Novel RNA Targets of the Spinal Muscular Atrophy Protein

Ribonucleoprotein complexes (RNPs) are involved in many essential cellular processes, of which the most prominent examples include the ribosome that functions in protein translation and the spliceosome which catalyzes pre-mRNA splicing. The biogenesis of RNPs often involves complex and elaborate pathways involving post-translational modifications, transit into specific cellular domains, and several auxiliary factors including assembly chaperones. One of the best-studied examples of such a chaperone is the survival motor neuron (SMN) protein, the disease gene in spinal muscular atrophy (SMA). SMN is part of a macromolecular protein complex and catalyzes the assembly of a heptameric core of Sm proteins onto small nuclear RNAs (snRNAs) to form spliceosomal snRNPs required for RNA splicing. The Sm and Sm-like (LSm) proteins are an evolutionarily conserved family of proteins that exhibit the propensity to form diverse heteromeric complexes with unique RNA-binding characteristics. The Sm/LSm proteins are thought to function as RNA chaperones whose association with their target RNAs plays a critical role in the maturation, transport, and stability of the resulting RNPs as well as modulation of RNA-RNA and RNA-protein interactions that are critical for RNP function. Sm/LSm containing RNPs have been shown to function in a variety of cellular pathways in addition to pre-mRNA splicing, including histone mRNA 3' end formation and mRNA decay. Interestingly, in addition to its direct binding to Sm proteins, SMN has been shown to associate in vitro with members of the LSm family as well as other RNA binding proteins, implicating the SMN complex in the biology of other cellular RNPs. The discovery of the full spectrum of RNPs that are dependent on SMN activity has important implications not only for our understanding of fundamental aspects of post-transcriptional gene regulation but also for SMA pathogenesis. To pursue this line of investigation, in this dissertation, I explore the hypothesis that SMN plays a general role in RNP assembly that extends to novel RNAs that function in diverse cellular pathways. First, I report the identification of a RNA polymerase III transcript of unknown function to be a novel cell type-specific RNA target of SMN function in ribonucleoprotein assembly. Second, I explore the role of SMN in the biology of the nuclear LSm2-8 complex active in splicing and the cytoplasmic LSm1-7 complex involved in mRNA decay. Finally, to facilitate the discovery of cellular pathways linked to SMN biology, I describe a novel cell-based model system for the phenotypic screening of genetic and pharmacological modifiers of SMN expression and function. Together, my studies significantly expand the repertoire of cellular RNAs that SMN is known to target and provide a unique platform for the identification of novel SMN-dependent cellular pathways, which have relevance for understanding RNA regulation and disease mechanisms and may help in the development of therapeutic approaches to SMA.