Membrane Stored Curvature Elastic Stress Modulates Recruitment of Maintenance Proteins PspA and Vipp1

Mechanism for Vipp1 spiral formation, ring biogenesis, and membrane repair

The ESCRT-III-like protein Vipp1 couples filament polymerization with membrane remodeling. It assembles planar sheets as well as 3D rings and helical polymers, all implicated in mitigating plastid-associated membrane stress. The architecture of Vipp1 planar sheets and helical polymers remains unknown, as do the geometric changes required to transition between polymeric forms. Here we show how cyanobacterial Vipp1 assembles into morphologically-related sheets and spirals on membranes in vitro. The spirals converge to form a central ring similar to those described in membrane budding. Cryo-EM structures of helical filaments reveal a close geometric relationship between Vipp1 helical and planar lattices. Moreover, the helical structures reveal how filaments twist-a process required for Vipp1, and likely other ESCRT-III filaments, to transition between planar and 3D architectures. Overall, our results provide a molecular model for Vipp1 ring biogenesis and a mechanism for Vipp1 membrane stabilization and repair, with implications for other ESCRT-III systems.

Evolutionary trends indicate a coherent organization of sap operons

Human hosts possess a complex network of immune responses against microbial pathogens. The production of antimicrobial peptides (AMPs), which target the pathogen cell membranes and inhibit them from inhabiting the hosts, is one such mechanism. However, pathogens have evolved systems that encounter these host-produced AMPs. The Sap (sensitivity to antimicrobial peptides) transporter uptakes AMPs inside the microbial cell and proteolytically degrades them. The Sap transporters comprise five subunits encoded by genes in an operon. Despite its ubiquitous nature, its subunits are not found to be in tandem with many organisms. In this study, a total of 421 Sap transporters were analyzed for their operonic arrangement. Out of 421, a total of 352 operons were found to be in consensus arrangement, while the remaining 69 show a varying arrangement of genes. The analysis of the intergenic distance between the subunits of the sap operon suggests a signature pattern with sapAB (-4), sapBC (-14), sapCD (-1), and sapDF (-4 to 1). An evolutionary analysis of these operons favors the consensus arrangement of the Sap transporter systems, substantiating its prevalence in most of the Gram-negative pathogens. Overall, this study provides insight into bacterial evolution, favoring the maintenance of the genetic organization of essential pathogenicity factors.

Structural basis for Vipp1 membrane binding: from loose coats and carpets to ring and rod assemblies

Vesicle-inducing protein in plastids 1 (Vipp1) is critical for thylakoid membrane biogenesis and maintenance. Although Vipp1 has recently been identified as a member of the endosomal sorting complexes required for transport III superfamily, it is still unknown how Vipp1 remodels membranes. Here, we present cryo-electron microscopy structures of Synechocystis Vipp1 interacting with membranes: seven structures of helical and stacked-ring assemblies at 5-7-Å resolution engulfing membranes and three carpet structures covering lipid vesicles at ~20-Å resolution using subtomogram averaging. By analyzing ten structures of N-terminally truncated Vipp1, we show that helix α0 is essential for membrane tubulation and forms the membrane-anchoring domain of Vipp1. Lastly, using a conformation-restrained Vipp1 mutant, we reduced the structural plasticity of Vipp1 and determined two structures of Vipp1 at 3.0-Å resolution, resolving the molecular details of membrane-anchoring and intersubunit contacts of helix α0. Our data reveal membrane curvature-dependent structural transitions from carpets to rings and rods, some of which are capable of inducing and/or stabilizing high local membrane curvature triggering membrane fusion.

