Distinct roles of LARP1 and 4EBP1/2 in regulating translation and stability of 5′TOP mRNAs

Vertebrate cells have evolved a simple, yet elegant, mechanism for coordinated regulation of ribosome biogenesis mediated by the 5′ terminal oligopyrimidine (5′TOP) motif. This motif allows cells to rapidly adapt to changes in the environment by specifically modulating the translation of messenger RNAs (mRNAs) encoding the translation machinery. The core signaling pathway which links nutrient availability with the production of ribosomes is the mechanistic target of rapamycin complex 1
(mTORC1), which integrates a variety of nutrient cues to modulate the translation of 5′TOP mRNAs. While the role ofmTORC1 in regulating the translation of 5′TOP mRNA has been well established, the identity of the key factor which binds to the 5′TOP motif to regulate translation has been challenging to elucidate.
Recently, La-related protein 1 (LARP1) was proposed to be the specific regulator of 5′TOP mRNA translation downstream of mTORC1, while eIF4E-binding proteins 1 and 2 (4EBP1/2) were suggested to have a general role in translational repression of all transcripts. To address these questions, we employed single-molecule translation site imaging of 5′TOP and canonical mRNAs to study the translation of single mRNAs in living cells. We reveal that 4EBP1/2 play a dominant role in the translational repression of both 5′TOP and canonical mRNAs during pharmacological inhibition of mTOR. In contrast, we find that LARP1 selectively protects 5′TOP mRNAs from degradation in a transcriptome-wide analysis of mRNA half-lives. Our results clarify the roles of 4EBP1/2 and LARP1 in regulating 5′TOP mRNAs and provide a framework to further study how these factors control cell growth during development and disease.