Mechanistic studies of the RNA chaperone activities of the DEAD-box RNA helicase CYT-19
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Structured RNAs are pervasive in biology, spanning a functional repertoire that includes messengers, regulators of gene expression and catalysts of translation and splicing. From the relatively simple tRNAs and riboswitches to the highly structured ribosomal RNAs, the ability of RNAs to function is dependent on well-defined secondary and tertiary structures. However, studies of RNA folding in vitro have revealed an extreme propensity to form alternative structures, which can be long-lived and interfere with function. In the cell, a diverse array of RNA binding proteins and RNA chaperones guide RNAs towards the correct structure and disrupt misfolded intermediates. Among these proteins, DEAD-box protein family stands out as one of the largest groups, with its members ubiquitously involved in RNA metabolism across all domains of life. DEAD-box proteins can function as both specific and general RNA chaperones by disrupting RNA structures in an ATP-dependent manner. Here I describe my work studying the general RNA chaperone mechanism of the Neurospora crassa protein CYT-19, a model DEAD-box protein and a biological RNA chaperone that is required for efficient folding of self-splicing group I intron RNAs in vivo. After an introduction to DEAD-box proteins and their mechanisms as RNA remodelers (Chapter 1), I will first describe studies of group I intron unfolding by CYT-19, focusing on the effects of RNA tertiary structure stability on CYT-19 activity and targeting to RNA substrates (Chapter 2). I will then describe the characterization of ATP-dependent mechanisms during CYT-19-mediated refolding of the misfolded group I intron (Chapter 3). In Chapter 4, I will present small-angle X-ray scattering (SAXS) studies of structural features of DEAD-box proteins that allow them to efficiently interact with large structured RNA substrates. Finally, I will turn to studies of DEAD-box protein involvement during early steps of RNA compaction and folding, using SAXS and activity-based approaches (Chapter 5). I will conclude with a general discussion of superfamily 2 RNA helicases, which include DEAD-box and related proteins, and their functions and mechanisms as remodelers of structured RNAs and RNPs.