Interaction between ESCRT-III proteins and the yeast SERINC homolog Tms1

The endosomal sorting complex required for transport (ESCRT)-III is involved in membrane remodeling and abscission during intraluminal vesicle (ILV) formation at endosomes. Our data now suggest that ESCRT-III function could be connected to lipid remodeling of the endosomal membrane. This notion is based on our finding that ESCRT-III proteins bind to the yeast serine incorporator (SERINC) homolog Tms1. Human SERINC3 and SERINC5 are HIV-1 restriction factors and have been shown to act as scramblases, flipping phospholipids between membrane leaflets. Due to the extraordinarily high sequence conservation between Tms1 and human SERINCs, it is likely that Tms1 is also a scramblase. While deletion of TMS1 had only a moderate effect on the sorting of multivesicular body (MVB) cargo proteins, the simultaneous deletion of a component of the Vps55/Vps68 complex led to a strong synergistic phenotype. This pronounced synergism suggests that Tms1 and Vps55/Vps68 perform a parallel function at endosomes. Vps55/Vps68 loosely resembles Tms1 in its overall structure. Thus, it is possible that Vps55/Vps68 is also a scramblase. Since both Vps55 and Tms1 physically interact with ESCRT-III proteins, we propose that the recruitment of a scramblase plays a crucial role in ESCRT-III-dependent membrane remodeling at endosomes.

Structure, biogenesis, and evolution of thylakoid membranes

Cyanobacteria and chloroplasts of algae and plants harbor specialized thylakoid membranes (TMs) that convert sunlight into chemical energy. These membranes house PSII and I, the vital protein-pigment complexes that drive oxygenic photosynthesis. In the course of their evolution, TMs have diversified in structure. However, the core machinery for photosynthetic electron transport remained largely unchanged, with adaptations occurring primarily in the light-harvesting antenna systems. Whereas TMs in cyanobacteria are relatively simple, they become more complex in algae and plants. The chloroplasts of vascular plants contain intricate networks of stacked grana and unstacked stroma thylakoids. This review provides an in-depth view of TM architectures in phototrophs and the determinants that shape their forms, as well as presenting recent insights into the spatial organization of their biogenesis and maintenance. Its overall goal is to define the underlying principles that have guided the evolution of these bioenergetic membranes.

The phage shock protein (PSP) envelope stress response: discovery of novel partners and evolutionary history

Bacterial phage shock protein (PSP) systems stabilize the bacterial cell membrane and protect against envelope stress. These systems have been associated with virulence, but despite their critical roles, PSP components are not well characterized outside proteobacteria. Using comparative genomics and protein sequence-structure-function analyses, we systematically identified and analyzed PSP homologs, phyletic patterns, domain architectures, and gene neighborhoods. This approach underscored the evolutionary significance of the system, revealing that its core protein PspA (Snf7 in ESCRT outside bacteria) was present in the last universal common ancestor and that this ancestral functionality has since diversified into multiple novel, distinct PSP systems across life. Several novel partners of the PSP system were identified: (i) the Toastrack domain, likely facilitating assembly of sub-membrane stress-sensing and signaling complexes, (ii) the newly defined HTH-associated α-helical signaling domain-PadR-like transcriptional regulator pair system, and (iii) multiple independent associations with ATPase, CesT/Tir-like chaperone, and Band-7 domains in proteins thought to mediate sub-membrane dynamics. Our work also uncovered links between the PSP components and other domains, such as novel variants of SHOCT-like domains, suggesting roles in assembling membrane-associated complexes of proteins with disparate biochemical functions. Results are available at our interactive web app, https://jravilab.org/psp.IMPORTANCEPhage shock proteins (PSP) are virulence-associated, cell membrane stress-protective systems. They have mostly been characterized in Proteobacteria and Firmicutes. We now show that a minimal PSP system was present in the last universal common ancestor that evolved and diversified into newly identified functional contexts. Recognizing the conservation and evolution of PSP systems across bacterial phyla contributes to our understanding of stress response mechanisms in prokaryotes. Moreover, the newly discovered PSP modularity will likely prompt new studies of lineage-specific cell envelope structures, lifestyles, and adaptation mechanisms. Finally, our results validate the use of domain architecture and genetic context for discovery in comparative genomics.

