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Brown and beige adipocytes combust nutrients for thermogenesis and through their metabolic activity decrease pro-atherogenic remnant lipoproteins in hyperlipidemic mice. However, whether the activation of thermogenic adipocytes affects the metabolism and anti-atherogenic properties of high-density lipoproteins (HDL) is unknown. Here, we report a reduction in atherosclerosis in response to pharmacological stimulation of thermogenesis linked to increased HDL levels in APOE*3-Leiden.CETP mice. Both cold-induced and pharmacological thermogenic activation enhances HDL remodelling, which is associated with specific lipidomic changes in mouse and human HDL. Furthermore, thermogenic stimulation promotes HDL-cholesterol clearance and increases macrophage-to-faeces reverse cholesterol transport in mice. Mechanistically, we show that intravascular lipolysis by adipocyte lipoprotein lipase and hepatic uptake of HDL by scavenger receptor B-I are the driving forces of HDL-cholesterol disposal in liver. Our findings corroborate the notion that high metabolic activity of thermogenic adipocytes confers atheroprotective properties via increased systemic cholesterol flux through the HDL compartment. Activation of brown adipose tissue (BAT) reduces the development of atherosclerosis in animal models. Here the authors show that BAT activation also increases reverse cholesterol transport and turnover of high-density lipoprotein, which likely contributes to the anti-atherosclerotic effect of BAT activation.

CsgG and CgsE form an encaging translocon for selective, iterative diffusion of curli subunits across the non-energized bacterial outer membrane. Amyloid transporter CsgG structure This manuscript reports the atomic structure of the bacterial amyloid transporter CsgG, an outer-membrane lipoprotein that forms the translocation channel for curli (amyloid fibre) subunits that once secreted, polymerize into cross β-fibres that mediate biofilm formation. Curli fibres constitute the major protein component of the extracellular matrix in biofilms formed by Bacteroidetes and Proteobacteria. The CsgG structure reveals a 36-stranded β-barrel that forms a channel spanning the membrane bilayer and suggests a potential mechanism for guiding substrates through the secretion pore via a channel constriction. Curli are functional amyloid fibres that constitute the major protein component of the extracellular matrix in pellicle biofilms formed by Bacteroidetes and Proteobacteria (predominantly of the α and γ classes) 1 , 2 , 3 . They provide a fitness advantage in pathogenic strains and induce a strong pro-inflammatory response during bacteraemia 1 , 4 , 5 . Curli formation requires a dedicated protein secretion machinery comprising the outer membrane lipoprotein CsgG and two soluble accessory proteins, CsgE and CsgF 6 , 7 . Here we report the X-ray structure of Escherichia coli CsgG in a non-lipidated, soluble form as well as in its native membrane-extracted conformation. CsgG forms an oligomeric transport complex composed of nine anticodon-binding-domain-like units that give rise to a 36-stranded β-barrel that traverses the bilayer and is connected to a cage-like vestibule in the periplasm. The transmembrane and periplasmic domains are separated by a 0.9-nm channel constriction composed of three stacked concentric phenylalanine, asparagine and tyrosine rings that may guide the extended polypeptide substrate through the secretion pore. The specificity factor CsgE forms a nonameric adaptor that binds and closes off the periplasmic face of the secretion channel, creating a 24,000 Å 3 pre-constriction chamber. Our structural, functional and electrophysiological analyses imply that CsgG is an ungated, non-selective protein secretion channel that is expected to employ a diffusion-based, entropy-driven transport mechanism.

