Characterization of Ars2 nuclear and cytoplasmic isoforms as a scaffold for transcriptional regulation

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The life of an messenger RNA (mRNA) is highly coordinated from its beginnings during transcriptional initiation to its inevitable degradation. This coordination requires the recruitment, assembly and disassembly of numerous transient RNA-protein complexes and plays a critical role in cellular responses to stress. Precisely how this is co-ordinated is far from understood and misregulation of these processes has profound effects on development. My lab discovered the gene encoding ARS2, which is a nexus for RNA Pol II RNA processing and turnover. Through my NSERC-funded program, we have led the characterization of both the biology of ARS2 and how it functions as an RNA and protein interaction hub. Recently, we discovered that the Ars2 gene generates 2 isoforms: a nuclear isoform (nARS2) and a newly identified cytoplasmic isoform (cARS2). nARS2 is a key member of the cap binding complex (CBC), which coordinates the nuclear processing, export, and degradation of RNAPII RNAs. We have now found that cARS2 replaces nARS2 in cytoplasmic CBC complexes. This `ARS2 switching' has dramatic functional consequences, changing the CBC from a nonsense mediated decay (NMD) inhibitor to NMD promoter in the cytoplasm. Exactly how this is accomplished is unclear. Understanding how ARS2 isoforms orchestrate the correct NMD response has a broad impact in diverse areas of biology. This is because, in addition to limiting the production of aberrant proteins from mRNAs with premature stop codons (PTCs), NMD has a critical physiological role in all eukaryotes by regulating endoplasmic reticulum (ER) stress response. This occurs through the unfolded protein response (UPR) pathway. Intriguingly, Ars2 was first identified as a gene involved in the cellular response to ER stress caused by arsenic through the UPR pathway. Thus, we think we have discovered a mechanistic link between the longstanding knowledge of ARS2 in arsenic sensitivity and its role as a RNA chaperone. We postulate that nARS2 and cARS2 function in a CBC dependent, non-redundant manner to guide RNAPII transcript metabolism from transcription to degradation, within the nucleus and cytoplasm, respectively. Secondly, we hypothesize the roles of ARS2 isoforms in NMD are important for ARS2's role in ER stress caused by arsenic treatment. In the next funding period the short-term goals of my research program are focused on two core aspects of ARS2 isoform biology: 1) How do ARS2 isoforms differentially coordinate NMD, and 2) How do the roles of ARS2 isoforms in NMD affect response to ER stress? These questions directly feed into the long-term goal of my research program which is to understand at the molecular level the control processes governing RNAPII transcripts from their biogenesis to degradation and how these processes contribute to stem and progenitor cell fate decisions.