Conserved structures of ESCRT-III superfamily members across domains of life

Structural and evolutionary studies of cyanobacterial phage shock protein A (PspA) and inner membrane-associated protein of 30 kDa (IM30) have revealed that these proteins belong to the endosomal sorting complex required for transport-III (ESCRT-III) superfamily, which is conserved across all three domains of life. PspA and IM30 share secondary and tertiary structures with eukaryotic ESCRT-III proteins, whilst also oligomerizing via conserved interactions. Here, we examine the structures of bacterial ESCRT-III-like proteins and compare the monomeric and oligomerized forms with their eukaryotic counterparts. We discuss conserved interactions used for self-assembly and highlight key hinge regions that mediate oligomer ultrastructure versatility. Finally, we address the differences in nomenclature assigned to equivalent structural motifs in both the bacterial and eukaryotic fields and suggest a common nomenclature applicable across the ESCRT-III superfamily.

TurboID reveals the proxiomes of Chlamydomonas proteins involved in thylakoid biogenesis and stress response

In Chlamydomonas (Chlamydomonas reinhardtii), the VESICLE-INDUCING PROTEIN IN PLASTIDS 1 and 2 (VIPP1 and VIPP2) play roles in the sensing and coping with membrane stress and in thylakoid membrane biogenesis. To gain more insight into these processes, we aimed to identify proteins interacting with VIPP1/2 in the chloroplast and chose proximity labeling (PL) for this purpose. We used the transient interaction between the nucleotide exchange factor CHLOROPLAST GRPE HOMOLOG 1 (CGE1) and the stromal HEAT SHOCK PROTEIN 70B (HSP70B) as test system. While PL with APEX2 and BioID proved to be inefficient, TurboID resulted in substantial biotinylation in vivo. TurboID-mediated PL with VIPP1/2 as baits under ambient and H2O2 stress conditions confirmed known interactions of VIPP1 with VIPP2, HSP70B, and the CHLOROPLAST DNAJ HOMOLOG 2 (CDJ2). Proteins identified in the VIPP1/2 proxiomes can be grouped into proteins involved in the biogenesis of thylakoid membrane complexes and the regulation of photosynthetic electron transport, including PROTON GRADIENT REGULATION 5-LIKE 1 (PGRL1). A third group comprises 11 proteins of unknown function whose genes are upregulated under chloroplast stress conditions. We named them VIPP PROXIMITY LABELING (VPL). In reciprocal experiments, we confirmed VIPP1 in the proxiomes of VPL2 and PGRL1. Our results demonstrate the robustness of TurboID-mediated PL for studying protein interaction networks in the chloroplast of Chlamydomonas and pave the way for analyzing functions of VIPPs in thylakoid biogenesis and stress responses.

Membrane Lipid Reshaping Underlies Oxidative Stress Sensing by the Mitochondrial Proteins UCP1 and ANT1

Oxidative stress and ROS are important players in the pathogenesis of numerous diseases. In addition to directly altering proteins, ROS also affects lipids with negative intrinsic curvature such as phosphatidylethanolamine (PE), producing PE adducts and lysolipids. The formation of PE adducts potentiates the protonophoric activity of mitochondrial uncoupling proteins, but the molecular mechanism remains unclear. Here, we linked the ROS-mediated change in lipid shape to the mechanical properties of the membrane and the function of uncoupling protein 1 (UCP1) and adenine nucleotide translocase 1 (ANT1). We show that the increase in the protonophoric activity of both proteins occurs due to the decrease in bending modulus in lipid bilayers in the presence of lysophosphatidylcholines (OPC and MPC) and PE adducts. Moreover, MD simulations showed that modified PEs and lysolipids change the lateral pressure profile of the membrane in the same direction and by the similar amplitude, indicating that modified PEs act as lipids with positive intrinsic curvature. Both results indicate that oxidative stress decreases stored curvature elastic stress (SCES) in the lipid bilayer membrane. We demonstrated that UCP1 and ANT1 sense SCES and proposed a novel regulatory mechanism for the function of these proteins. The new findings should draw the attention of the scientific community to this important and unexplored area of redox biochemistry.

