Every cell in the human body contains almost the same genetic material, therefore, cellular identity is derived from the selection of genes transcribed into RNA and the mRNAs that are made into proteins. To achieve precise control of gene expression, the transcription of messenger RNAs by RNA polymerase II is regulated at multiple checkpoints. A major control point within this system is the P-TEFb dependent transition from paused to productive elongating polymerase complexes. Reversible inhibition of P-TEFb by the 7SK small nuclear ribonucleoprotein (snRNP) is the key step in the control of transcription elongation. Due to the importance of the regulation of P-TEFb, this research investigates the structure of 7SK RNA and the interactions within the 7SK snRNP. Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) was used to demonstrate a magnesium-dependent conformational change of in vitro transcribed 7SK RNA folding including a switch in the pairing of the 7SK motif, which is required for P-TEFb regulation. SHAPE was also used to determine that the 5′ end of 7SK pairs alternatively with two different regions within the RNA resulting in open and closed conformations. Moreover, SHAPE was used to show a similar conformational change in cellular 7SK snRNP complexes after the loss of P-TEFb. Assembly of the 7SK snRNP in vitro, using recombinant HEXIM1, P-TEFb, LARP7, MEPCE, and in vitro transcribed 7SK RNA were combined under optimized conditions, resulted in a complete and functional complex. These complexes demonstrated a reversible inhibition of the activity of P-TEFb as well as a similar structure to cellular complexes. LARP7 was found to contain a C-terminal MEPCE interaction domain (MID) that associates with and inhibits MEPCE after binding to the 3′ stem loop of 7SK. The inhibition of MEPCE was determined to be dependent on the overall conformation of 7SK and structural elements of the 3′ stem loop. Use of a highly selective degrader of Brd4, dBET6, also revealed a possible alternative mechanism for P-TEFb sequestration into the 7SK snRNP. Collectively, these findings aid in the understanding of gene regulation through the control of P-TEFb by the 7SK snRNP.

The U5 snRNP is a major component of the yeast spliceosome, being part of the U4/U6.U5 tri-snRNP, the precatalytic spliceosome and the catalytically activated spliceosome. The U5 snRNP includes, at its heart, the U5 snRNA which contains the invariant Loop 1 that functions in tethering and aligning exons during splicing. The major protein components of the U5 snRNP are the highly conserved Prp8p, the GTPase Snu114p and the helicase Brr2p. These proteins and the U5 snRNA are integral in forming the active site of the spliceosome and regulating the dynamic changes of the spliceosome. The first part of this study aimed to express and purify specific domains of Snu114p to define the structure and function of Snu114p. The N-terminal region of Snu114p was successfully expressed and purified from bacteria. Addition of the Snu114p N-terminal fragment to in vitro splicing assays resulted in a first step splicing defect, indicating a role for the N-terminus in pre-mRNA splicing. NMR studies revealed that the N-terminus of Snu114p exists as an unstructured protein domain. Mutagenesis indicated that the N-terminus of Snu114p is tolerant to mutation. A novel genetic interaction between amino acids in the N-terminus of Snu114p and the 3’ side of the U5 snRNA IL1 was identified. It is proposed here that the N-terminus of Snu114p functions to stabilise interactions of Snu114p with other proteins or snRNAs, possibly the U5 snRNA. Alternatively, the N-terminus of Snu114p may form intramolecular interactions with other regions of Snu114p to regulate Snu114p function in pre-mRNA splicing.Prp8p, Snu114p and Brr2p are known to form a stable complex but their interactions with the specific domains of the U5 snRNA are not known. The second part of this study aimed to investigate the association of Brr2p, Snu114p and Prp8p with the U5 snRNA. Mutants of the U5 snRNA were constructed in the conserved Loop 1 and the Internal Loop 1 (IL1). The influences of the U5 snRNA mutations on interactions of Prp8p, Snu114p or Brr2p with the snRNA were investigated. It was revealed that Loop 1 and both sides of IL1 of the U5 snRNA are important in association of Brr2p, Snu114p and Prp8p. Mutations in the 3’ side of IL1 drastically reduce association of Brr2p, Snu114p and Prp8p with the U5 snRNA, highlighting this region as a potential ‘protein docking’ site within the U5 snRNP. Differences seen in the associations of Brr2p, Snu114p and Prp8p with U5 snRNA mutations demonstrate that although there are intimate interactions between Brr2p, Snu114p and Prp8p, they do not associate with the U5 snRNA as a tri-protein complex. Genetic screening of BRR2 and U5 snRNA mutants reveals an interaction between the N-terminal half of Brr2p and the 3’ side of U5 snRNA IL1. This supports the proposed ‘protein docking’ site at the 3’ side of the U5 snRNA IL1.Data presented in this study increases our understanding of the regions in the U5 snRNA required for association of the essential U5 snRNP proteins, Brr2p, Snu114p and Prp8p, and goes some way to elucidating the organisation of essential proteins within the U5 snRNP.

