Spatially Distinct Pools of TORC1 Balance Protein Homeostasis

Abstract

Graphical AbstractHighlights d Rag GTPases and TORC1 assemble at the surfaces of both endosomes and vacuoles d These spatially and functionally distinct pools of TORC1 target specific effectors d Vacuolar TORC1 promotes protein synthesis through its proximal effector Sch9 d Endosomal TORC1 curbs macro-and microautophagy via Atg13 and Vps27, respectively SUMMARY The eukaryotic TORC1 kinase is a homeostatic controller of growth that integrates nutritional cues and mediates signals primarily from the surface of lysosomes or vacuoles. Amino acids activate TORC1 via the Rag GTPases that combine into structurally conserved multi-protein complexes such as the EGO complex (EGOC) in yeast. Here we show that Ego1, which mediates membrane-anchoring of EGOC via lipid modifications that it acquires while traveling through the trans-Golgi network, is separately sorted to vacuoles and perivacuolar endosomes. At both surfaces, it assembles EGOCs, which regulate spatially distinct pools of TORC1 that impinge on functionally divergent effectors: vacuolar TORC1 predominantly targets Sch9 to promote protein synthesis, whereas endosomal TORC1 phosphorylates Atg13 and Vps27 to inhibit macroautophagy and ESCRT-driven microautophagy, respectively. Thus, the coordination of three key regulatory nodes in protein synthesis and degradation critically relies on a division of labor between spatially sequestered populations of TORC1.

TORC1 regulates vacuole membrane composition through ubiquitin- and ESCRT-dependent microautophagy

“…It is intriguing to observe that in yeast, after TORC1 inactivation, Vph1 is downregulated by ~30-50%. Two other recent publications also made a similar observation (Hatakeyama et al, 2019;Oku et al, 2017). This observation is surprising because, in mammalian cells, it is well established that the v-ATPase components are transcriptionally upregulated by TFEB after MTORC1 inactivation (Sardiello et al, 2009).…”

“…S2A-D and S3). These results showed that the ESCRT machinery, but not macroautophagy, is required for the degradation of vacuole membrane proteins (Hatakeyama et al, 2019;Oku et al, 2017;Zhu et al, 2017).…”

“…What are the underlying mechanisms for TORC1 to downregulate vacuole membrane proteins? We reasoned that there might be three different levels of regulation: (1) (Hatakeyama et al, 2019). Upon starvation, Vps27 will be dephosphorylated to promote ESCRT assembly.…”

“…Interestingly, De Virgilio and colleagues reported that active TORC1 directly phosphorylates Vps27 to inhibit its function on the vacuole membrane. Upon starvation, the dephosphorylation of Vps27 can lead to more efficient assembly of the ESCRT complex on vacuole membrane (Hatakeyama et al, 2019).…”

“…Sakai and colleagues observed that glucose depletion leads to the invagination and degradation of two vacuole membrane proteins, Vph1 (a V0 subunit of the vacuolar ATPase) and Pho8 (vacuolar phosphatase), in an ESCRT-dependent manner (Oku et al, 2017). Similarly, De Virgilio and colleagues observed that rapamycin treatment leads to the ESCRT-dependent degradation of Pho8 (Hatakeyama et al, 2019). These observations indicated that remodeling the vacuole proteome after TORC1 inactivation might be more complex than previously anticipated, and raised several interesting questions: First, is the downregulation of vacuole membrane proteins a general response to TORC1 inactivation?…”