Regulation of XBP1 Splicing in Mammalian Cells

Unlu, Irem

doi:https://doi.org/10.21985/n2-3s10-1n46

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Regulation of XBP1 Splicing in Mammalian Cells

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RNA repair pathways exist in all three domains of life. In eukaryotes, they play key roles in fundamental biological processes such as tRNA splicing and endoplasmic reticulum (ER) stress induced noncanonical splicing of a master transcription factor mRNA (named XBP1 in mammalian cells). Even though most living organisms contain an RNA repair pathway based on RtcB RNA ligase, fungi and plants evolved to have an entirely different RNA repair system, the main component being Trl1 RNA cyclic phosphodiesterase/kinase/ligase. Mammalian cells, while sharing the same type of RNA repair pathway with prokaryotes, archaea, and metazoans as a main mechanism, were also shown to have remnants of a yeast-like RNA repair mechanism. In this thesis work, we investigated the function of this putative yeast-like RNA repair pathway in mammalian cells. First, we showed that mammalian cells can also survive on the yeast Trl1-mediated RNA repair system by genetic rescue of RtcB depleted cells with Trl1. Then we investigated the function of the yeast-like RNA repair components of mammalian cells. We showed that these yeast-like RNA repair proteins do not work redundantly to the RtcB pathway. Instead, one of these proteins, CNP, negatively regulates the RtcB-mediated XBP1 splicing. Additionally, we found that RtcA, a protein with an opposing function to CNP, facilitates XBP1 splicing. Lastly, we showed that CNP and RtcA mediated modulation of XBP1 affect the expressions of its downstream proteins. Since XBP1 has many downstream targets with various functions such as ER associated protein degradation, vesicular trafficking, autophagy, glycosylation, ER expansion and lipid metabolism, the regulation by CNP and RtcA is expected to have expansive physiological consequences. Taken together, our results reveal a new regulatory mechanism for XBP1 levels by the RNA repair proteins CNP and RtcA in mammalian cells.

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