Cryo-EM architecture of a near-native stretch-sensitive membrane microdomain.

Biological membranes are partitioned into functional zones termed membrane microdomains, which contain specific lipids and proteins1-3. The composition and organization of membrane microdomains remain controversial because few techniques are available that allow the visualization of lipids in situ without disrupting their native behaviour3,4. The yeast eisosome, composed of the BAR-domain proteins Pil1 and Lsp1 (hereafter, Pil1/Lsp1), scaffolds a membrane compartment that senses and responds to mechanical stress by flattening and releasing sequestered factors5-9. Here we isolated near-native eisosomes as helical tubules made up of a lattice of Pil1/Lsp1 bound to plasma membrane lipids, and solved their structures by helical reconstruction. Our structures reveal a striking organization of membrane lipids, and, using in vitro reconstitutions and molecular dynamics simulations, we confirmed the positioning of individual PI(4,5)P2, phosphatidylserine and sterol molecules sequestered beneath the Pil1/Lsp1 coat. Three-dimensional variability analysis of the native-source eisosomes revealed a dynamic stretching of the Pil1/Lsp1 lattice that affects the sequestration of these lipids. Collectively, our results support a mechanism in which stretching of the Pil1/Lsp1 lattice liberates lipids that would otherwise be anchored by the Pil1/Lsp1 coat, and thus provide mechanistic insight into how eisosome BAR-domain proteins create a mechanosensitive membrane microdomain.

Dynamic metabolome profiling uncovers potential TOR signaling genes.

Although the genetic code of the yeast Saccharomyces cerevisiae was sequenced 25 years ago, the characterization of the roles of genes within it is far from complete. The lack of a complete mapping of functions to genes hampers systematic understanding of the biology of the cell. The advent of high-throughput metabolomics offers a unique approach to uncovering gene function with an attractive combination of cost, robustness, and breadth of applicability. Here, we used flow-injection time-of-flight mass spectrometry to dynamically profile the metabolome of 164 loss-of-function mutants in TOR and receptor or receptor-like genes under a time course of rapamycin treatment, generating a dataset with >7000 metabolomics measurements. In order to provide a resource to the broader community, those data are made available for browsing through an interactive data visualization app hosted at https://rapamycin-yeast.ethz.ch. We demonstrate that dynamic metabolite responses to rapamycin are more informative than steady-state responses when recovering known regulators of TOR signaling, as well as identifying new ones. Deletion of a subset of the novel genes causes phenotypes and proteome responses to rapamycin that further implicate them in TOR signaling. We found that one of these genes, CFF1, was connected to the regulation of pyrimidine biosynthesis through URA10. These results demonstrate the efficacy of the approach for flagging novel potential TOR signaling-related genes and highlight the utility of dynamic perturbations when using functional metabolomics to deliver biological insight.

EGOC inhibits TOROID polymerization by structurally activating TORC1.

Target of rapamycin complex 1 (TORC1) is a protein kinase controlling cell homeostasis and growth in response to nutrients and stresses. In Saccharomyces cerevisiae, glucose depletion triggers a redistribution of TORC1 from a dispersed localization over the vacuole surface into a large, inactive condensate called TOROID (TORC1 organized in inhibited domains). However, the mechanisms governing this transition have been unclear. Here, we show that acute depletion and repletion of EGO complex (EGOC) activity is sufficient to control TOROID distribution, independently of other nutrient-signaling pathways. The 3.9-Å-resolution structure of TORC1 from TOROID cryo-EM data together with interrogation of key interactions in vivo provide structural insights into TORC1-TORC1' and TORC1-EGOC interaction interfaces. These data support a model in which glucose-dependent activation of EGOC triggers binding to TORC1 at an interface required for TOROID assembly, preventing TORC1 polymerization and promoting release of active TORC1.

Ultrastructure expansion microscopy reveals the cellular architecture of budding and fission yeast.

The budding and fission yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe have served as invaluable model organisms to study conserved fundamental cellular processes. Although super-resolution microscopy has in recent years paved the way to a better understanding of the spatial organization of molecules in cells, its wide use in yeasts has remained limited due to the specific know-how and instrumentation required, contrasted with the relative ease of endogenous tagging and live-cell fluorescence microscopy. To facilitate super-resolution microscopy in yeasts, we have extended the ultrastructure expansion microscopy (U-ExM) method to both S. cerevisiae and S. pombe, enabling a 4-fold isotropic expansion. We demonstrate that U-ExM allows imaging of the microtubule cytoskeleton and its associated spindle pole body, notably unveiling the Sfi1p-Cdc31p spatial organization on the appendage bridge structure. In S. pombe, we validate the method by monitoring the homeostatic regulation of nuclear pore complex number through the cell cycle. Combined with NHS-ester pan-labelling, which provides a global cellular context, U-ExM reveals the subcellular organization of these two yeast models and provides a powerful new method to augment the already extensive yeast toolbox. This article has an associated First Person interview with Kerstin Hinterndorfer and Felix Mikus, two of the joint first authors of the paper.