The APOE4-mediated proinflammatory pathway is shown to initiate blood–brain barrier breakdown and resulting neurodegeneration in transgenic mice. Restoring the blood–brain barrier There are known connections between the Alzheimer's-disease-linked APOE4 gene and cerebrovascular integrity. However, the mechanisms that drive known blood–brain-barrier dysfunction both in rodent models and in APOE4-associated neurological disorders are unknown. Here, Berislav Zlokovic and colleagues report that APOE4 activates a matrix metalloproteinase pathway in cells forming the blood–brain barrier in mice, leading to its breakdown and the neuronal uptake of blood-derived neurotoxic proteins. In turn, microvascular and cerebral blood flow are reduced; together, these deficits can initiate neurodegenerative changes in rodents. The authors suggest that cyclophilin A (CypA), a component of the APOE4-activated pathway, is a potential target for treating APOE4-mediated neuronal dysfunction. Treatment with the CypA inhibitor cyclosporine A restores the blood–brain barrier in APOE4 mice. Human apolipoprotein E has three isoforms: APOE2, APOE3 and APOE4 1 . APOE4 is a major genetic risk factor for Alzheimer’s disease 2 , 3 and is associated with Down’s syndrome dementia and poor neurological outcome after traumatic brain injury and haemorrhage 3 . Neurovascular dysfunction is present in normal APOE4 carriers 4 , 5 , 6 and individuals with APOE4 -associated disorders 3 , 7 , 8 , 9 , 10 . In mice, lack of Apoe leads to blood–brain barrier (BBB) breakdown 11 , 12 , whereas APOE4 increases BBB susceptibility to injury 13 . How APOE genotype affects brain microcirculation remains elusive. Using different APOE transgenic mice, including mice with ablation and/or inhibition of cyclophilin A (CypA), here we show that expression of APOE4 and lack of murine Apoe, but not APOE2 and APOE3, leads to BBB breakdown by activating a proinflammatory CypA–nuclear factor-κB–matrix-metalloproteinase-9 pathway in pericytes. This, in turn, leads to neuronal uptake of multiple blood-derived neurotoxic proteins, and microvascular and cerebral blood flow reductions. We show that the vascular defects in Apoe- deficient and APOE4 -expressing mice precede neuronal dysfunction and can initiate neurodegenerative changes. Astrocyte-secreted APOE3, but not APOE4, suppressed the CypA–nuclear factor-κB–matrix-metalloproteinase-9 pathway in pericytes through a lipoprotein receptor. Our data suggest that CypA is a key target for treating APOE4-mediated neurovascular injury and the resulting neuronal dysfunction and degeneration.

How proteins are targeted to lipid droplets (LDs) and distinguish the LD surface from the surfaces of other organelles is poorly understood, but many contain predicted amphipathic helices (AHs) that are involved in targeting. We have focused on human perilipin 4 (Plin4), which contains an AH that is exceptional in terms of length and repetitiveness. Using model cellular systems, we show that AH length, hydrophobicity, and charge are important for AH targeting to LDs and that these properties can compensate for one another, albeit at a loss of targeting specificity. Using synthetic lipids, we show that purified Plin4 AH binds poorly to lipid bilayers but strongly interacts with pure triglycerides, acting as a coat and forming small oil droplets. Because Plin4 overexpression alleviates LD instability under conditions where their coverage by phospholipids is limiting, we propose that the Plin4 AH replaces the LD lipid monolayer, for example during LD growth. Lipid droplets are cellular organelles important for cellular homeostasis and their disruption has been implicated in many diseases. Here the authors use a large amphipathic helix from perilipin 4 to uncover parameters important for specific lipid droplet targeting and stabilization of the oil core.

Coordination of outer membrane constriction with septation is critical to faithful division in Gram-negative bacteria and vital to the barrier function of the membrane. This coordination requires the recruitment of the peptidoglycan-binding outer-membrane lipoprotein Pal at division sites by the Tol system. Here, we show that Pal accumulation at Escherichia coli division sites is a consequence of three key functions of the Tol system. First, Tol mobilises Pal molecules in dividing cells, which otherwise diffuse very slowly due to their binding of the cell wall. Second, Tol actively captures mobilised Pal molecules and deposits them at the division septum. Third, the active capture mechanism is analogous to that used by the inner membrane protein TonB to dislodge the plug domains of outer membrane TonB-dependent nutrient transporters. We conclude that outer membrane constriction is coordinated with cell division by active mobilisation-and-capture of Pal at division septa by the Tol system. The lipoprotein Pal participates in the coordination of outer-membrane constriction with septation in Gram-negative bacteria. Here, the authors show that this coordination is mediated by active mobilisation-and-capture of Pal at division septa by the Tol system.

