mTOR-Dependent Autophagy Is Conserved in the Ciliate Tetrahymena thermophila

In Tetrahymena, autophagy appears to be involved in the integrity of the mitochondria which supply energy for active ciliary movement on the cell surface, the maintenance of various structures such as cilia, and cell shape changes associated with starvation. In this organism, which is evolutionarily distant from yeast and animal cells, the mechanisms controlling autophagy remain largely unknown. mTOR is a major regulatory factor that suppresses autophagy induction when cells are under nutrient-rich conditions. Previous studies suggest that T. thermophila contains two mTOR orthologs, but lacks the canonical TORC1 subunits, while retaining TORC2-associated components. To determine whether mTOR controls autophagy in T. thermophila, we examined Torin1, an ATP-competitive inhibitor that blocks mTOR kinase activity regardless of complex composition. It markedly triggered ATG8 puncta formation and mitochondrial degradation under nutrient-rich conditions. In contrast, rapamycin, a TORC1-specific inhibitor widely used to induce autophagy, did not affect cell growth or autophagy. These findings demonstrate that mTOR functions as a negative regulator of autophagy in T. thermophila, likely through a rapamycin-insensitive TORC2 or noncanonical complex. Our results highlight lineage-specific divergence in mTOR signaling architecture and expand our understanding of the diversity of autophagy regulation across eukaryotes.