Sequestration of a dual function DNA-binding protein by Vibrio cholerae CRP

Although the mechanism by which the cyclic AMP receptor protein (CRP) regulates global gene transcription has been intensively studied for decades, new discoveries remain to be made. Here, we report that, during rapid growth, CRP associates with both the well-conserved, dual-function DNA-binding protein peptidase A (PepA) and the cell membrane. These interactions are not present under nutrient-limited growth conditions, due to post-translational modification of three lysines on a single face of CRP. Although coincident DNA binding is rare, dissociation from CRP results in increased PepA occupancy at many chromosomal binding sites and differential regulation of hundreds of genes, including several encoding cyclic dinucleotide phosphodiesterases. We show that PepA represses biofilm formation and activates motility/chemotaxis. We propose a model in which membrane-bound CRP interferes with PepA DNA binding. Under nutrient limitation, PepA is released. Together, CRP and free PepA activate a transcriptional response that impels the bacterium to seek a more hospitable environment. This work uncovers a function for CRP in the sequestration of a regulatory protein. More broadly, it describes a paradigm of bacterial transcriptome modulation through metabolically regulated association of transcription factors with the cell membrane.

Envelope Stress Activates Expression of the Twin Arginine Translocation (Tat) System in Salmonella

The twin arginine translocation system (Tat) is a protein export system that is conserved in bacteria, archaea, and plants. In Gram-negative bacteria, it is required for the export of folded proteins from the cytoplasm to the periplasm. In Salmonella, there are 30 proteins that are predicted substrates of Tat, and among these are enzymes required for anaerobic respiration and peptidoglycan remodeling. We have demonstrated that some conditions that induce bacterial envelope stress activate expression of a ΔtatABC-lacZ fusion in Salmonella enterica serovar Typhimurium. Particularly, the addition of bile salts to the growth medium causes a 3-fold induction of a ΔtatABC-lacZ reporter fusion. Our data demonstrate that this induction is mediated via the phage shock protein (Psp) stress response system protein PspA. Further, we show that deletion of tatABC increases the induction of tatABC expression in bile salts. Indeed, the data suggest significant interaction between PspA and the Tat system in the regulatory response to bile salts. Although we have not identified the precise mechanism of Psp regulation of tatABC, our work shows that PspA is involved in the activation of tatABC expression by bile salts and adds another layer of complexity to the Salmonella response to envelope stress. IMPORTANCE Salmonella species cause an array of diseases in a variety of hosts. This research is significant in showing induction of the Tat system as a defense against periplasmic stress. Understanding the underlying mechanism of this regulation broadens our understanding of the Salmonella stress response, which is critical to the ability of the organism to cause infection.

Membrane destabilization and pore formation induced by the Synechocystis IM30 protein

The inner membrane-associated protein of 30 kDa (IM30) is essential in chloroplasts and cyanobacteria. The spatio-temporal cellular localization of the protein appears to be highly dynamic and triggered by internal as well as external stimuli, mainly light intensity. The soluble fraction of the protein is localized in the cyanobacterial cytoplasm or the chloroplast stroma, respectively. Additionally, the protein attaches to the thylakoid membrane as well as to the chloroplast inner envelope or the cyanobacterial cytoplasmic membrane, respectively, especially under conditions of membrane stress. IM30 is involved in thylakoid membrane biogenesis and/or maintenance, where it either stabilizes membranes and/or triggers membrane-fusion processes. These apparently contradicting functions have to be tightly controlled and separated spatiotemporally in chloroplasts and cyanobacteria. IM30's fusogenic activity depends on Mg2+ binding to IM30; yet, it still is unclear how Mg2+-loaded IM30 interacts with membranes and promotes membrane fusion. Here, we show that the interaction of Mg2+ with IM30 results in increased binding of IM30 to native, as well as model, membranes. Via atomic force microscopy in liquid, IM30-induced bilayer defects were observed in solid-supported bilayers in the presence of Mg2+. These structures differ dramatically from the membrane-stabilizing carpet structures that were previously observed in the absence of Mg2+. Thus, Mg2+-induced alterations of the IM30 structure switch the IM30 activity from a membrane-stabilizing to a membrane-destabilizing function, a crucial step in membrane fusion.

