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Acetyl-CoA carboxylase catalyses the ATP-dependent carboxylation of acetyl-CoA, a rate-limiting step in fatty acid biosynthesis 1 , 2 . Eukaryotic acetyl-CoA carboxylases are large, homodimeric multienzymes. Human acetyl-CoA carboxylase occurs in two isoforms: the metabolic, cytosolic ACC1, and ACC2, which is anchored to the outer mitochondrial membrane and controls fatty acid β-oxidation 1 , 3 . ACC1 is regulated by a complex interplay of phosphorylation, binding of allosteric regulators and protein–protein interactions, which is further linked to filament formation 1 , 4 – 8 . These filaments were discovered in vitro and in vivo 50 years ago 7 , 9 , 10 , but the structural basis of ACC1 polymerization and regulation remains unknown. Here, we identify distinct activated and inhibited ACC1 filament forms. We obtained cryo-electron microscopy structures of an activated filament that is allosterically induced by citrate (ACC–citrate), and an inactivated filament form that results from binding of the BRCT domains of the breast cancer type 1 susceptibility protein (BRCA1). While non-polymeric ACC1 is highly dynamic, filament formation locks ACC1 into different catalytically competent or incompetent conformational states. This unique mechanism of enzyme regulation via large-scale conformational changes observed in ACC1 has potential uses in engineering of switchable biosynthetic systems. Dissecting the regulation of acetyl-CoA carboxylase opens new paths towards counteracting upregulation of fatty acid biosynthesis in disease. Cryo-electron microscopy studies of distinct, catalytically active and inactive filaments of human acetyl-CoA carboxylase 1 reveal the structural basis of its regulation.

Mammalian fatty acid synthase is a large multienzyme that catalyzes all steps of fatty acid synthesis. We have determined its crystal structure at 3.2 angstrom resolution covering five catalytic domains, whereas the flexibly tethered terminal acyl carrier protein and thioesterase domains remain unresolved. The structure reveals a complex architecture of alternating linkers and enzymatic domains. Substrate shuttling is facilitated by flexible tethering of the acyl carrier protein domain and by the limited contact between the condensing and modifying portions of the multienzyme, which are mainly connected by linkers rather than direct interaction. The structure identifies two additional nonenzymatic domains: (i) a pseudo-ketoreductase and (ii) a peripheral pseudo-methyltransferase that is probably a remnant of an ancestral methyltransferase domain maintained in some related polyketide synthases. The structural comparison of mammalian fatty acid synthase with modular polyketide synthases shows how their segmental construction allows the variation of domain composition to achieve diverse product synthesis.

Eukaryotic cells can undergo different forms of programmed cell death, many of which culminate in plasma membrane rupture as the defining terminal event 1 – 7 . Plasma membrane rupture was long thought to be driven by osmotic pressure, but it has recently been shown to be in many cases an active process, mediated by the protein ninjurin-1 8 (NINJ1). Here we resolve the structure of NINJ1 and the mechanism by which it ruptures membranes. Super-resolution microscopy reveals that NINJ1 clusters into structurally diverse assemblies in the membranes of dying cells, in particular large, filamentous assemblies with branched morphology. A cryo-electron microscopy structure of NINJ1 filaments shows a tightly packed fence-like array of transmembrane α-helices. Filament directionality and stability is defined by two amphipathic α-helices that interlink adjacent filament subunits. The NINJ1 filament features a hydrophilic side and a hydrophobic side, and molecular dynamics simulations show that it can stably cap membrane edges. The function of the resulting supramolecular arrangement was validated by site-directed mutagenesis. Our data thus suggest that, during lytic cell death, the extracellular α-helices of NINJ1 insert into the plasma membrane to polymerize NINJ1 monomers into amphipathic filaments that rupture the plasma membrane. The membrane protein NINJ1 is therefore an interactive component of the eukaryotic cell membrane that functions as an in-built breaking point in response to activation of cell death. Structural, biochemical and mutagenesis studies indicate that, in dying cells, the membrane protein NINJ1 assembles into filaments, disrupting the cell membrane.