Cryo-EM structure of the SEA complex.

The SEA complex (SEAC) is a growth regulator that acts as a GTPase-activating protein (GAP) towards Gtr1, a Rag GTPase that relays nutrient status to the Target of Rapamycin Complex 1 (TORC1) in yeast1. Functionally, the SEAC has been divided into two subcomplexes: SEACIT, which has GAP activity and inhibits TORC1, and SEACAT, which regulates SEACIT2. This system is conserved in mammals: the GATOR complex, consisting of GATOR1 (SEACIT) and GATOR2 (SEACAT), transmits amino acid3 and glucose4 signals to mTORC1. Despite its importance, the structure of SEAC/GATOR, and thus molecular understanding of its function, is lacking. Here, we solve the cryo-EM structure of the native eight-subunit SEAC. The SEAC has a modular structure in which a COPII-like cage corresponding to SEACAT binds two flexible wings, which correspond to SEACIT. The wings are tethered to the core via Sea3, which forms part of both modules. The GAP mechanism of GATOR1 is conserved in SEACIT, and GAP activity is unaffected by SEACAT in vitro. In vivo, the wings are essential for recruitment of the SEAC to the vacuole, primarily via the EGO complex. Our results indicate that rather than being a direct inhibitor of SEACIT, SEACAT acts as a scaffold for the binding of TORC1 regulators.

Flipper Probes for the Community.

This article describes four fluorescent membrane tension probes that have been designed, synthesized, evaluated, commercialized and applied to current biology challenges in the context of the NCCR Chemical Biology. Their names are Flipper-TR®, ER Flipper-TR®, Lyso Flipper-TR®, and Mito Flipper-TR®. They are available from Spirochrome.

Chemical-Biology-derived in vivo Sensors: Past, Present, and Future.

To understand the complex biochemistry and biophysics of biological systems, one needs to be able to monitor local concentrations of molecules, physical properties of macromolecular assemblies and activation status of signaling pathways, in real time, within single cells, and at high spatio-temporal resolution. Here we look at the tools that have been / are being / need to be provided by chemical biology to address these challenges. In particular, we highlight the utility of molecular probes that help to better measure mechanical forces and flux through key signalling pathways. Chemical biology can be used to both build biosensors to visualize, but also actuators to perturb biological processes. An emergent theme is the possibility to multiplex measurements of multiple cellular processes. Advances in microscopy automation now allow us to acquire datasets for 1000's of cells. This produces high dimensional datasets that require computer vision approaches that automate image analysis. The high dimensionality of these datasets are often not immediately accessible to human intuition, and, similarly to 'omics technologies, require statistical approaches for their exploitation. The field of biosensor imaging is therefore experiencing a multidisciplinary transition that will enable it to realize its full potential as a tool to provide a deeper appreciation of cell physiology.

Passive coupling of membrane tension and cell volume during active response of cells to osmosis.

During osmotic changes of their environment, cells actively regulate their volume and plasma membrane tension that can passively change through osmosis. How tension and volume are coupled during osmotic adaptation remains unknown, as their quantitative characterization is lacking. Here, we performed dynamic membrane tension and cell volume measurements during osmotic shocks. During the first few seconds following the shock, cell volume varied to equilibrate osmotic pressures inside and outside the cell, and membrane tension dynamically followed these changes. A theoretical model based on the passive, reversible unfolding of the membrane as it detaches from the actin cortex during volume increase quantitatively describes our data. After the initial response, tension and volume recovered from hypoosmotic shocks but not from hyperosmotic shocks. Using a fluorescent membrane tension probe (fluorescent lipid tension reporter [Flipper-TR]), we investigated the coupling between tension and volume during these asymmetric recoveries. Caveolae depletion and pharmacological inhibition of ion transporters and channels, mTORCs, and the cytoskeleton all affected tension and volume responses. Treatments targeting mTORC2 and specific downstream effectors caused identical changes to both tension and volume responses, their coupling remaining the same. This supports that the coupling of tension and volume responses to osmotic shocks is primarily regulated by mTORC2.

Identification of a Covalent Importin-5 Inhibitor, Goyazensolide, from a Collective Synthesis of Furanoheliangolides.

Sesquiterpenes are a rich source of covalent inhibitors with a long history in traditional medicine and include several important therapeutics and tool compounds. Herein, we report the total synthesis of 16 sesquiterpene lactones via a build/couple/pair strategy, including goyasensolide. Using an alkyne-tagged cellular probe and proteomics analysis, we discovered that goyazensolide selectively targets the oncoprotein importin-5 (IPO5) for covalent engagement. We further demonstrate that goyazensolide inhibits the translocation of RASAL-2, a cargo of IPO5, into the nucleus and perturbs the binding between IPO5 and two specific viral nuclear localization sequences.