Distinctive in vitro ATP Hydrolysis Activity of AtVIPP1, a Chloroplastic ESCRT-III Superfamily Protein in Arabidopsis

Vesicle-inducing protein in plastid 1 (VIPP1), characteristic to oxygenic photosynthetic organisms, is a membrane-remodeling factor that forms homo-oligomers and functions in thylakoid membrane formation and maintenance. The cyanobacterial VIPP1 structure revealed a monomeric folding pattern similar to that of endosomal sorting complex required for transport (ESCRT) III. Characteristic to VIPP1, however, is its own GTP and ATP hydrolytic activity without canonical domains. In this study, we found that histidine-tagged Arabidopsis VIPP1 (AtVIPP1) hydrolyzed GTP and ATP to produce GDP and ADP in vitro, respectively. Unexpectedly, the observed GTPase and ATPase activities were biochemically distinguishable, because the ATPase was optimized for alkaline conditions and dependent on Ca²⁺ as well as Mg²⁺, with a higher affinity for ATP than GTP. We found that a version of AtVIPP1 protein with a mutation in its nucleotide-binding site, as deduced from the cyanobacterial structure, retained its hydrolytic activity, suggesting that Arabidopsis and cyanobacterial VIPP1s have different properties. Negative staining particle analysis showed that AtVIPP1 formed particle or rod structures that differed from those of cyanobacteria and Chlamydomonas. These results suggested that the nucleotide hydrolytic activity and oligomer formation of VIPP1 are common in photosynthetic organisms, whereas their properties differ among species.

An Stomatin, Prohibitin, Flotillin, and HflK/C-Domain Protein Required to Link the Phage-Shock Protein to the Membrane in Bacillus subtilis

Membrane surveillance and repair is of utmost importance to maintain cellular integrity and allow cellular life. Several systems detect cell envelope stress caused by antimicrobial compounds and abiotic stresses such as solvents, pH-changes and temperature in bacteria. Proteins containing an Stomatin, Prohibitin, Flotillin, and HflK/C (SPFH)-domain, including bacterial flotillins have been shown to be involved in membrane protection and membrane fluidity regulation. Here, we characterize a bacterial SPFH-domain protein, YdjI that is part of a stress induced complex in Bacillus subtilis. We show that YdjI is required to localize the ESCRT-III homolog PspA to the membrane with the help of two membrane integral proteins, YdjG/H. In contrast to classical flotillins, YdjI resides in fluid membrane regions and does not enrich in detergent resistant membrane fractions. However, similarly to FloA and FloT from B. subtilis, deletion of YdjI decreases membrane fluidity. Our data reveal a hardwired connection between phage shock response and SPFH proteins.

Does membrane curvature elastic energy play a role in mediating oxidative stress in lipid membranes?