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The splicing of pre-mRNA transcripts is catalyzed by a huge and dynamic machinery called spliceosome. The spliceosomal complex consists of five small nuclear ribonucleoprotein (snRNP) particles and hundreds of non-snRNP proteins. Biogenesis of spliceosomal snRNPs is a multi-step process, the final steps of which take place in a specialized sub-nuclear compartment, the Cajal body. However, molecular details of snRNP targeting to the Cajal body remain mostly unclear. Our previous results revealed that SART3 protein is important for accumulation of U4, U5 and U6 snRNPs in Cajal bodies, but how SART3 binds snRNP particles is elusive. SART3 has been identified as a U6 snRNP interaction partner and U4/U6 di-snRNP assembly factor. Here, we show that SART3 interacts with U2 snRNP as well, and that it binds specifically immature U2 particles. Next, we provide evidence that SART3 associates with U2 snRNP via Sm proteins, which are components of the stable snRNP core and are present in four out of five major snRNPs (i.e. in U1, U2, U4 and U5). We propose that the interaction between SART3 and Sm proteins represents a general SART3-snRNP binding mechanism, how SART3 recognizes immature snRNPs and quality controls the snRNP assembly process in Cajal bodies.

U5 small ribonucleoprotein particle (U5 snRNP) is a crucial component of the spliceosome, the complex responsible for pre-mRNA splicing. Despite the importance of U5 snRNP, not much is known about its biogenesis. When we depleted one of the core U5 components, protein PRPF8, the other U5-specific proteins do not associate with U5 snRNA and the incomplete U5 was accumulated in nuclear structures known as Cajal bodies. To further clarify the role of PRPF8 in U5 snRNP assembly, we studied PRPF8 mutations that cause an autosomal dominant retinal disorder, retinitis pigmentosa (RP). We prepared eight different PRPF8 variants carrying RP-associated mutations and expressed them stably in human cell culture. We showed that most mutations interfere with the assembly of snRNPs which consequently leads to reduced efficiency of splicing. The mutant PRPF8 together with EFTUD2 are stalled in the cytoplasm in a form of U5 snRNP assembly intermediate. Strikingly, we identified several chaperons including the HSP90/R2TP complex and ZNHIT2 as new PRPF8's interactors and potential U5 snRNP assembly factors. Our results further imply that these chaperons preferentially bind the unassembled U5 complexes and that HSP90 is required for stability of...

In the cell we can find a lot of small noncoding RNAs, which are important for many processes. Among those RNAs are small nuclear RNA uridin rich, which with proteins create U snRNP.These particles play important role in pre-mRNA splicing. In this process are noncoding sequences (introns) removed and coding sequences (exons) are joined. It is catalyzed by spliceosome. The core of this spliceosome is created by U1, U2, U4, U5 and U6 snRNP. They are essential for this process. Some steps of U snRNP biogenesis proceed in nuclear structures called Cajal bodies (CB). In my thesis I focused on factors, which are important for targeting U snRNA into CB. I used U2 snRNA like a model. With the aid of microinjection of fluorescently labeled U2 snRNA mutants I found, that the Sm binding site on U2 snRNA is essential for targeting to CB. Knock down of Sm B/B'showed us, that Sm proteins are necessary for transport U2 snRNA to CB. Sm proteins are formed on U2 snRNA by SMN complex. Deletion of SMN binding site on U2 snRNA had the same inhibition effect. From these results we can see, that Sm proteins and SMN complex are important for U2 snRNA biogenesis espacially for targeting into CB. Key words: U snRNP, Cajal body, U snRNA, cell nucleus

1 ABSTRACT There are many structures, suborganelles and bodies in the eukaryotic cell nucleus. These domains provide the nucleus with many specific functions. Nucleolus is specialized compartment serves to ribosomes assembly, nuclear speckles or Splicing Factors Compartment play an important role in RNA processing and best studied of them, Cajal bodies (CBs), are involved in snRNP maturation. However, non-membrane substructures are not unique for cell nucleus; processing bodies (P bodies) found in the cytoplasm are proposed to be important places in mRNA degradation pathway. This work is a compilation of four projects focused on non-membrane cellular bodies; namely, nuclear CBs and cytoplasmic P bodies. Both CBs and P bodies are dynamic structures that continuously exchange their components with surrounding environment. In addition to a widely accepted role of CBs in snRNP biogenesis, we show that the CB serves as a place where snRNPs are regenerated after each round of splicing. Thus, CBs are important nuclear compartment involved in snRNP recycling. To further characterize tri-snRNP assembly in CBs we applied kinetic experiments combined with mathematical modeling and created a kinetic model of tri- snRNP formation in the CB that determined kinetic parameters of tri-snRNP formation. Moreover, our kinetic...

