Search

Target of rapamycin complex 1 (TORC1) promotes cellular anabolism and suppresses macroautophagy/autophagy. In mammalian cells starved of amino acid, the GATOR1 complex, a negative regulator of TORC1, is released from its inhibitor GATOR2 and inactivates TORC1. We have recently identified the evolutionarily conserved GATOR2 components in fission yeast including Sea3, an ortholog of mammalian WDR59, but, unexpectedly, Sea3 acts as a part of GATOR1 to suppress TORC1. Moreover, fission yeast GATOR1 is not required for the amino-acid starvation-induced TORC1 attenuation, which is instead mediated by the Gcn2 pathway. Conversely, absence of a nitrogen source suppresses TORC1 in a manner dependent on GATOR1 as well as the Tsc1-Tsc2 complex, whose mammalian equivalent functions as a growth-factor sensitive TORC1 inhibitor. Thus, the evolutionarily conserved signaling modules are utilized differently between fission yeast and mammals to control TORC1 activity and autophagy.

Mammalian target of rapamycin complex 1 (TORC1) is controlled by the GATOR complex composed of the GATOR1 subcomplex and its inhibitor, the GATOR2 subcomplex, sensitive to amino acid starvation. Previously, we identified fission yeast GATOR1 that prevents deregulated activation of TORC1 (Chia et al., 2017). Here, we report identification and characterization of GATOR2 in fission yeast. Unexpectedly, the GATOR2 subunit Sea3, an ortholog of mammalian WDR59, is physically and functionally proximal to GATOR1, rather than GATOR2, attenuating TORC1 activity. The fission yeast GATOR complex is dispensable for TORC1 regulation in response to amino acid starvation, which instead activates the Gcn2 pathway to inhibit TORC1 and induce autophagy. On the other hand, nitrogen starvation suppresses TORC1 through the combined actions of the GATOR1-Sea3 complex, the Gcn2 pathway, and the TSC complex, another conserved TORC1 inhibitor. Thus, multiple, parallel signaling pathways implement negative regulation of TORC1 to ensure proper cellular starvation responses.

Mitochondria undergo fission and fusion, and their coordinated balance is crucial for maintaining mitochondrial homeostasis. In yeast, the dynamin-related protein Dnm1 is a mitochondrial fission factor acting from outside the mitochondria. We recently reported the mitochondrial intermembrane space protein Atg44/mitofissin/Mdi1/Mco8 as a novel fission factor, but the relationship between Atg44 and Dnm1 remains elusive. Here, we show that Atg44 is required to complete Dnm1-mediated mitochondrial fission under homeostatic conditions. Atg44-deficient cells often exhibit enlarged mitochondria with accumulated Dnm1 and rosary-like mitochondria with Dnm1 foci at constriction sites. These mitochondrial constriction sites retain the continuity of both the outer and inner membranes within an extremely confined space, indicating that Dnm1 is unable to complete mitochondrial fission without Atg44. Moreover, accumulated Atg44 proteins are observed at mitochondrial constriction sites. These findings suggest that Atg44 and Dnm1 cooperatively execute mitochondrial fission from inside and outside the mitochondria, respectively. Abbreviation: ATG: autophagy related; CLEM: correlative light and electron microscopy; EM: electron microscopy; ER: endoplasmic reticulum; ERMES: endoplasmic reticulum-mitochondria encounter structure; GA: glutaraldehyde; GFP: green fluorescent protein; GTP: guanosine triphosphate: IMM: inner mitochondrial membrane; IMS: intermembrane space; OMM: outer mitochondrial membrane; PB: phosphate buffer; PBS: phosphate-buffered saline; PFA: paraformaldehyde; RFP: red fluorescent protein; WT: wild type. [ABSTRACT FROM AUTHOR]

Target of rapamycin complex 1 (TORC1) is an evolutionarily conserved protein kinase complex, whose activation in response to nutrients suppresses autophagy. In mammalian cells, amino-acid stimuli induce lysosomal translocation and activation of MTORC1 through the RRAG GTPase heterodimer, which is tethered to the surface of lysosomes by the Ragulator complex. Our recent study demonstrated that the fission yeast Schizosaccharomyces pombe also has a Ragulator complex that anchors the Gtr1-Gtr2 Rag GTPase heterodimer to the vacuole, a lysosome-like organelle. Unexpectedly, however, neither vacuolar localization nor activation of TORC1 is dependent on the Rag-Ragulator complex, which instead plays a critical role in attenuating TORC1 signaling. Our findings suggest dual functionality of the Rag GTPase in both activation and inactivation of TORC1.

TOR complex 1 (TORC1) is an evolutionarily conserved protein kinase complex that promotes cellular macromolecular synthesis and suppresses autophagy. Amino-acid-induced activation of mammalian TORC1 is initiated by its recruitment to the RagA/B-RagC/D GTPase heterodimer, which is anchored to lysosomal membranes through the Ragulator complex. We have identified in the model organism Schizosaccharomyces pombe a Ragulator-like complex that tethers the Gtr1-Gtr2 Rag heterodimer to the membranes of vacuoles, the lysosome equivalent in yeasts. Unexpectedly, the Ragulator-Rag complex is not required for the vacuolar targeting of TORC1, but the complex plays a crucial role in attenuating TORC1 activity independently of the Tsc1-Tsc2 complex, a known negative regulator of TORC1 signaling. The GATOR1 complex, which functions as Gtr1 GAP, is essential for the TORC1 attenuation by the Ragulator-Rag complex, suggesting that Gtr1GDP-Gtr2 on vacuolar membranes moderates TORC1 signaling for optimal cellular response to nutrients.

We talk to corresponding author Tomotake Kanki and co-first authors Tomoyuki Fukuda and Kentaro Furukawa about their paper "The mitochondrial intermembrane space protein mitofissin drives mitochondrial fission required for mitophagy" (this issue of Molecular Cell), their career paths, interests outside of their fields, and how they strike a work-life balance. [ABSTRACT FROM AUTHOR]

One of the most powerful techniques for attributing functions to genes in uni- and multicellular organisms is comprehensive analysis of mutant traits. In this study, systematic and quantitative analyses of mutant traits are achieved in the budding yeast Saccharomyces cerevisiae by investigating morphological phenotypes. Analysis of fluorescent microscopic images of triple-stained cells makes it possible to treat morphological variations as quantitative traits. Deletion of nearly half of the yeast genes not essential for growth affects these morphological traits. Similar morphological phenotypes are caused by deletions of functionally related genes, enabling a functional assignment of a locus to a specific cellular pathway. The high-dimensional phenotypic analysis of defined yeast mutant strains provides another step toward attributing gene function to all of the genes in the yeast genome. cell morphology | functional genomics | high-dimensional phenotyping | phenome

The meiotic roles of Cdc7 kinase in the initiation of meiotic recombination like DNA double-strand breaks (DSBs) mediated by Spo11 and other coactivating proteins are investigated. Results provide evidence that Cdc7 kinase participates in the dual meiotic regulations for the initiation of meiotic recombination and then meiotic progression into meiosis I.