The Reaction Mechanism of Cellular U snRNP Assembly Dissertation zur Erlangung des naturwissenschaftlichen Doktorgrades der Bayerischen Julius-Maximilians-Universität Würzburg vorgelegt von Ashwin Chari Aus Bangalore (Indien) Würzburg 2009 Eingereicht am: Mitglieder der Promotionskommission: Vorsitzender: Prof. Dr. M. Müller 1. Gutachter: Prof. Dr. U. Fischer 2. Gutachter: Prof. Dr. U. Scheer Tag des Promotionskolloquiums: Doktorurkunde ausgehändigt am: Erklärung Erklärung gemäss §4 Absatz 3 der Promotionsordnung der Fakultät für Biologie der Bayerischen Julius-Maximilians-Universität Würzburg vom 15. März 1999 1. Hiermit erkläre ich ehrenwörtlich, dass ich die vorliegende Dissertation selbstständig angefertigt und keine anderen als die angegebenen Quellen und Hilfsmittel benutzt habe. 2. Ich erkläre, dass die vorliegende Dissertation weder in gleicher noch in ähnlicher Form bereits in einem Prüfungsverfahren vorgelegen hat. 3. Ich erkläre, dass ich ausser den mit dem Zulassungsantrag urkundlich vorgelegten Graden keine weiteren akademischen Grade erworben oder zu erwerben versucht habe. Würzburg, 2009 Ashwin Chari Table of Contents 1. Summary 1 2. Zusammenfassung 5 3. Introduction 9 3.1 Principles Governing Macromolecular Complex Assembly in Vivo 9 3.2 Pre-mRNA Splicing 12 3.3 Architecture of Spliceosomal U snRNPs 14 3.4 The Cell Biology of U snRNP Biogenesis 16 3.5 U snRNP Assembly in Vivo is an Active, Factor-Mediated Process 19 3.6 References 22 4. Goals of this Thesis 29 5. Results 31 5.1 Taking an Inventory of the Subunits of the Human SMN-Complex 31 5.2 Definition of the Basic Architecture of the Human SMN-Complex 49 5.3 Mechanistic Aspects of Cellular U snRNP Assembly 65 5.4 Evolution of the SMN-Complex 115 6. Discussion 129 6.1 The Etiology of Spinal Muscular Atrophy 131 6.2 The Mechanistic Basis of Cellular U snRNP Formation 147 7. Conclusions 169 8. Appendix 171 8.1 The Role of the LSm 1-7 Complex in the Translation an Replication of Positive-Strand RNA Virus Genomes 173 8.2 Arginine Methylation of Mammalian Pre-mRNA Cleavage Factor I 235 8.3 IGHMBP2 is a Ribosome-Associated Helicase Inactive in the Neuromuscular Disorder Distal SMA Type 1 291 8.4 A 5´-Fluorobenzoyladenosine-Based Method to Identify Physiological Substrates of a Drosophila p21-Activated Kinase 325 1. Summary Macromolecular complexes, also termed molecular machines, facilitate a large spectrum of biological reactions and tasks crucial to the survival of cells. These complexes are composed of either protein only, or proteins bound to nucleic acids (DNA or RNA). Prominent examples for each class are the proteosome, the nucleosome and the ribosome. How such units are assembled within the context of a living cell is a central question in molecular biology. Earlier studies had indicated that even very large complexes such as ribosomes could be reconstituted from purified constituents in vitro. The structural information required for the formation of macromolecular complexes, hence, lies within the subunits itself and, thus, allow for self- assembly. However, increasing evidence suggests that in vivo many macromolecular complexes do not form spontaneously but require assisting factors (“assembly chaperones”) for their maturation. In this thesis the assembly of RNA-protein (RNP) complexes has been studied by a combination of biochemical and structural approaches. A resourceful model system to study this process is the biogenesis pathway of the uridine-rich small nuclear ribonucleoproteins (U snRNPs) of the spliceosome. This molecular machine catalyzes pre-mRNA splicing, i.e. the removal of non-coding introns and the joining of coding exons to functional mRNA. The composition and architecture of U snRNPs is well defined, also, the nucleo-cytoplasmic transport events enabling the formation of these particles in vivo have been analyzed in some detail. Furthermore, recent studies suggest that the formation of U snRNPs in vivo is mediated by an elaborate assembly machinery consisting of protein arginine methyltransferase (PRMT5)- and survival motor neuron (SMN)-complexes. The elucidation of the reaction mechanism of cellular U snRNP assembly would serve as a paradigm for our understanding of how RNA-protein complexes are formed in the cellular environment. The following key findings were obtained as part of this study: 1) Efforts were made to establish a full inventory of the subunits of the SMN-complex. This was achieved by the biochemical definition and characterization of an atypical component of this complex, the unrip protein. This protein is associated with the SMN- complex exclusively in the cytoplasm and influences its subcellular localization. 