Nutrient-regulated Phosphorylation of ATG13 Inhibits Starvation-induced Autophagy

Autophagy is a conserved catabolic process that utilizes a defined series of membrane trafficking events to generate a de novo double-membrane vesicle termed the autophagosome, which matures by fusing to the lysosome. Subsequently, the lysosome facilitates the degradation and recycling of the cytoplasmic cargo. In yeast, the upstream signals that regulate the induction of starvation-induced autophagy are clearly defined. The nutrient-sensing kinase Tor inhibits the activation of autophagy by regulating the formation of the Atg1-Atg13-Atg17 complex, through hyperphosphorylation of Atg13. However, in mammals, the ortholog complex ULK1-ATG13-FIP200 is constitutively formed. As such, the molecular mechanism by which mTOR regulates mammalian autophagy is unknown. Here we report the identification and characterization of novel nutrient-regulated phosphorylation sites on ATG13: Ser-224 and Ser-258. mTOR directly phosphorylates ATG13 on Ser-258 while Ser-224 is modulated by the AMPK pathway. In ATG13 knock-out cells reconstituted with an unphosphorylatable mutant of ATG13, ULK1 kinase activity is more potent, and amino acid starvation induced more rapid ATG13 and ULK1 translocation. These events culminated in a more rapid starvation-induced autophagy response. Therefore, ATG13 phosphorylation plays a crucial role in autophagy regulation.     

Chronic rapamycin treatment or lack of S6K1 does not reduce ribosome activity in vivo

Reducing activity of the mTORC1/S6K1 pathway has been shown to extend lifespan in both vertebrate and invertebrate models. For instance, both pharmacological inhibition of mTORC1 with the drug rapamycin or S6K1 knockout extends lifespan in mice. Since studies with invertebrate models suggest that reducing translational activity can increase lifespan, we reasoned that the benefits of decreased mTORC1 or S6K1 activity might be due, at least in part, to a reduction of general translational activity. Here, we report that mice given a single dose of rapamycin have reduced translational activity, while mice receiving multiple injections of rapamycin over 4 weeks show no difference in translational activity compared with vehicle-injected controls. Furthermore, mice lacking S6K1 have no difference in global translational activity compared with wild-type littermates as measured by the percentage of ribosomes that are active in multiple tissues. Translational activity is reduced in S6K1-knockout mice following single injection of rapamycin, demonstrating that rapamycin’s effects on translation can occur independently of S6K1. Taken together, these data suggest that benefits of chronic rapamycin treatment or lack of S6K1 are dissociable from potential benefits of reduced translational activity, instead pointing to a model whereby changes in translation of specific subsets of mRNAs and/or translation-independent effects of reduced mTOR signaling underlie the longevity benefits.     

PP2A regulates autophagy in two alternative ways in Drosophila

Protein phosphatase 2A (PP2A) holoenzyme is a heterotrimeric complex, consisting of A, B and C subunits. The catalytic subunit PP2A-C (microtubule star/mts) binds to the C-terminal part of the scaffold protein PP2A-A (PP2A-29B). In Drosophila, there are three different forms of B subunits (widerborst/wdb, twins/tws and PP2A-B'), which determine the subcellular localization and substrate specificity of the holoenzyme. Previous studies demonstrated that PP2A is involved in the control of TOR-dependent autophagy both in yeast and mammals. Furthermore, in Drosophila, wdb genetically interacts with the PtdIns3K/PTEN/Akt signaling cascade, which is a main upstream regulatory system of dTOR. Here we demonstrate that in Drosophila, two different PP2A complexes (containing B' or wdb subunit) play essential roles in the regulation of starvation-induced autophagy. The PP2A-A/wdb/C complex acts upstream of dTOR, whereas the PP2A-A/B'/C complex functions as a target of dTOR and may regulate the elongation of autophagosomes and their subsequent fusion with lysosomes. We also identified three Drosophila Atg orthologs (Atg14, Atg17 and Atg101), which represent potential targets of the PP2A-A/B'/C complex during autophagy.     

植物TOR激酶响应上游信号的研究进展

雷帕霉素靶蛋白(TOR)是真核生物中高度保守的丝氨酸/苏氨酸蛋白激酶, 能整合营养、能量、生长因子及环境信号, 协调细胞增殖、生长和代谢等过程, 是真核生物生长发育的核心调控因子。近年来, 随着相关研究系统的建立, 植物TOR的功能和机制研究取得了众多突破, 发现其进化上保守的生物学功能及植物中特有的信号通路。该文概述了TOR蛋白复合体的构成, 以及植物TOR响应糖、营养元素(氮、磷和硫)、激素及逆境胁迫信号来调控下游基因转录、蛋白翻译、代谢、细胞自噬和胁迫应答等生物学过程的分子机制, 并提出了植物TOR领域一些亟待解决的科学问题, 以期为全面揭示植物TOR的生物学功能提供参考。     

Identification and characterization of a functional Candida albicans homolog of the Saccharomyces cerevisiae TCO89 gene

As one of the components of target of rapamycin complex 1 (TORC1), ScTco89p is involved in rapamycin sensitivity and cellular integrity in Saccharomyces cerevisiae. Here we provide evidence showing that deletion of ScTCO89 causes yeast cells to be hypersensitive to salt stress in a high osmolarity glycerol pathway-independent fashion. In addition, we have identified and characterized a functional Candida albicans homolog (CaTCO89) of ScTCO89, which encodes a protein of 708 amino acids that shows overall 15% identity with ScTco89p at the amino acid level. However, CaTCO89 could complement the functions of ScTCO89 in rapamycin sensitivity, salt tolerance, and cellular integrity. Candida albicans cells disrupted for CaTCO89 are also sensitive to rapamycin and lithium salt, but not susceptible to challenges to cellular integrity.