The spliceosomal U5 snRNP contains several proteins with well characterised functions in splicing, including: Brr2, an ATPase/RNA helicase that disrupts U4/U6 and U2/U6 snRNA base pairing during activation of the spliceosome; Snu114, a GTPase that controls the action of Brr2; and Prp8, the largest and most conserved protein considered to have a central role in the spliceosome, which interacts directly with Snu114 and Brr2. Yeast Cwc21 is one of twelve Bact complex proteins that associate with spliceosomes shortly before the first step of splicing catalysis. Cwc21 interacts directly with Prp8 and Snu114, as does its human orthologue, the SR protein SRm300/SRRM2. Although, Cwc21 is not essential for yeast cell viability, it is required for sporulation. This work aims to identify the function of Cwc21 during meiosis. PP1 is a protein phosphatase required for both steps of splicing. Multiple sequence alignments of Snu114 and Prp8 revealed the presence of putative PP1 binding motifs that are well conserved among different species. This led me to hypothesize that PP1 may interact with Snu114 and/or Prp8 to regulate these or other interacting proteins. By screening intron-containing genes that are expressed in meiosis, I found that Cwc21 is required for splicing HRB1 transcripts. In addition, I show that HRB1 is also required during meiosis. The HRB1 intron contains an unusual branchsite sequence, TACTAATG, which when changed to the consensus branchsite sequence restores sporulation in the absence of Cwc21. Therefore, it is likely that Cwc21 promotes the expression of HRB1 during an early stage of meiosis by stabilising its pre-mRNA in the catalytic centre of the spliceosome. This study demonstrates a novel function for Cwc21 during meiosis. Using yeast two hybrid assay I have identified the interacting regions of Cwc21, PP1 and Brr2 in Snu114. Through biochemical studies I provide evidence for mutually exclusive interaction of Cwc21 and PP1 in the putative PP1 binding motif situated in Snu114 domain ‘IVa’. In the case of yeast Snu114, the PP1 binding motif has a novel sequence ‘YGVQYK’. I also show that the affinity of Cwc21 and PP1 for Snu114 is influenced by the different nucleotide-bound states of Snu114. Furthermore, I show that mutations in Snu114 domain ‘IVa’ restrict Snu114 function during meiosis and affect the MER1 splicing regulatory network. Therefore, Snu114 may play a role in modulating the conformational state of the catalytic spliceosome through its interactions with Cwc21/PP1 in regulating subsets of genes during meiosis. Finally, I show that PP1 is a putative regulator of Prp8.