The worldwide dissemination of metallo-β-lactamases (MBLs), mediating resistance to carbapenem antibiotics, is a major public health problem. The extent of dissemination of MBLs such as VIM-2, SPM-1 and NDM among Gram-negative pathogens cannot be explained solely based on the associated mobile genetic elements or the resistance phenotype. Here, we report that MBL host range is determined by the impact of MBL expression on bacterial fitness. The signal peptide sequence of MBLs dictates their adaptability to each host. In uncommon hosts, inefficient processing of MBLs leads to accumulation of toxic intermediates that compromises bacterial growth. This fitness cost explains the exclusion of VIM-2 and SPM-1 from Escherichia coli and Acinetobacter baumannii , and their confinement to Pseudomonas aeruginosa . By contrast, NDMs are expressed without any apparent fitness cost in different bacteria, and are secreted into outer membrane vesicles. We propose that the successful dissemination and adaptation of MBLs to different bacterial hosts depend on protein determinants that enable host adaptability and carbapenem resistance. Metallo-β-lactamases (MBLs) confer resistance to carbapenem antibiotics. Here, López et al. show that the host range of MBLs depends on the efficiency of MBL signal peptide processing and secretion into outer membrane vesicles, which affects bacterial fitness.

Vitellogenin (Vg) is the main yolk precursor lipoprotein in almost all egg-laying animals. In addition, along its evolutionary history, Vg has developed a range of new functions in different taxa. In the honey bee, Vg has functions related to immunity, antioxidant protection, social behavior and longevity. However, the molecular mechanisms underlying Vg functionalities are still poorly understood. Here, we report the cryo-EM structure of full-length honey bee Vg, one-step purified directly from hemolymph. The structure provides structural insights into the overall domain architecture, including the lipid binding cavity and the previously uncharacterized von Willebrand factor type D domain. A domain of unknown function has been identified as a C-terminal cystine knot domain based on structural homology. Information about post-translational modifications, cleavage products, metal and lipid binding allow an improved understanding of the mechanisms underlying the range of Vg functionalities. The findings have numerous implications for the structure-function relationship of vitellogenins of other species as well as members of the same protein superfamily, which share the same structural elements. Vitellogenin is present in almost all egg-laying animals and is known for its multiple functions in reproduction, immunity and social organization. Here, the authors present the cryo-EM structure of native Vitellogenin from the honey bee.

Lipid-anchored proteins are integral components of cell surfaces. In bacteria, lipidation of proteins with a conserved lipobox motif ([L/V/I] −3 [A/S/T/V/I] −2 [G/A/S] −1 [C] +1 ) is catalyzed by prolipoprotein diacylglyceryl transferase (Lgt). Although lipobox-containing proteins, or lipoproteins, are predicted to be abundant in several archaeal species, no archaeal homologs of Lgt have been identified, suggesting distinct lipidation enzymes evolved in archaea to accommodate their unique membrane lipids. Here, we predicted lipoprotein presence for all major archaeal lineages and revealed a high prevalence of lipoproteins across the domain Archaea. Using comparative genomics, we identified a comprehensive set of candidates for archaeal lipoprotein biogenesis components (Ali). Genetic and biochemical characterization in the archaeon Haloferax volcanii confirmed that two paralogous genes, aliA and aliB , are important for lipoprotein lipidation. Moreover, deletion of both genes led to a complete absence of diphytanylglyceryl thioether from lipoprotein extracts, revealing the chemical nature of lipid anchors in Hfx. volcanii lipoproteins. Disruption of AliA- and AliB-mediated lipoprotein lipidation caused severe growth defects, decreased motility, and cell-shape alterations, underscoring the importance of lipoproteins in archaeal cell physiology. Notably, AliA and AliB exhibit distinct, non-redundant enzymatic activities with potential substrate selectivity, uncovering a new layer of regulation in prokaryotic lipoprotein lipidation. Lipoproteins are major cell-surface components in archaea, but their functions and the lipidation mechanisms are unclear. Here, Hong et al. identify two proteins required for attachment of proteins to unique archaeal membrane lipids via thioether bonds, and demonstrate their importance in archaeal physiology.

Recent advances in understanding intracellular amino acid transport and mechanistic target of rapamycin complex 1 (mTORC1) signaling shed light on solute carrier 38, family A member 9 (SLC38A9), a lysosomal transporter responsible for the binding and translocation of several essential amino acids. Here we present the first crystal structure of SLC38A9 from Danio rerio in complex with arginine. As captured in the cytosol-open state, the bound arginine was locked in a transitional state stabilized by transmembrane helix 1 (TM1) of drSLC38A9, which was anchored at the groove between TM5 and TM7. These anchoring interactions were mediated by the highly conserved WNTMM motif in TM1, and mutations in this motif abolished arginine transport by drSLC38A9. The underlying mechanism of substrate binding is critical for sensitizing the mTORC1 signaling pathway to amino acids and for maintenance of lysosomal amino acid homeostasis. This study offers a first glimpse into a prototypical model for SLC38 transporters.