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Controlling ribosome biogenesis by mTORC1-mediated phosphorylation of LARP1

Research output: Book/anthology/dissertation/reportPh.D. thesis

Documents

  • Michael Solgaard Jensen
In human cells, the control of ribosome biogenesis is in part owed to a 5’ terminal oligo-pyrimidine tract (TOP), which is harbored by mRNAs encoding e.g. ribosomal proteins. The TOP-motif enables La-Related Protein 1 (LARP1) to specifically stabilize TOP mRNAs. In addition, conditions such as shortage of nutrition or abrogated growth signaling, inactivate Mammalian Target Of Rapamycin Complex 1 (mTORC1), thereby inducing a prominent LARP1-mediated translational repression of TOP mRNAs. TOP mRNA translation is suggestively overseen by several TOP-specific regulators. Moreover, the mTORC1-targeted sites in LARP1 and the mechanism and diversity of LARP1-mediated stabilization of transcripts remain poorly characterized. Therefore, this thesis aims to reevaluate TOP-specific regulators, while focusing on how mTORC1 controls LARP1. Additionally, the present study investigates a decay-related mechanism that targets TOP mRNAs, and it also assesses the diversity and biological relevance of LARP1-mediated control of non-TOP mRNA expression.
I emphasize that, among known regulators, LARP1 is the most prominent TOP-specific translational regulator, although other uncharacterized mechanisms likely complement LARP1. To efficiently control LARP1, mTORC1 may depend on interacting with an interface on LARP1 that comprises residues S550, S554, S689, T692 and S697. I verify that 26 serines and threonine residues in LARP1 are likely targeted for phosphorylation by mTORC1. Among these, the phosphorylation of LARP1 at S747, T768, S770, S772, S774, S776, T779, S784, T788, and S791 results in the release of LARP1 from the TOP-motif and rescues TOP mRNA translation, thus strongly suggesting that mTORC1 targets these sites to control LARP1-mediated translational repression of TOP mRNAs. Such detailed characterization of mTORC1-mediated control of LARP1 is unprecedented.
I verify that select TOP mRNAs likely experience an endoribonucleolytic cleavage immediately downstream of the TOP-motif, thereby committing the cleaved transcripts to decay. These TOP mRNAs are targeted by an yet unknown endoribonuclease in a co-translational manner that also depends on the TOP-motif. Moreover, the presented data suggests that LARP1 is capable of stabilizing TOP mRNAs by inhibiting the endoribonucleolytic event, at least during mTORC1 inactivation. This thesis presents the first identification of TOP mRNA decay intermediates, which arise in a LARP1-regulated TOP-dependent manner.
Finally, I verify that numerous non-TOP transcripts experience a LARP1-dependent post-transcriptional regulation of their abundance and translation, during proliferative conditions and mTORC1 inactivation. I emphasize that select LARP1-regulated candidates should be investigated in future studies, due to their relation to ribosome biogenesis (NPM1), mTORC1 signaling (YWHAE, MID1, Rheb, NDRG1) and resource homeostasis (PKM, MTR, QARS). In parallel, this bioinformatical study also assesses TOP mRNAs, thereby providing the first individual analysis of LARP1-dependent changes in both transcript abundance and translation for TOP and non-TOP mRNAs.
Original languageEnglish
PublisherAarhus Universitet
Number of pages212
Publication statusPublished - 2020

Note re. dissertation

Termination date: 19.10.2020

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