TORC1 phosphorylates and inhibits the ribosome preservation factor Stm1 to activate dormant ribosomes

Target of rapamycin complex 1 (TORC1) promotes biogenesis and inhibits degradation of ribosomes in response to nutrient availability. To ensure a basal supply of ribosomes, cells preserve a small pool of dormant ribosomes under nutrient-limited conditions. The regulation of dormant ribosomes is poorly characterized. Here, we show that upon inhibition of TORC1 by rapamycin or nitrogen starvation, Stm1 (suppressor of target of Myb protein 1) forms non-translating, dormant 80S ribosomes. Furthermore, Stm1-bound 80S ribosomes are protected from proteasomal degradation. Upon re-feeding, TORC1 directly phosphorylates and inhibits Stm1, thereby reactivating translation. Finally, SERBP1 (SERPINE1 mRNA binding protein), a mammalian ortholog of Stm1, forms dormant 80S ribosomes upon mTORC1 inhibition in mammalian cells. Thus, TORC1 regulates ribosomal dormancy in an evolutionarily conserved manner via a ribosome preservation factor.