The Transcription Factor TFEB Links mTORC1 Signaling to Transcriptional Control of Lysosome Homeostasis

The nuclear translocation of a transcription factor that promotes lysosomal biogenesis is inhibited when lysosomal function is adequate. Sensing Lysosomal Status Lysosomes clear cells of damaged organelles, cellular debris, and internalized materials. Lysosomal biogenesis requires the transcription factor TFEB, and Roczniak-Ferguson et al. investigated the mechanisms by which lysosomal status controlled the activity of TFEB. They found that TFEB interacted with mTOR (mechanistic target of rapamycin), a kinase that localizes to lysosomes. The mTOR-dependent phosphorylation of TFEB caused TFEB to interact with 14-3-3 proteins, which led to the retention of the transcription factor in the cytoplasm. When lysosomal function was inhibited, TFEB no longer interacted with mTOR, was dephosphorylated, and translocated to the nucleus. Thus, the localization (and thus activity) of TFEB is determined by mTOR-mediated phosphorylation, which in turn reflects lysosomal status. Lysosomes are the major cellular site for clearance of defective organelles and digestion of internalized material. Demand on lysosomal capacity can vary greatly, and lysosomal function must be adjusted to maintain cellular homeostasis. Here, we identified an interaction between the lysosome-localized mechanistic target of rapamycin complex 1 (mTORC1) and the transcription factor TFEB (transcription factor EB), which promotes lysosome biogenesis. When lysosomal activity was adequate, mTOR-dependent phosphorylation of TFEB on Ser211 triggered the binding of 14-3-3 proteins to TFEB, resulting in retention of the transcription factor in the cytoplasm. Inhibition of lysosomal function reduced the mTOR-dependent phosphorylation of TFEB, resulting in diminished interactions between TFEB and 14-3-3 proteins and the translocation of TFEB into the nucleus, where it could stimulate genes involved in lysosomal biogenesis. These results identify TFEB as a target of mTOR and suggest a mechanism for matching the transcriptional regulation of genes encoding proteins of autophagosomes and lysosomes to cellular need. The closely related transcription factors MITF (microphthalmia transcription factor) and TFE3 (transcription factor E3) also localized to lysosomes and accumulated in the nucleus when lysosome function was inhibited, thus broadening the range of physiological contexts under which this regulatory mechanism may prove important.