The effects of oxidative stress on cells are associated with a wide range of pathologies. Oxidative stress is predominantly initiated by the action of reactive oxygen species and/or lipoxygenases on polyunsaturated fatty acid containing lipids. The downstream products are oxidised phospholipids, bioactive aldehydes and a range of Schiff base by-products between aldehydes and lipids, or other biomacromolecules. In this review we assess the impact of oxidative stress on lipid membranes, focusing on the changes that occur to the curvature preference (lipid spontaneous curvature) and elastic properties of membranes, since these biophysical properties modulate phospholipid homeostasis. Studies show that the lipid products of oxidative stress reduce stored curvature elastic energy in membranes. Based upon this observation, we hypothesize that the effects of oxidative stress on lipid membranes will be reduced by compounds that increase stored curvature elastic energy. We find a strong correlation appears across literature studies that we have reviewed, such that many compounds like vitamin E, Curcumin, Coenzyme Q10 and vitamin A show behaviour consistent with this hypothesis. Finally, we consider whether age-related changes in lipid composition represent the homeostatic response of cells to compensate for the accumulation of in vivo lipid oxidation products.

Surface Sensing and Adaptation in Bacteria

Bacteria thrive both in liquids and attached to surfaces. The concentration of bacteria on surfaces is generally much higher than in the surrounding environment, offering bacteria ample opportunity for mutualistic, symbiotic, and pathogenic interactions. To efficiently populate surfaces, they have evolved mechanisms to sense mechanical or chemical cues upon contact with solid substrata. This is of particular importance for pathogens that interact with host tissue surfaces. In this review we discuss how bacteria are able to sense surfaces and how they use this information to adapt their physiology and behavior to this new environment. We first survey mechanosensing and chemosensing mechanisms and outline how specific macromolecular structures can inform bacteria about surfaces. We then discuss how mechanical cues are converted to biochemical signals to activate specific cellular processes in a defined chronological order and describe the role of two key second messengers, c-di-GMP and cAMP, in this process.

Bacterial Vipp1 and PspA are members of the ancient ESCRT-III membrane-remodeling superfamily

Membrane remodeling and repair are essential for all cells. Proteins that perform these functions include Vipp1/IM30 in photosynthetic plastids, PspA in bacteria, and ESCRT-III in eukaryotes. Here, using a combination of evolutionary and structural analyses, we show that these protein families are homologous and share a common ancient evolutionary origin that likely predates the last universal common ancestor. This homology is evident in cryo-electron microscopy structures of Vipp1 rings from the cyanobacterium Nostoc punctiforme presented over a range of symmetries. Each ring is assembled from rungs that stack and progressively tilt to form dome-shaped curvature. Assembly is facilitated by hinges in the Vipp1 monomer, similar to those in ESCRT-III proteins, which allow the formation of flexible polymers. Rings have an inner lumen that is able to bind and deform membranes. Collectively, these data suggest conserved mechanistic principles that underlie Vipp1, PspA, and ESCRT-III-dependent membrane remodeling across all domains of life.

Insights into complex nanopillar-bacteria interactions: Roles of nanotopography and bacterial surface proteins

Nanopillared surfaces have emerged as a promising strategy to combat bacterial infections on medical devices. However, the mechanisms that underpin nanopillar-induced rupture of the bacterial cell membrane remain speculative. In this study, we have tested three medically relevant poly(ethylene terephthalate) (PET) nanopillared-surfaces with well-defined nanotopographies against both Gram-negative and Gram-positive bacteria. Focused ion beam scanning electron microscopy (FIB-SEM) and contact mechanics analysis were utilised to understand the nanobiophysical response of the bacterial cell envelope to a single nanopillar. Given their importance to bacterial adhesion, the contribution of bacterial surface proteins to nanotopography-mediated cell envelope damage was also investigated. We found that, whilst cell envelope deformation was affected by the nanopillar tip diameter, the nanopillar density affected bacterial metabolic activities. Moreover, three different types of bacterial cell envelope deformation were observed upon contact of bacteria with the nanopillared surfaces. These were attributed to bacterial responses to cell wall stresses resulting from the high intrinsic pressure caused by the engagement of nanopillars by bacterial surface proteins. Such influences of bacterial surface proteins on the antibacterial action of nanopillars have not been previously reported. Our findings will be valuable to the improved design and fabrication of effective antibacterial surfaces.

ESCRTs got your Bac!

