LARP1 facilitates translational recovery after amino acid refeeding by preserving long poly(A)-tailed TOP mRNAs

Occasionally, cells must adapt to an inimical growth conditions like amino acid starvation (AAS) by downregulating protein synthesis. A class of transcripts containing 5’terminal oligopyrimidine (5’TOP) motif encodes translation-related proteins such as ribosomal proteins (RPs) and elongation factors, and therefore, their translation is severely repressed during AAS to conserve energy. The RNA-binding protein LARP1 transduces amino acid signaling to TOP gene expression by controlling translation and stability of TOP mRNAs. When released from AAS, translation machineries in turn have to be restored, however, the underlying mechanism of such re-adaptation is largely unknown. Here we show that LARP1 preserves TOP mRNAs in a long polyadenylated state during long-term AAS. We found that TOP mRNAs become highly polyadenylated when cells are in AAS or treated with the mTOR (mechanistic target of rapamycin) inhibitor Torin1. Importantly, depletion of LARP1 completely abrogated the polyadenylation of TOP mRNAs. Comprehensive analysis of poly(A) tail length using the Nanopore direct RNA sequencing revealed that TOP mRNAs are selectively polyadenylated under mTOR inhibition. Since a long poly(A) tail confers increased stability and polysome formation of TOP mRNAs, we predict that LARP1-dependent preservation of TOP mRNAs enables rapid translational resumption after the release from AAS.