As a result of continuing volume and hence surface-area growth, the skins of most fruit species suffer ongoing strain throughout development. Maintenance of surface integrity is essential to protect ...the underlying tissues from desiccation and pathogen attack. Fruit skins are commonly "primary" in structure. They comprise a polymeric cuticle overlying an epidermis and a hypodermis. The cuticle is responsible for the skin's barrier function and the cellular layers for the skin's load-bearing functions. Skin failure can be just of the cuticle layer (microcracking) resulting in barrier impairment or it can involve cuticle and cellular layers (macrocracking) resulting in both barrier and structural impairment. Fruit skin failure is associated with a number of disorders including shriveling, cracking, russeting, and skin spots. All result in reduced market value. Our objective is to review the literature on the strategies adopted by fruit to cope with the challenge of continuing skin expansion. We uncover a multistep strategy to prevent or minimize the risk of fruit skin failure. This comprises: (1) area expansion of the load-bearing skin-cell layer(s) by ongoing cell division and (2) the avoidance of skin stress or strain concentrations by matching patterns of skin-cell division to those of area expansion. Also involved, (3) are the partitioning of cuticle strain into plastic and viscoelastic components at the expense of the elastic one. For this, wax and cutin are deposited in the cuticle during growth. Wax and cutin deposition "fix" the strain in the cuticle. Cutin is preferentially deposited on the inner surface of the cuticle, which fixes the strain, but it leaves the outer cuticle surface more strained. Last, (4) if the primary skin is damaged, the barrier functions are restored by the formation of a "secondary" fruit surface (periderm). Lignin can also be used to strengthen the underlying cells following structural failure.
Anthrax toxin is the major virulence factor secreted by Bacillus anthracis, causing high mortality in humans and other mammals. It consists of a membrane translocase, known as protective antigen ...(PA), that catalyzes the unfolding of its cytotoxic substrates lethal factor (LF) and edema factor (EF), followed by translocation into the host cell. Substrate recruitment to the heptameric PA pre-pore and subsequent translocation, however, are not well understood. Here, we report three high-resolution cryo-EM structures of the fully-loaded anthrax lethal toxin in its heptameric pre-pore state, which differ in the position and conformation of LFs. The structures reveal that three LFs interact with the heptameric PA and upon binding change their conformation to form a continuous chain of head-to-tail interactions. As a result of the underlying symmetry mismatch, one LF binding site in PA remains unoccupied. Whereas one LF directly interacts with a part of PA called alpha-clamp, the others do not interact with this region, indicating an intermediate state between toxin assembly and translocation. Interestingly, the interaction of the N-terminal domain with the alpha-clamp correlates with a higher flexibility in the C-terminal domain of the protein. Based on our data, we propose a model for toxin assembly, in which the relative position of the N-terminal alpha-helices in the three LFs determines which factor is translocated first.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The lysine‐specific demethylase 6A/UTX (gene name KDM6A) acts as a component of the COMPASS complex to control gene activation. UTX demethylates H3K27me2/3 at genes and enhancers. Deleterious ...mutations in KDM6A are found in many cancer types, prominently urothelial carcinoma and certain T‐cell leukemias. In certain cancers, however, UTX supports oncogenic transcription factors, e.g. steroid hormone receptors in breast and prostate cancer. In fetal development, UTX regulates lineage choice and cell differentiation. Analogously, loss of UTX function in cancer may lead to metaplasia or impede differentiation. Likely because its function is contingent on its interacting transcription factors, the effects of UTX inactivation are not uniform and require detailed investigation in each cancer type. In urothelial carcinoma, in particular, the functional consequences of the frequent mutations in KDM6A and other COMPASS component genes are poorly understood. Nevertheless, UTX inactivation appears to sensitize many cancers to inhibitors of the H3K27 methyltransferase EZH2. Conversely, inhibitors of UTX enzymatic activity may be applicable in cancers with an oncogenic UTX function. Intriguingly, the fact that KDM6A is localized on the X‐chromosome, but both copies are expressed, may account for gender‐specific differences in cancer susceptibility. In conclusion, despite recent progress, many open questions need to be addressed, most importantly, the detailed mechanisms by which KDM6A inactivation promotes various cancers, but also with which proteins UTX interacts in and apart from the COMPASS complex, and to which extent its catalytic function is required for its tumor‐suppressive function.