Phosphoproteomic Effects of Acute Depletion of PP2A Regulatory Subunit Cdc55.

Protein phosphatase regulatory subunits are increasingly recognized as promising drug targets. In the absence of an existing drug, inducible degradation provides a means of predicting candidate targets. Here auxin-inducible degradation of Saccharomyces cerevisiae PP2A regulatory subunit Cdc55 in combination with quantitative phosphoproteomics is employed. A prevalence of hyperphosphorylated phosphopeptides indicates that the approach successfully identified direct PP2ACdc55 targets. PRM follow up of data-dependent acquisition results confirmed that vacuolar amino acid transporters are among the proteins most strongly affected by Cdc55 depletion.

Resolving the Communication GAPs Upstream of TORC1.

In this issue of Developmental Cell, Yang et al. (2020) report that both nutrient- and growth factor-dependent signaling impinge upon the RAG GTPases which in turn control TSC residency time on the lysosome membrane and ultimately mTORC1 activity.

Structural Insights into TOR Signaling.

The Target of Rapamycin (TOR) is a highly conserved serine/threonine protein kinase that performs essential roles in the control of cellular growth and metabolism. TOR acts in two distinct multiprotein complexes, TORC1 and TORC2 (mTORC1 and mTORC2 in humans), which maintain different aspects of cellular homeostasis and orchestrate the cellular responses to diverse environmental challenges. Interest in understanding TOR signaling is further motivated by observations that link aberrant TOR signaling to a variety of diseases, ranging from epilepsy to cancer. In the last few years, driven in large part by recent advances in cryo-electron microscopy, there has been an explosion of available structures of (m)TORC1 and its regulators, as well as several (m)TORC2 structures, derived from both yeast and mammals. In this review, we highlight and summarize the main findings from these reports and discuss both the fascinating and unexpected molecular biology revealed and how this knowledge will potentially contribute to new therapeutic strategies to manipulate signaling through these clinically relevant pathways.

TOR complex 2 (TORC2) signaling and the ESCRT machinery cooperate in the protection of plasma membrane integrity in yeast.

The endosomal sorting complexes required for transport (ESCRT) mediate evolutionarily conserved membrane remodeling processes. Here, we used budding yeast (Saccharomyces cerevisiae) to explore how the ESCRT machinery contributes to plasma membrane (PM) homeostasis. We found that in response to reduced membrane tension and inhibition of TOR complex 2 (TORC2), ESCRT-III/Vps4 assemblies form at the PM and help maintain membrane integrity. In turn, the growth of ESCRT mutants strongly depended on TORC2-mediated homeostatic regulation of sphingolipid (SL) metabolism. This was caused by calcineurin-dependent dephosphorylation of Orm2, a repressor of SL biosynthesis. Calcineurin activity impaired Orm2 export from the endoplasmic reticulum (ER) and thereby hampered its subsequent endosome and Golgi-associated degradation (EGAD). The ensuing accumulation of Orm2 at the ER in ESCRT mutants necessitated TORC2 signaling through its downstream kinase Ypk1, which repressed Orm2 and prevented a detrimental imbalance of SL metabolism. Our findings reveal compensatory cross-talk between the ESCRT machinery, calcineurin/TORC2 signaling, and the EGAD pathway important for the regulation of SL biosynthesis and the maintenance of PM homeostasis.

The flipside of the TOR coin - TORC2 and plasma membrane homeostasis at a glance.

Target of rapamycin (TOR) is a serine/threonine protein kinase conserved in most eukaryote organisms. TOR assembles into two multiprotein complexes (TORC1 and TORC2), which function as regulators of cellular growth and homeostasis by serving as direct transducers of extracellular biotic and abiotic signals, and, through their participation in intrinsic feedback loops, respectively. TORC1, the better-studied complex, is mainly involved in cell volume homeostasis through regulating accumulation of proteins and other macromolecules, while the functions of the lesser-studied TORC2 are only now starting to emerge. In this Cell Science at a Glance article and accompanying poster, we aim to highlight recent advances in our understanding of TORC2 signalling, particularly those derived from studies in yeast wherein TORC2 has emerged as a major regulator of cell surface homeostasis.

Chemical Genetics of AGC-kinases Reveals Shared Targets of Ypk1, Protein Kinase A and Sch9.

