RNA-binding protein that regulates the translation of specific target mRNA species downstream of the mTORC1 complex, in function of growth signals and nutrient availability (PubMed:20430826, PubMed:23711370, PubMed:24532714, PubMed:25940091, PubMed:28650797, PubMed:28673543, PubMed:29244122). Interacts on the one hand with the 3' poly-A tails that are present in all mRNA molecules, and on the other hand with the 7-methylguanosine cap structure of mRNAs containing a 5' terminal oligopyrimidine (5'TOP) motif, which is present in mRNAs encoding ribosomal proteins and several components of the translation machinery (PubMed:23711370, PubMed:25940091, PubMed:28650797, PubMed:29244122, PubMed:26206669, PubMed:28379136). The interaction with the 5' end of mRNAs containing a 5'TOP motif leads to translational repression by preventing the binding of EIF4G1 (PubMed:25940091, PubMed:28650797, PubMed:29244122, PubMed:28379136). When mTORC1 is activated, LARP1 is phosphorylated and dissociates from the 5' untranslated region (UTR) of mRNA (PubMed:25940091, PubMed:28650797). Does not prevent binding of EIF4G1 to mRNAs that lack a 5'TOP motif (PubMed:28379136). Interacts with the free 40S ribosome subunit and with ribosomes, both monosomes and polysomes (PubMed:20430826, PubMed:24532714, PubMed:25940091, PubMed:28673543). Under normal nutrient availability, interacts primarily with the 3' untranslated region (UTR) of mRNAs encoding ribosomal proteins and increases protein synthesis (PubMed:23711370, PubMed:28650797). Associates with actively translating ribosomes and stimulates translation of mRNAs containing a 5'TOP motif, thereby regulating protein synthesis, and as a consequence, cell growth and proliferation (PubMed:20430826, PubMed:24532714). Stabilizes mRNAs species with a 5'TOP motif, which is required to prevent apoptosis (PubMed:20430826, PubMed:23711370, PubMed:25940091, PubMed:28673543). (Microbial infection) Positively regulates the replication of dengue virus (DENV). Interacts with PABPC1/PABP (PubMed:20430826, PubMed:24532714, PubMed:25940091, PubMed:23711370, PubMed:28650797). Interacts with EIF4A1 (PubMed:24532714). Interacts with RPTOR (PubMed:24532714, PubMed:25940091, PubMed:28650797). Recruited to the active mTORC1 complex via interaction with RPTOR (PubMed:25940091, PubMed:28650797). Inhibition of mTORC1 activity strongly reduces interaction with RPTOR and the mTORC1 complex (PubMed:25940091, PubMed:28650797). Identified in a complex with mRNA, PABPC1, EIF4E and EIF4G1 (PubMed:20430826, PubMed:28650797). Found in a complex with PABPC1 and SHFL (PubMed:26735137). Colocalizes with RPTOR and PABPC1 in cytoplasmic granules that resemble stress granules. Up-regulated in a number of hepatocellular carcinoma cell lines and liver cancer lesions, as well as in patients with hepatocellular carcinoma with a lower survival rate (at protein level). The C-terminal region mediates interaction with the mRNA and polysomes (PubMed:24532714, PubMed:26206669, PubMed:28379136). It is required for translational repression of mRNAs with a 5'TOP motif (PubMed:29244122). The N-terminal region mediates interaction with PABPC1. Phosphorylated on multiple Ser and Thr residues in response to active mTORC1. Phosphorylation is important for interaction with RPTOR and the mTORC1 complex. Phosphorylation promotes dissociation from the 5'UTR of mRNA molecules with a 5'TOP motif. Belongs to the LARP family. Conflicting results are reported regarding the interaction with PABPC1. Some studies found that the interaction depends on the presence of mRNA (PubMed:23711370, PubMed:28650797). Others found that the interaction is direct and does not depend on the presence of mRNA (PubMed:20430826, PubMed:24532714, PubMed:25940091). Truncated N-terminus. Aberrant splicing.