Pre-mRNA splicing by the spliceosome requires the precise and regulated efforts of the five snRNAs (U1, U2, U4, U5, and U6) and numerous associated proteins. Following assembly and activation of the spliceosome, two consecutive reactions result in intron removal and exon ligation from pre-mRNA substrates. It has been established that several members of the DExH/D-box family of helicases act transiently on the spliceosome prior to the chemical steps to authorize the successive reactions by hydrolyzing ATP and consequently inducing structural rearrangements. While it has been suggested that these changes produced in the structure of the spliceosome result in optimal positioning of the reactive species, the mechanisms and products of these reorganizations remain uncharacterized. The work presented here describes the genetic strategy for accumulating and purifying spliceosomes arrested in vivo, during the catalytic steps of the splicing cycle. Using these complexes, we have defined the components required to proceed through the first and second steps of splicing, in addition to the factors required for the release of the spliced message. Analysis of these functional, synchronized particles has also allowed us to define a function for Prp2p in initiating the first step of pre-mRNA splicing. Our data suggest that Prp2p may act in an ATP-independent manner to remodel the spliceosome prior to using its ATPase function to displace the SF3 complex. We propose that the SF3 complex, in addition to its role in identification of the branchpoint, also acts to sequester the reactive 2’OH of the branchpoint adenosine to prevent premature reactivity. Following the two catalytic steps of the splicing cycle, the spliceosome must disassemble and recycle its snRNPs for further rounds of splicing. The essential U6 snRNP component Prp24p, mediates one of the early assembly events - the annealing between the U4 and U6 snRNAs. We have discovered that although Prp24p is essential for viability, its function(s) can be bypassed by overexpressing the U6 snRNA. Additionally, biochemical characterizations of various forms of the U4/U6 snRNP provide evidence that Prp24p must be released before other components of the U4/U6 snRNP are permitted to interact and facilitate tri-snRNP formation. / text

In mammals, a major fraction of the genome is transcribed as non-coding RNAs. An increasing amount of evidence has accumulated showing that non-coding RNAs play important roles both for normal cell function and in disease processes such as cancer or neurodegeneration. Interpreting the functions of non-coding RNAs and the molecular mechanisms through which they act is one of the most important challenges facing RNA biology today. In my Ph.D. thesis, I have been investigating the role of 7SK, one of the most abundant non-coding RNAs, in the development and function of motoneurons. 7SK is a highly structured 331 nt RNA transcribed by RNA polymerase III. It forms four stem-loop (SL) structures that serve as binding sites for different proteins. Larp7 binds to SL4 and protects the 3' end from exonucleolytic degradation. SL1 serves as a binding site for HEXIM1, which recruits the pTEFb complex composed of CDK9 and cyclin T1. pTEFb has a stimulatory role for transcription and is regulated through sequestration by 7SK. More recently, a number of heterogeneous nuclear ribonucleoproteins (hnRNPs) have been identified as 7SK interactors. One of these is hnRNP R, which has been shown to have a role in motoneuron development by regulating axon growth. Taken together, 7SK’s function involves interactions with RNA binding proteins, and different RNA binding proteins interact with different regions of 7SK, such that 7SK can be considered as a hub for recruitment and release of different proteins. The questions I have addressed during my Ph.D. are as follows: 1) which region of 7SK interacts with hnRNP R, a main interactor of 7SK? 2) What effects occur in motoneurons after the protein binding sites of 7SK are abolished? 3) Are there additional 7SK binding proteins that regulate the functions of the 7SK RNP? Using in vitro and in vivo experiments, I found that hnRNP R binds both the SL1 and SL3 region of 7SK, and also that pTEFb cannot be recruited after deleting the SL1 region but is able to bind to a 7SK mutant with deletion of SL3. In order to answer the question of how the 7SK mutations affect axon outgrowth and elongation in mouse primary motoneurons, we proceeded to conduct rescue experiments in motoneurons by using lentiviral vectors. The constructs were designed to express 7SK deletion mutants under the mouse U6 promoter and at the same time to drive expression of a 7SK shRNA from an H1 promoter for the depletion of endogenous 7SK. Using this system we found that 7SK mutants harboring deletions of either SL1 