The endoplasmic reticulum-mitochondria encounter structure (ERMES) complex tethers the endoplasmic reticulum and the mitochondria. It is thought to facilitate interorganelle lipid exchange and ...influence mitochondrial dynamics and mitochondrial DNA maintenance. Despite this important role, ERMES is not found in metazoans. Here, we identified single amino acid substitutions in Vps13 (vacuolar protein sorting 13), a large universally conserved eukaryotic protein, which suppress all measured phenotypic consequences of ERMES deficiency. Combined loss of VPS13 and ERMES is lethal, indicating that Vps13 and ERMES function in redundant pathways. Vps13 dynamically localizes to vacuole-mitochondria and to vacuole-nucleus contact sites depending on growth conditions, suggesting that ERMES function can be bypassed by the activity of other contact sites, and that contact sites establish a growth condition-regulated organelle network.
Various bacterial protein toxins and effectors target the actin cytoskeleton. At least three groups of toxins/effectors can be identified, which directly modify actin molecules. One group of ...toxins/effectors causes ADP‐ribosylation of actin at arginine‐177, thereby inhibiting actin polymerization. Members of this group are numerous binary actin–ADP‐ribosylating exotoxins (e.g. Clostridium botulinum C2 toxin) as well as several bacterial ADP‐ribosyltransferases (e.g. Salmonella enterica SpvB) which are not binary in structure. The second group includes toxins that modify actin to promote actin polymerization and the formation of actin aggregates. To this group belongs a toxin from the Photorhabdus luminescens Tc toxin complex that ADP‐ribosylates actin at threonine‐148. A third group of bacterial toxins/effectors (e.g. Vibrio cholerae multifunctional, autoprocessing RTX toxin) catalyses a chemical crosslinking reaction of actin thereby forming oligomers, while blocking the polymerization of actin to functional filaments. Novel findings about members of these toxin groups are discussed in detail.
Various bacterial protein toxins and effectors target the actin cytoskeleton. Binary actin‐ADP‐ribosylating exotoxins cause ADP‐ribosylation of actin at Arg177, thereby inhibiting actin polymerization. Photorhabdus luminescens toxin complex that ADP‐ribosylates actin at Thr148, promotes actin polymerization and forms actin aggregates. A third group of bacterial toxins/effectors (e.g. Vibrio cholerae RTX toxin) catalyses a chemical cross‐linking reaction of actin thereby forming oligomers, while blocking the polymerization of actin to functional filaments
A standout feature of eukaryotic cells is the presence of organelles with distinct chemical compositions and physical properties, which aid in the accomplishment of specialized metabolic tasks. This ...complex topology, however, makes a permanent crosstalk between the organelles a necessity for the coordination of cellular function. While molecule exchange between organelles via the vesicular transport system has been extensively studied, communication via direct connections has only recently become a new matter of interest. These direct connections termed membrane contact sites (MCSs) represent zones of close proximity (10–30 nm) between two organelles. Research in the past years has revealed a number of MCSs especially between the ER and almost every other organelle 1 . In particular, the MCSs between the ER and the mitochondria have undergone intense investigation. While the quest for ER–mitochondria MCS components in human cells has led to the revelation of an ever growing number of potential factors, studies in the simpler eukaryote Saccharomyces cerevisiae revealed the actual existence of a molecular tether between the two organelles 2 .
Tripartite Tc toxin complexes of bacterial pathogens perforate the host membrane and translocate toxic enzymes into the host cell, including in humans. The underlying mechanism is complex but poorly ...understood. Here we report the first, to our knowledge, high-resolution structures of a TcA subunit in its prepore and pore state and of a complete 1.7 megadalton Tc complex. The structures reveal that, in addition to a translocation channel, TcA forms four receptor-binding sites and a neuraminidase-like region, which are important for its host specificity. pH-induced opening of the shell releases an entropic spring that drives the injection of the TcA channel into the membrane. Binding of TcB/TcC to TcA opens a gate formed by a six-bladed β-propeller and results in a continuous protein translocation channel, whose architecture and properties suggest a novel mode of protein unfolding and translocation. Our results allow us to understand key steps of infections involving Tc toxins at the molecular level.