Protein phosphorylation cascades play a central role in the regulation of cell growth and protein kinases PKA, Sch9 and Ypk1 take center stage in regulating this process in S. cerevisiae To understand how these kinases co-ordinately regulate cellular functions we compared the phospho-proteome of exponentially growing cells without and with acute chemical inhibition of PKA, Sch9 and Ypk1. Sites hypo-phosphorylated upon PKA and Sch9 inhibition were preferentially located in RRxS/T-motifs suggesting that many are directly phosphorylated by these enzymes. Interestingly, when inhibiting Ypk1 we not only detected several hypo-phosphorylated sites in the previously reported RxRxxS/T-, but also in an RRxS/T-motif. Validation experiments revealed that neutral trehalase Nth1, a known PKA target, is additionally phosphorylated and activated downstream of Ypk1. Signaling through Ypk1 is therefore more closely related to PKA- and Sch9-signaling than previously appreciated and may perform functions previously only attributed to the latter kinases.

The Aspartic Protease Ddi1 Contributes to DNA-Protein Crosslink Repair in Yeast.

Naturally occurring or drug-induced DNA-protein crosslinks (DPCs) interfere with key DNA transactions if not repaired in a timely manner. The unique family of DPC-specific proteases Wss1/SPRTN targets DPC protein moieties for degradation, including stabilized topoisomerase-1 cleavage complexes (Top1ccs). Here, we describe that the efficient DPC disassembly requires Ddi1, another conserved predicted protease in Saccharomyces cerevisiae. We found Ddi1 in a genetic screen of the tdp1 wss1 mutant defective in Top1cc processing. Ddi1 is recruited to a persistent Top1cc-like DPC lesion in an S phase-dependent manner to assist in the eviction of crosslinked protein from DNA. Loss of Ddi1 or its putative protease activity hypersensitizes cells to DPC trapping agents independently from Wss1 and 26S proteasome, implying its broader role in DPC repair. Among the potential Ddi1 targets, we found the core component of Pol II and show that its genotoxin-induced degradation is impaired in ddi1. We propose that the Ddi1 protease contributes to DPC proteolysis.

Tricalbin-Mediated Contact Sites Control ER Curvature to Maintain Plasma Membrane Integrity.

Membrane contact sites (MCS) between the endoplasmic reticulum (ER) and the plasma membrane (PM) play fundamental roles in all eukaryotic cells. ER-PM MCS are particularly abundant in Saccharomyces cerevisiae, where approximately half of the PM surface is covered by cortical ER (cER). Several proteins, including Ist2, Scs2/22, and Tcb1/2/3 are implicated in cER formation, but the specific roles of these molecules are poorly understood. Here, we use cryo-electron tomography to show that ER-PM tethers are key determinants of cER morphology. Notably, Tcb proteins (tricalbins) form peaks of extreme curvature on the cER membrane facing the PM. Combined modeling and functional assays suggest that Tcb-mediated cER peaks facilitate the transport of lipids between the cER and the PM, which is necessary to maintain PM integrity under heat stress. ER peaks were also present at other MCS, implying that membrane curvature enforcement may be a widespread mechanism to regulate MCS function.

TOR Signaling Is Going through a Phase.

Recently in Cell, Kato et al. (2019) and Yang et al. (2019) report that reversible oxidation of multiple methionines in a region of Pbp1, the yeast paralog of ataxin-2 protein, couples metabolic redox status to phase separation of Pbp1 into liquid-like condensates. In turn, Pbp1 condensates inhibit target of rapamycin complex 1 (TORC1) signaling and thereby induce autophagy and restore metabolic homeostasis.

TORC2 controls endocytosis through plasma membrane tension.

Target of rapamycin complex 2 (TORC2) is a conserved protein kinase that regulates multiple plasma membrane (PM)-related processes, including endocytosis. Direct, chemical inhibition of TORC2 arrests endocytosis but with kinetics that is relatively slow and therefore inconsistent with signaling being mediated solely through simple phosphorylation cascades. Here, we show that in addition to and independently from regulation of the phosphorylation of endocytic proteins, TORC2 also controls endocytosis by modulating PM tension. Elevated PM tension, upon TORC2 inhibition, impinges on endocytosis at two different levels by (1) severing the bonds between the PM adaptor proteins Sla2 and Ent1 and the actin cytoskeleton and (2) hindering recruitment of Rvs167, an N-BAR-containing protein important for vesicle fission to endocytosis sites. These results underline the importance of biophysical cues in the regulation of cellular and molecular processes.

Sphingolipids and membrane targets for therapeutics.

Lipids and membranes are often strongly altered in various diseases and pathologies, but are not often targeted for therapeutic advantage. In particular, the sphingolipids are particularly sensitive to altered physiology and have been implicated as important players in not only several rare hereditary diseases, but also other major pathologies, including cancer. This review discusses some potential targets in the sphingolipid pathway and describes how the initial drug compounds have been evolved to create potentially improved therapeutics. This reveals how lipids and their interactions with proteins can be used for therapeutic advantage. We also discuss the possibility that modification of the physical properties of membranes could also affect intracellular signaling and be of therapeutic interest.