or SL3 could not rescue the axon growth defect of 7SK-depleted motoneurons suggesting that 7SK/hnRNP R complexes are integral for this process. In order to identify novel 7SK binding proteins and investigate their functions, I proceeded to conduct pull-down experiments by using a biotinylated RNA antisense oligonucleotide that targets the U17-C33 region of 7SK thereby purifying endogenous 7SK complexes. Following mass spectrometry of purified 7SK complexes, we identified a number of novel 7SK interactors. Among these is the Smn complex. Deficiency of the Smn complex causes the motoneuron disease spinal muscular atrophy (SMA) characterized by loss of lower motoneurons in the spinal cord. Smn has previously been shown to interact with hnRNP R. Accordingly, we found Smn as part of 7SK/hnRNP R complexes. These proteomics data suggest that 7SK potentially plays important roles in different signaling pathways in addition to transcription. / Bei Säugetieren wird ein großer Teil des Genoms als nicht-kodierende RNAs transkribiert. Es gibt immer mehr Hinweise darauf, dass nicht-kodierende RNAs eine wichtige Rolle sowohl für die normale Zellfunktion als auch bei Krankheitsprozessen wie Krebs oder Neurodegeneration spielen. Die Interpretation der Funktionen nicht-kodierender RNAs und der molekularen Mechanismen, über die sie wirken, ist eine der wichtigsten Herausforderungen, denen die RNA-Biologie heute gegenübersteht. In meiner Promotionsarbeit habe ich die Rolle von 7SK, einer der am häufigsten vorkommenden nicht-kodierenden RNAs, bei der Entwicklung und Funktion von Motoneuronen untersucht. 7SK ist eine RNA, die aus 331 Nukleotiden besteht und deren Struktur bekannt ist. Sie wird von der RNA-Polymerase III transkribiert. Sie bildet vier Stem-Loop (SL)-Strukturen, die als Bindungsstellen für verschiedene Proteine dienen. LARP7 bindet an SL4 und schützt das 3'-Ende vor exonukleolytischem Abbau. SL1 dient als Bindungsstelle für HEXIM1, das den P-TEFb-Komplex rekrutiert, der aus CDK9 und Cyclin T1 besteht. P-TEFb hat eine stimulierende Rolle für die Transkription und wird durch Sequestrierung durch 7SK reguliert. In jüngerer Zeit wurde eine Reihe von heterogenen nukleären Ribonukleoproteinen (hnRNPs) als 7SK-Interaktoren identifiziert. Eines davon ist hnRNP R, von dem gezeigt wurde, dass es eine Rolle bei der Entwicklung von Motoneuronen spielt, indem es das Axonwachstum reguliert. Durch die Interaktion mit P-TEFb und RNA-bindenden Proteinen kann 7SK als Drehscheibe für die Rekrutierung und Freisetzung verschiedener Proteine betrachtet werden. Die Fragen, mit denen ich mich während meiner Doktorarbeit beschäftigt habe, lauten wie folgt: 1) Welche Region von 7SK interagiert mit hnRNP R, einem Hauptinteraktor von 7SK? 2) Welche Effekte treten in Motoneuronen auf, wenn die Bindung von hnRNP R an 7SK inhibiert wird? 3) Gibt es zusätzliche 7SK-bindende Proteine, die die Funktionen des 7SK RNPs regulieren? Mit Hilfe von in vitro und in vivo Experimenten fand ich heraus, dass hnRNP R sowohl die SL1- als auch die SL3-Region von 7SK bindet, und dass P-TEFb nach der Deletion der SL1-Region nicht rekrutiert werden kann, aber in der Lage ist, an eine 7SK-Mutante mit Deletion von SL3 zu binden. Um die Frage zu beantworten, wie sich die 7SK-Mutationen auf Axonwachstum in primären Motoneuronen der Maus auswirken, führten wir Rettungsexperimente an Motoneuronen unter Verwendung lentiviraler Vektoren durch. Die Konstrukte wurden so konzipiert, dass sie 7SK-Deletionsmutanten durch den U6-Promotor der Maus exprimieren und gleichzeitig eine 7SK-shRNA von einem H1-Promotor für die Depletion von endogenem 7SK transkribieren. Mit diesem System fanden wir heraus, dass 7SK-Mutanten, die Deletionen von SL1 oder SL3 beherbergen, den Axon-Wachstumsdefekt von 7SK-depletierten Motoneuronen nicht retten konnten, was darauf hindeutet, dass 7SK/hnRNP R-Komplexe für diesen Prozess von Bedeutung sind. Um neue 7SK-Bindungsproteine zu identifizieren und ihre Funktionen zu untersuchen, führte ich Pulldown-Experimente durch, bei denen ich ein biotinyliertes RNA-Antisense-Oligonukleotid verwendete, das an die U17-C33-Region von 7SK bindet und dadurch Aufreinigung endogener 7SK-Komplexe erlaubt. Nach der Massenspektrometrie der gereinigten 7SK-Komplexe identifizierten wir eine Reihe neuer 7SK-Interaktoren. Einer davon ist der Smn-Komplex. Ein Mangel des Smn-Komplexes verursacht die Motoneuronerkrankung Spinale Muskelatrophie (SMA), die durch den Verlust der unteren Motoneuronen im Rückenmark gekennzeichnet ist. Es wurde bereits gezeigt, dass Smn mit hnRNP R interagiert. Dementsprechend fanden wir Smn als Teil des 7SK/hnRNP R-Komplexes. Diese Proteom-Daten deuten darauf hin, dass 7SK neben der Transkription möglicherweise auch in anderen Signalwegen wie der spliceosomalen snRNP Biogenese eine wichtige Rolle spielt.