Protein synthesis is one of the most energy consuming processes in the cell. The mammalian/mechanistic target of rapamycin (mTOR) is a serine/threonine kinase that integrates a multitude of extrace...

The mammalian target of rapamycin complex 1 (mTORC1) is a critical regulator of protein synthesis. The best-studied targets of mTORC1 in translation are the eukaryotic initiation factor binding protein 1 (4E-BP1) and ribosomal protein S6 kinase 1 (S6K1). In this study, we identify the La related protein 1 (LARP1) as a key novel target of mTORC1 with a fundamental role in terminal oligopyrimidine (TOP) mRNA translation. Recent genome-wide studies indicate that TOP and TOP-like mRNAs comprise a large portion of the mTORC1 translatome, but the mechanism by which mTORC1 controls TOP mRNA translation is incompletely understood. Here, we report that LARP1 functions as a key repressor of TOP mRNA translation downstream of mTORC1. Our data show that: (i) LARP1 associates with mTORC1 via RAPTOR, (ii) LARP1 interacts with TOP mRNAs in an mTORC1-dependent manner, (iii) LARP1 binds the 5'TOP motif to repress TOP mRNA translation, and (iv) LARP1 competes with the eukaryotic initiation factor (eIF) 4G for TOP mRNA binding. Importantly, from a drug resistance stand-point, our data also show that (v) reducing LARP1 protein levels by RNA interference attenuates the inhibitory effect of rapamycin, Torin1 and amino acid deprivation on TOP mRNA translation. Collectively, our findings demonstrate that LARP1 functions as an important repressor of TOP mRNA translation downstream of mTORC1.