Ligand–receptor interactions that are reinforced by mechanical stress, so-called catch-bonds, play a major role in cell–cell adhesion. They critically contribute to widespread urinary tract infections by pathogenic Escherichia coli strains. These pathogens attach to host epithelia via the adhesin FimH, a two-domain protein at the tip of type I pili recognizing terminal mannoses on epithelial glycoproteins. Here we establish peptide-complemented FimH as a model system for fimbrial FimH function. We reveal a three-state mechanism of FimH catch-bond formation based on crystal structures of all states, kinetic analysis of ligand interaction and molecular dynamics simulations. In the absence of tensile force, the FimH pilin domain allosterically accelerates spontaneous ligand dissociation from the FimH lectin domain by 100,000-fold, resulting in weak affinity. Separation of the FimH domains under stress abolishes allosteric interplay and increases the affinity of the lectin domain. Cell tracking demonstrates that rapid ligand dissociation from FimH supports motility of piliated E. coli on mannosylated surfaces in the absence of shear force. Catch bonds have a role in bacterial adhesion and infection by uropathogenic E. coli. Here, the authors report crystal structures, molecular dynamics simulations, ligand binding analysis and cell tracking to characterise the catch bond interaction between the adhesin FimH and carbohydrate receptors.

Target of rapamycin (TOR), a conserved protein kinase and central controller of cell growth, functions in two structurally and functionally distinct complexes: TORC1 and TORC2. Dysregulation of mammalian TOR (mTOR) signaling is implicated in pathologies that include diabetes, cancer, and neurodegeneration. We resolved the architecture of human mTORC1 (mTOR with subunits Raptor and mLST8) bound to FK506 binding protein (FKBP)–rapamycin, by combining cryo–electron microscopy at 5.9 angstrom resolution with crystallographic studies of Chaetomium thermophilum Raptor at 4.3 angstrom resolution. The structure explains how FKBP-rapamycin and architectural elements of mTORC1 limit access to the recessed active site. Consistent with a role in substrate recognition and delivery, the conserved amino-terminal domain of Raptor is juxtaposed to the kinase active site.

Urease converts urea into ammonia and carbon dioxide and makes urea available as a nitrogen source for all forms of life except animals. In human bacterial pathogens, ureases also aid in the invasion of acidic environments such as the stomach by raising the surrounding pH. Here, we report the structure of urease from the pathogen Yersinia enterocolitica at 2 Å resolution from cryo-electron microscopy. Y. enterocolitica urease is a dodecameric assembly of a trimer of three protein chains, ureA, ureB and ureC. The high data quality enables detailed visualization of the urease bimetal active site and of the impact of radiation damage. The obtained structure is of sufficient quality to support drug development efforts. Urease is a nickel enzyme responsible for catalyzing the conversion of urea into ammonia and carbon dioxide. Here the authors report a high resolution cryo-EM structure of urease from the bacterial pathogen Yersinia enterocolitica , providing a detailed visualization of the urease bimetal active site and a basis for drug development.

The TMEM175 family constitutes recently discovered K+channels that are important for autophagosome turnover and lysosomal pH regulation and are associated with the early onset of Parkinson Disease. TMEM175 channels lack a P-loop selectivity filter, a hallmark of all known K+ channels, raising the question how selectivity is achieved. Here, we report the X-ray structure of a closed bacterial TMEM175 channel in complex with a nanobody fusion-protein disclosing bound K+ ions. Our analysis revealed that a highly conserved layer of threonine residues in the pore conveys a basal K+ selectivity. An additional layer comprising two serines in human TMEM175 increases selectivity further and renders this channel sensitive to 4-aminopyridine and Zn2+. Our findings suggest that large hydrophobic side chains occlude the pore, forming a physical gate, and that channel opening by iris-like motions simultaneously relocates the gate and exposes the otherwise concealed selectivity filter to the pore lumen.