ESCRT-III proteins, which form filaments that deform, bud, and sever membranes, are found in eukaryotes and some archaea. Three studies in this issue of Cell reveal that PspA and Vipp1 are bacterial and cyanobacterial members of the ESCRT-III superfamily, indicating it is even more ubiquitous and ancient than previously thought.

Structural basis for VIPP1 oligomerization and maintenance of thylakoid membrane integrity

Vesicle-inducing protein in plastids 1 (VIPP1) is essential for the biogenesis and maintenance of thylakoid membranes, which transform light into life. However, it is unknown how VIPP1 performs its vital membrane-remodeling functions. Here, we use cryo-electron microscopy to determine structures of cyanobacterial VIPP1 rings, revealing how VIPP1 monomers flex and interweave to form basket-like assemblies of different symmetries. Three VIPP1 monomers together coordinate a non-canonical nucleotide binding pocket on one end of the ring. Inside the ring's lumen, amphipathic helices from each monomer align to form large hydrophobic columns, enabling VIPP1 to bind and curve membranes. In vivo mutations in these hydrophobic surfaces cause extreme thylakoid swelling under high light, indicating an essential role of VIPP1 lipid binding in resisting stress-induced damage. Using cryo-correlative light and electron microscopy (cryo-CLEM), we observe oligomeric VIPP1 coats encapsulating membrane tubules within the Chlamydomonas chloroplast. Our work provides a structural foundation for understanding how VIPP1 directs thylakoid biogenesis and maintenance.

PspA adopts an ESCRT-III-like fold and remodels bacterial membranes

PspA is the main effector of the phage shock protein (Psp) system and preserves the bacterial inner membrane integrity and function. Here, we present the 3.6 Å resolution cryoelectron microscopy (cryo-EM) structure of PspA assembled in helical rods. PspA monomers adopt a canonical ESCRT-III fold in an extended open conformation. PspA rods are capable of enclosing lipids and generating positive membrane curvature. Using cryo-EM, we visualized how PspA remodels membrane vesicles into μm-sized structures and how it mediates the formation of internalized vesicular structures. Hotspots of these activities are zones derived from PspA assemblies, serving as lipid transfer platforms and linking previously separated lipid structures. These membrane fusion and fission activities are in line with the described functional properties of bacterial PspA/IM30/LiaH proteins. Our structural and functional analyses reveal that bacterial PspA belongs to the evolutionary ancestry of ESCRT-III proteins involved in membrane remodeling.

Binding and/or hydrolysis of purine-based nucleotides is not required for IM30 ring formation

IM30, the inner membrane-associated protein of 30 kDa, is conserved in cyanobacteria and chloroplasts. Although its exact physiological function is still mysterious, IM30 is clearly essential for thylakoid membrane biogenesis and/or dynamics. Recently, a cryptic IM30 GTPase activity has been reported, albeit thus far no physiological function has been attributed to this. Yet, it is still possible that GTP binding/hydrolysis affects formation of the prototypical large homo-oligomeric IM30 ring and rod structures. Here, we show that the Synechocystis sp. PCC 6803 IM30 protein in fact is an NTPase that hydrolyzes GTP and ATP, but not CTP or UTP, with about identical rates. While IM30 forms large oligomeric ring complexes, nucleotide binding and/or hydrolysis are clearly not required for ring formation.

Mini Review: Bacterial Membrane Composition and Its Modulation in Response to Stress

Antibiotics and other agents that perturb the synthesis or integrity of the bacterial cell envelope trigger compensatory stress responses. Focusing on Bacillus subtilis as a model system, this mini-review summarizes current views of membrane structure and insights into how cell envelope stress responses remodel and protect the membrane. Altering the composition and properties of the membrane and its associated proteome can protect cells against detergents, antimicrobial peptides, and pore-forming compounds while also, indirectly, contributing to resistance against compounds that affect cell wall synthesis. Many of these regulatory responses are broadly conserved, even where the details of regulation may differ, and can be important in the emergence of antibiotic resistance in clinical settings.