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Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Photorhabdus luminescens is an insect pathogenic bacterium that is symbiotic with entomopathogenic nematodes. On invasion of insect larvae, P. luminescens is released from the nematodes and kills the ...insect through the action of a variety of virulence factors including large tripartite ABC-type toxin complexes (Tcs). Tcs are typically composed of TcA, TcB and TcC proteins and are biologically active only when complete. Functioning as ADP-ribosyltransferases, TcC proteins were identified as the actual functional components that induce actin-clustering, defects in phagocytosis and cell death. However, little is known about the translocation of TcC into the cell by the TcA and TcB components. Here we show that TcA in P. luminescens (TcdA1) forms a transmembrane pore and report its structure in the prepore and pore state determined by cryoelectron microscopy. We find that the TcdA1 prepore assembles as a pentamer forming an α-helical, vuvuzela-shaped channel less than 1.5 nanometres in diameter surrounded by a large outer shell. Membrane insertion is triggered not only at low pH as expected, but also at high pH, explaining Tc action directly through the midgut of insects. Comparisons with structures of the TcdA1 pore inserted into a membrane and in complex with TcdB2 and TccC3 reveal large conformational changes during membrane insertion, suggesting a novel syringe-like mechanism of protein translocation. Our results demonstrate how ABC-type toxin complexes bridge a membrane to insert their lethal components into the cytoplasm of the host cell. We believe that the proposed mechanism is characteristic of the whole ABC-type toxin family. This explanation of toxin translocation is a step towards understanding the host-pathogen interaction and the complex life cycle of P. luminescens and other pathogens, including human pathogenic bacteria, and serves as a strong foundation for the development of biopesticides.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
9.
Organization and function of membrane contact sites Helle, Sebastian C.J.; Kanfer, Gil; Kolar, Katja ...
Biochimica et biophysica acta,
November 2013, 2013-Nov, 2013-11-00, Letnik:
1833, Številka:
11
Journal Article
Recenzirano
Odprti dostop
Membrane-bound organelles are a wonderful evolutionary acquisition of the eukaryotic cell, allowing the segregation of sometimes incompatible biochemical reactions into specific compartments with ...tailored microenvironments. On the flip side, these isolating membranes that crowd the interior of the cell, constitute a hindrance to the diffusion of metabolites and information to all corners of the cell. To ensure coordination of cellular activities, cells use a network of contact sites between the membranes of different organelles. These membrane contact sites (MCSs) are domains where two membranes come to close proximity, typically less than 30nm. Such contacts create microdomains that favor exchange between two organelles. MCSs are established and maintained in durable or transient states by tethering structures, which keep the two membranes in proximity, but fusion between the membranes does not take place. Since the endoplasmic reticulum (ER) is the most extensive cellular membrane network, it is thus not surprising to find the ER involved in most MCSs within the cell. The ER contacts diverse compartments such as mitochondria, lysosomes, lipid droplets, the Golgi apparatus, endosomes and the plasma membrane. In this review, we will focus on the common organizing principles underlying the many MCSs found between the ER and virtually all compartments of the cell, and on how the ER establishes a network of MCSs for the trafficking of vital metabolites and information. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.
► The endoplasmic reticulum makes several membrane contacts with various cellular organelles ► These contact sites typically serve in privileged calcium and lipid exchange reactions between compartments ► Tethering structures are required to establish and maintain such contacts ► Contact sites may be regulated in time and space to fulfill cellular needs ► Contact sites impact on the biogenesis, physiology and dynamics of most organelles.
Tc toxins are widely distributed among different gram-negative and gram-positive bacteria, where they act as pathogenicity factors. The toxins are composed of different components that form oligomers ...for biological activity. Lipid bilayer experiments were performed with the TcdA1 component of the Tc toxin from Photorhabdus luminescens, which preferentially kills insects by actin polymerization. TcdA1 was able to increase the specific conductance of artificial lipid bilayer membranes by the formation of ion-permeable channels. The channels had on average a single-channel conductance of 125 pS in 150 mM KCl and were found to be cation selective. The single-channel conductance of the TcdA1-channels was only moderately dependent on the bulk aqueous KCl concentration, which indicated point-charge effects on the channel properties. Experiments to study the voltage dependence of the TcdA1 channel demonstrated that it is reconstituted in a fully oriented way when it is added to only one side of the lipid bilayer membrane. A combination of biologically active components (TccC3) and a possible chaperone (TcdB2) blocked the TcdA1-mediated conductance efficiently in a dose-dependent manner when they were added to the cis side of the membrane. The half-saturation constant for binding of TcdB2-TccC3 to TcdA1 is in the low nanomolar range.
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