present near the C terminus of Prp8p. Ubiquitination regulates a broad range of cellular pathways, and its functions generally require ubiquitin recognition by one or more ubiquitin-binding domains (UBDs). No precise role for ubiquitin has been defined in the pre-mRNA splicing pathway, and no known UBDs have been found within splicing proteins. Here we show that a Prp8p fragment containing the Jab1/MPN domain binds directly to ubiquitin with an affinity comparable to other known UBDs. Several mutations within this domain that compromise splicing also reduce interaction of the fragment with ubiquitin-Sepharose. Our results define a new UBD and suggest functional links between ubiquitin and the pre-mRNA splicing machinery.

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.

Yeast shuttling SR proteins Npl3p, Gbp2p, and Hrb1p are part of the translating mRNPs, and Npl3p can function as a translational repressor

A major challenge in current molecular biology is to understand how sequential steps in gene expression are coupled. Recently, much attention has been focused on the linkage of transcription, processing, and mRNA export. Here we describe the cytoplasmic rearrangement for shuttling mRNA binding proteins in Saccharomyces cerevisiae during translation. While the bulk of Hrp1p, Nab2p, or Mex67p is not associated with polysome containing mRNAs, significant amounts of the serine/arginine (SR)-type shuttling mRNA binding proteins Npl3p, Gbp2p, and Hrb1p remain associated with the mRNA-protein complex during translation. Interestingly, a prolonged association of Npl3p with polysome containing mRNAs results in translational defects, indicating that Npl3p can function as a negative translational regulator. Consistent with this idea, a mutation in NPL3 that slows down translation suppresses growth defects caused by the presence of translation inhibitors or a mutation in eIF5A. Moreover, using sucrose density gradient analysis, we provide evidence that the import receptor Mtr10p, but not the SR protein kinase Sky1p, is involved in the timely regulated release of Npl3p from polysome-associated mRNAs. Together, these data shed light onto the transformation of an exporting to a translating mRNP.

Dependence of pre-mRNA introns on PRP17, a non-essential splicing factor: implications for efficient progression through cell cycle transitions

Saccharomyces cerevisiae PRP17 (CDC40) encodes a second-step pre-mRNA splicing factor with a role in cell division. The functions of Prp17 in specific cell cycle transitions were examined using temperature-sensitive alleles in arrest/release experiments. We find that G(1)/S and G(2)/M transitions depend on Prp17. G(1)-synchronized prp17::LEU2 cells arrest at non-permissive temperatures as unbudded haploid cells with low levels of CLN1, CLB5 and RNR1 transcripts. This indicates a Prp17 execution point at or prior to Start. Reduced levels of alpha-tubulin protein, a mitotic spindle component, underlie the benomyl sensitivity of prp17 mutants and possibly their G(2)/M arrest. Splicing of TUB1 and TUB3 transcripts, which encode alpha-tubulin, was analyzed in prp17 and other second-step factor mutants. TUB1 splicing is inefficient in prp17, prp16 and prp22, and marginally affected in prp18, slu7-1 and psf1-1. TUB3 splicing is similarly affected. In vitro splicing with TUB3 pre-mRNA demonstrates a compromised second step in prp17::LEU2 extracts, implicating a direct role for Prp17 in its efficient splicing. Genomic replacement of an intronless TUB1 gene relieves the benomyl sensitivity of prp17 mutants; however, they remain temperature sensitive, implying multiple limiting factors for mitosis. The data suggest that integration of splicing with the cell cycle is important for G(1)/S and G(2)/M transitions.

Distinct domains of splicing factor Prp8 mediate different aspects of spliceosome activation

Prp8 is the largest and most highly conserved protein in the spliceosome yet its mechanism of function is poorly understood. Our previous studies implicate Prp8 in control of spliceosome activation for the first catalytic step of splicing, because substitutions in five distinct regions (a-e) of Prp8 suppress a cold-sensitive block to activation caused by a mutation in U4 RNA. Catalytic activation of the spliceosome is thought to require unwinding of the U1 RNA/5' splice site and U4/U6 RNA helices by the Prp28 and Prp44/Brr2 DExD/H-box helicases, respectively. Here we show that mutations in regions a, d, and e of Prp8 exhibit allele-specific genetic interactions with mutations in Prp28, Prp44/Brr2, and U6 RNA, respectively. These results indicate that Prp8 coordinates multiple processes in spliceosome activation and enable an initial correlation of Prp8 structure and function. Furthermore, additional genetic interactions with U4-cs1 support a two-state model for this RNA conformational switch and implicate another splicing factor, Prp31, in Prp8-mediated spliceosome activation.

The yeast THO complex and mRNA export factors link RNA metabolism with transcription and genome instability

The THO complex is a multimeric factor containing four polypeptides, Tho2, Hpr1, Mft1 and Thp2. Mutations in any of the genes encoding THO confer impairment of transcription and a transcription-dependent hyper-recombination phenotype, suggesting that THO has a functional role in gene expression. Using an in vivo assay developed to study expression of long and G+C-rich DNA sequences, we have isolated SUB2, a gene involved in mRNA splicing and export, as a multicopy suppressor of the gene expression defect of hpr1 Delta. Further investigation of a putative functional relationship between mRNA metabolism and THO revealed that mRNA export mutants sub2, yra1, mex67 and mtr2 have similar defective transcription and hyper-recombination phenotypes as THO mutants. In addition, THO becomes essential in cells with a defective Mex67 mRNA export er. Finally, we have shown that THO has the ability to associate with RNA and DNA in vitro. These results indicate a functional link between the processes of elongation and metabolism of nascent mRNA mediated by THO and mRNA export proteins, which have important consequences for the maintenance of genome stability.