The mammalian target of rapamycin (mTOR) is a key protein kinase controlling cellular metabolism and growth. It is part of the two structurally and functionally distinct multiprotein complexes mTORC1 and mTORC2. Dysregulation of mTOR occurs in diabetes, cancer and neurological disease. We report the architecture of human mTORC2 at intermediate resolution, revealing a conserved binding site for accessory proteins on mTOR and explaining the structural basis for the rapamycin insensitivity of the complex. To grow and multiply, a living cell must take a variety of factors into account, such as its own energy levels and the availability of nutrients. A protein called mTOR sits at the core of a signaling pathway that integrates these and other sources information. Problems with the mTOR pathway contribute to several diseases including diabetes and cancer. The mTOR protein occurs in two distinct protein complexes, called mTORC1 and mTORC2. These complexes contain a mix of other proteins – known as accessory proteins. They also sense different cues and act upon distinct targets in the cell. Recent research reported the structure of mTORC1, which provided clues about how this complex works. Yet, much less was known about the mTORC2 complex. Stuttfeld, Aylett et al. have now used a technique called cryo-electron microscopy to reveal the three-dimensional architecture of the human version of mTORC2. Comparing the new mTORC2 structure to the existing one for mTORC1 showed that they have many features in common but important differences too. The overall shape of both complexes is similar and each complex contains two copies of mTOR arranged in a similar way. Also, the main accessory proteins in each complex interact with almost the exact same parts of mTOR, but the accessory proteins in mTORC2 are organized differently from those of mTORC1. The different accessory proteins also have distinct shapes. These differences could help to explain why the complexes respond to different cues and recognize different targets. These new findings provide an entry point for further studies on how mTORC2 works in cells. The next step is to get a higher resolution image of the structure of this complex to see the finer details of all the components. This may in the future help scientists to develop drugs that inhibit mTORC2 to treat cancer and other diseases.

The vertebrate-specific DEP domain-containing mTOR interacting protein (DEPTOR), an oncoprotein or tumor suppressor, has important roles in metabolism, immunity, and cancer. It is the only protein that binds and regulates both complexes of mammalian target of rapamycin (mTOR), a central regulator of cell growth. Biochemical analysis and cryo-EM reconstructions of DEPTOR bound to human mTOR complex 1 (mTORC1) and mTORC2 reveal that both structured regions of DEPTOR, the PDZ domain and the DEP domain tandem (DEPt), are involved in mTOR interaction. The PDZ domain binds tightly with mildly activating effect, but then acts as an anchor for DEPt association that allosterically suppresses mTOR activation. The binding interfaces of the PDZ domain and DEPt also support further regulation by other signaling pathways. A separate, substrate-like mode of interaction for DEPTOR phosphorylation by mTOR complexes rationalizes inhibition of non-stimulated mTOR activity at higher DEPTOR concentrations. The multifaceted interplay between DEPTOR and mTOR provides a basis for understanding the divergent roles of DEPTOR in physiology and opens new routes for targeting the mTOR-DEPTOR interaction in disease.

Acetyl-CoA carboxylases (ACCs) catalyse the committed step in fatty-acid biosynthesis: the ATP-dependent carboxylation of acetyl-CoA to malonyl-CoA. They are important regulatory hubs for metabolic control and relevant drug targets for the treatment of the metabolic syndrome and cancer. Eukaryotic ACCs are single-chain multienzymes characterized by a large, non-catalytic central domain (CD), whose role in ACC regulation remains poorly characterized. Here we report the crystal structure of the yeast ACC CD, revealing a unique four-domain organization. A regulatory loop, which is phosphorylated at the key functional phosphorylation site of fungal ACC, wedges into a crevice between two domains of CD. Combining the yeast CD structure with intermediate and low-resolution data of larger fragments up to intact ACCs provides a comprehensive characterization of the dynamic fungal ACC architecture. In contrast to related carboxylases, large-scale conformational changes are required for substrate turnover, and are mediated by the CD under phosphorylation control. Acetyl-CoA carboxylases are central regulatory hubs of fatty acid metabolism and are important targets for drug development in obesity and cancer. Here, the authors demonstrate that the regulation of these highly dynamic enzymes in fungi is governed by a mechanism based on phosphorylation-dependent conformational variability.