FUS contributes to mTOR-dependent inhibition of translation.

The amyotrophic lateral sclerosis (ALS) and frontotemporal degeneration (FTD)-linked RNA-binding protein called fused in sarcoma (FUS) has been implicated in several aspects of RNA regulation, including mRNA translation. The mechanism by which FUS affects the translation of polyribosomes has not been established. Here we show that FUS can associate with stalled polyribosomes and that this association is sensitive to mTOR (mammalian Target of Rapamycin) kinase activity. Specifically, we show that FUS association with polyribosomes is increased by Torin1 treatment or when cells are cultured in nutrient-deficient media, but not when cells are treated with rapamycin, the allosteric inhibitor of mTORC1. Moreover, we report that FUS is necessary for efficient stalling of translation as FUS-deficient cells are refractory to the inhibition of mTOR-dependent signaling by Torin1. We also show that ALS-linked FUS mutants, R521G and P525L, associate abundantly with polyribosomes and decrease global protein synthesis. Importantly, the inhibitory effect on translation by FUS is impaired by mutations that reduce its RNA binding affinity. These findings demonstrate that FUS is an important RNA-binding protein that mediates translational repression through mTOR-dependent signaling, and that ALS-linked FUS-mutants can cause a toxic gain-of-function in the cytoplasm by repressing the translation of mRNA at polyribosomes.