The LARP1 homolog Slr1p controls the stability and expression of proto-5’TOP mRNAs in fission yeast [RNA-seq]
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ABSTRACT: Messenger RNAs (mRNAs) in higher eukaryotes that encode highly expressed proteins important for the assembly of the translational apparatus (e.g. ribosomal proteins) often harbour a pyrimidine-rich motif at the extreme 5’ end known as a 5’ terminal oligopyrimidine (5’TOP) sequence. Members of the La-related protein 1 (LARP1) family control 5’TOP expression through a conserved DM15 motif, but the mechanism is not well understood. 5’TOP motifs have not been described in many lower organisms, but fission yeast (and many other single-celled eukaryotes) harbour a LARP1 homolog that also lacks a DM15 motif. In this work, we show that the fission yeast LARP1 homolog, Slr1p, controls the translation and stability of mRNAs encoding proteins analogous to 5’TOP mRNAs in higher eukaryotes, which we thus refer to as proto-5’TOPs. Our data suggest that the LARP1 DM15 motif and the mRNA 5’TOP motif may be features that were scaffolded over a more fundamental mechanism of LARP1 family member-associated control of gene expression that is still utilized in lower systems.
Project description:Messenger RNAs (mRNAs) in higher eukaryotes that encode highly expressed proteins important for the assembly of the translational apparatus (e.g. ribosomal proteins) often harbour a pyrimidine-rich motif at the extreme 5’ end known as a 5’ terminal oligopyrimidine (5’TOP) sequence. Members of the La-related protein 1 (LARP1) family control 5’TOP expression through a conserved DM15 motif, but the mechanism is not well understood. 5’TOP motifs have not been described in many lower organisms, but fission yeast (and many other single-celled eukaryotes) harbour a LARP1 homolog that also lacks a DM15 motif. In this work, we show that the fission yeast LARP1 homolog, Slr1p, controls the translation and stability of mRNAs encoding proteins analogous to 5’TOP mRNAs in higher eukaryotes, which we thus refer to as proto-5’TOPs. Our data suggest that the LARP1 DM15 motif and the mRNA 5’TOP motif may be features that were scaffolded over a more fundamental mechanism of LARP1 family member-associated control of gene expression that is still utilized in lower systems.
Project description:Terminal oligopyrimidine motif-containing (TOP) mRNAs encode all ribosomal proteins in mammals and are regulated to tune ribosome synthesis to cell state. Previous studies implicate LARP1 in 40S- or 80S-ribosome complexes that repress and stabilize TOP mRNAs. However, a mechanistic understanding of how LARP1 and TOP mRNAs interact with ribosomes to coordinate TOP mRNA outcomes is lacking. Here, we show that LARP1 senses the cellular supply of ribosomes by directly binding non-translating 80S ribosomes. Cryo-EM structures reveal a previously uncharacterized domain of LARP1 bound to and occluding the 40S mRNA channel and mutations at the LARP1-ribosome interface block formation of the 40S/80S-LARP1-TOP complexes. Free cytosolic ribosomes induce sequestration of TOP mRNAs in repressed 80S-LARP1-TOP complexes independent of alterations in mTOR signaling. Together, this work demonstrates a ribosome-sensing function of LARP1 that allows it to tune ribosome protein synthesis to the availability of free ribosomes.
Project description:La-related protein 1 (LARP1) has been identified as a key translational inhibitor of terminal oligopyrimidine tract (TOP) mRNAs downstream of the nutrient sensing protein kinase complex, mTORC1. LARP1 exerts this inhibitory effect on TOP mRNA translation by binding to the mRNA cap and the adjacent 5’TOP motif, resulting in the displacement of the eIF4E complex from TOP mRNAs. In the present study, we identify a second nutrient sensing kinase GCN2 that converges on LARP1 to control TOP mRNA translation. GCN2 inhibits TOP mRNA translation via ATF4-dependent transcriptional induction of LARP1 mRNAs and GCN1-mediated recruitment of LARP1 to stalled ribosomes. We performed ATF4 ChIP-seq experiments in both WT and GCN2 KO MEFs with or without leucine deprivation.
Project description:The mammalian target of rapamycin (mTOR) is a pivotal kinase responsible for transducing cellular energy signals to regulate a host of metabolic processes including protein synthesis, which in turn regulate cell growth and proliferation. All aspects of mRNA life cycle are controlled by protein/RNA interactions and although several effectors of mTOR signalling have been identified to date, how mTOR re-sculptures the mRNA interactome is unknown. Here we characterise mTOR regulated RNA-binding proteins, identifying LARP1, whose binding to RNA increases upon mTOR inhibition. We identified over 3800 LARP1 bound mRNAs, which can be broken down into two groups, those constantly bound by LARP1, or mRNAs that increase their interaction following mTOR inhibition. LARP1 has been implicated in the control of TOP mRNA translation and importantly we observe a large number of TOP mRNAs increasing association with LARP1 upon mTOR inhibition. Regarding the regulation of LARP1, we show that LARP1 and PABP show coordinated differential mRNA binding after mTOR inhibition. Importantly we find that LARP1-PABP interaction is important for LARP1 mRNA binding and mutations in the DM15 domain of LARP1 do not perturb its RNA interaction. Lastly we show that mRNAs bound by LARP1 and PABP are translationally repressed, including mRNAs encoding proteins critical for cell growth and survival.
Project description:To study the effect of Larp1 on the abundance and subcellular localization of 5'TOP containing mRNAs, Larp1 was depleted from mouse primary cortical neurons using shRNAs. RNA from subcellular compartments (neurite and soma cytoplasm) was isolated and sequenced in parallel with scrambled control shRNA expressing samples.
Project description:Translation of TOP mRNAs encoding protein synthesis machinery is strictly regulated by an amino acid sensing mTOR pathway. However, its regulatory mechanism remains elusive. Here, we demonstrate that TOP mRNA translation positively correlates with its poly(A) tail length under mTOR active/amino acid-rich condition, suggesting that TOP mRNAs are post-transcriptionally controlled by poly(A) tail length regulation. Consistent with this, tail length of TOP mRNAs dynamically fluctuates in response to amino acid availability. Poly(A) tail shortens under mTOR active/ amino acid-rich condition, whereas the long-tailed TOP mRNAs accumulate under mTOR inactive/amino acid-starved (AAS) condition. An RNA-binding protein LARP1 that specifically binds to TOP mRNAs is indispensable for the process. We also show that LARP1 interacts with non-canonical poly(A) polymerases, PAPD4, PAPD5 and PAPD7 and induces post-transcriptional polyadenylation of the target when tethered to the mRNA. Our findings illustrate that LARP1 contributes to the selective accumulation of TOP mRNAs with long poly(A) tail under AAS, resulting in accelerated ribosomal loading onto TOP mRNAs for the resumption of translation after AAS.