1 2) With a full inventory of the components in hand, the architecture of the SMN-complex was defined on the basis of an interaction map of all subunits. This study elucidated that the proteins SMN, Gemin7 and Gemin8 form a backbone, onto which the remaining subunits adhere in a modular manner. 3) The two studies mentioned above formed the basis to elucidate the reaction mechanism of cellular U snRNP assembly. Initially, an early phase in the SMN-assisted formation of U snRNPs was analyzed. Two subunits of the U7 snRNP (LSm10 and 11) were found to interact with the PRMT5-complex, without being methylated. This report suggests that the stimulatory role of the PRMT5-complex is independent of its methylation activity. 4) Key reaction intermediates in U snRNP assembly were found and characterized by a combination of biochemistry and structural studies. Initially, a precursor to U snRNPs with a sedimentation coefficient of 6S is formed by the pICln subunit of the PRMT5- complex and Sm proteins. This intermediate was shown to constitute a kinetic trap in the U snRNP assembly reaction. Progression towards the assembled U snRNP depends on the activity of the SMN-complex, which acts as a catalyst. The formation of U snRNPs is shown to be structurally similar to the way clamps are deposited onto DNA to tether poorly processive polymerases. 5) The human SMN-complex is composed of several subunits. However, it is unknown whether all subunits of this entity are essential for U snRNP assembly. A combination of bioinformatics and biochemistry was applied to tackle this question. By mining databases containing whole-genome assemblies, the SMN-Gemin2 heterodimer is recognized as the most ancestral form of the SMN-complex. Biochemical purification of the Drosophila melanogaster SMN-complex reveals that this complex is composed of the same two subunits. Furthermore, evidence is provided that the SMN-Gemin2 heterodimer is necessary and sufficient to promote faithful U snRNP assembly. Future studies will adress further details in the reaction mechanism of cellular U snRNP assembly. The results obtained in this thesis suggest that the SMN and Gemin2 subunits are sufficient to promote U snRNP formation. What then is the function of the remaining subunits of the SMN-complex? The reconstitution schemes established in this thesis will be 2 instrumental to address this question. Furthermore, additional mechanistic insights into the U snRNP assembly reaction will require the elucidation of structures of the assembly machinery trapped at various states. The prerequisite for these structural studies, the capability to generate homogenous complexes in sufficient amounts, has been accomplished in this thesis. 3 4 2. Zusammenfassung Makromolekulare Komplexe, auch molekulare Maschinen genannt, ermöglichen eine grosse Vielfalt biologischer Reaktionen und Aufgaben, die für das Überleben von Organismen kritisch sind. Diese Komplexe bestehen entweder nur aus Protein, oder setzen sich aus Protein und Nukleinsäure (DNA oder RNA) zusammen. Prominente Beispiele für diese Klassen molekularer Maschinen sind das Proteosom, das Nukleosom oder das Ribosom. Wie sich solche Einheiten innerhalb einer Zelle zusammenlagern ist eine grundlegende Frage der Molekularbiologie. Frühere Studien hatten angeduetet, dass es möglich ist sogar sehr grosse Komplexe wie das Ribosom in vitro aus gereinigten Bestandteilen zu einem aktiven Partikel zu rekonstruieren. Die Strukturinformation, die für die Bildung von makromolekularen Komplexen erforderlich ist, liegt also in den Untereinheiten selbst. Im Gegensatz dazu mehren sich heute die Hinweise dafür, dass sich viele makromolekulare Komplexe nicht spontan zusammenlagern, sondern die Aktivität assistierender Faktoren („Assembly Chaperone“) für ihre Reifung benötigen. In dieser Arbeit wurde der Zusammenbau von RNA-Protein (RNP) Partikeln durch eine Kombination aus Biochemie und Strukturbiologie untersucht. Ein ergiebiges System, um diesen Prozess zu studieren, ist die Biogenese der RNPs (U snRNPs) des Spleissosoms. Aufgabe dieser molekularen Maschine ist das Herausschneiden nicht-kodierender Introns und das Zusammenfügen kodiereneder Exons um so funktionelle mRNA zu bilden. Die Zusammensetzung und Architektur von U snRNPs sind gut definiert. Auch ist der Kern- Zytoplasma Transport, der für die Reifung dieser Partikel notwendig sind, detailliert beschrieben worden. Außerdem weisen neueste Studien darauf hin, dass die Bildung von U snRNPs in vivo durch eine komplexe Maschinerie, die aus den Protein-Arginin- Methyltransferase 5 (PRMT5)- und Survival-Motor-Neuron (SMN)- Komplexen besteht, vermittelt wird. Die Entschlüsselung des Reaktionsmechanismus des zellulärem U snRNP Zusammenbaus würde als Musterbeispiel für unser Verständnis dienen, wie RNPs in einer Zelle gebildet werden. Folgende Erkenntnisse wurden in dieser Arbeit