Mini Review: Bacterial Membrane Composition and Its Modulation in Response to Stress

Antibiotics and other agents that perturb the synthesis or integrity of the bacterial cell envelope trigger compensatory stress responses. Focusing on Bacillus subtilis as a model system, this mini-review summarizes current views of membrane structure and insights into how cell envelope stress responses remodel and protect the membrane. Altering the composition and properties of the membrane and its associated proteome can protect cells against detergents, antimicrobial peptides, and pore-forming compounds while also, indirectly, contributing to resistance against compounds that affect cell wall synthesis. Many of these regulatory responses are broadly conserved, even where the details of regulation may differ, and can be important in the emergence of antibiotic resistance in clinical settings.

How bacteria recognise and respond to surface contact

Bacterial biofilms can cause medical problems and issues in technical systems. While a large body of knowledge exists on the phenotypes of planktonic and of sessile cells in mature biofilms, our understanding of what happens when bacteria change from the planktonic to the sessile state is still very incomplete. Fundamental questions are unanswered: for instance, how do bacteria sense that they are in contact with a surface, and what are the very initial cellular responses to surface contact. Here, we review the current knowledge on the signals that bacteria could perceive once they attach to a surface, the signal transduction systems that could be involved in sensing the surface contact and the cellular responses that are triggered as a consequence to surface contact ultimately leading to biofilm formation. Finally, as the main obstacle in investigating the initial responses to surface contact has been the difficulty to experimentally study the dynamic response of single cells upon surface attachment, we also review recent experimental approaches that could be employed to study bacterial surface sensing, which ultimately could lead to an improved understanding of how biofilm formation could be prevented.

Proton Leakage Is Sensed by IM30 and Activates IM30-Triggered Membrane Fusion

The inner membrane-associated protein of 30 kDa (IM30) is crucial for the development and maintenance of the thylakoid membrane system in chloroplasts and cyanobacteria. While its exact physiological function still is under debate, it has recently been suggested that IM30 has (at least) a dual function, and the protein is involved in stabilization of the thylakoid membrane as well as in Mg2+-dependent membrane fusion. IM30 binds to negatively charged membrane lipids, preferentially at stressed membrane regions where protons potentially leak out from the thylakoid lumen into the chloroplast stroma or the cyanobacterial cytoplasm, respectively. Here we show in vitro that IM30 membrane binding, as well as membrane fusion, is strongly increased in acidic environments. This enhanced activity involves a rearrangement of the protein structure. We suggest that this acid-induced transition is part of a mechanism that allows IM30 to (i) sense sites of proton leakage at the thylakoid membrane, to (ii) preferentially bind there, and to (iii) seal leaky membrane regions via membrane fusion processes.

GTP hydrolysis by Synechocystis IM30 does not decisively affect its membrane remodeling activity

The function of IM30 (also known as Vipp1) is linked to protection and/or remodeling of the thylakoid membrane system in chloroplasts and cyanobacteria. Recently, it has been revealed that the Arabidopsis IM30 protein exhibits GTP hydrolyzing activity in vitro, which was unexpected, as IM30 does not show any classical GTPase features. In the present study, we addressed the question, whether an apparent GTPase activity is conserved in IM30 proteins and can also be observed for IM30 of the cyanobacterium Synechocystis sp. PCC 6803. We show that Synechocystis IM30 is indeed able to bind and hydrolyze GTP followed by the release of Pi. Yet, the apparent GTPase activity of Synechocystis IM30 does not depend on Mg2+, which, together with the lack of classical GTPase features, renders IM30 an atypical GTPase. To elucidate the impact of this cryptic GTPase activity on the membrane remodeling activity of IM30, we tested whether GTP hydrolysis influences IM30 membrane binding and/or IM30-mediated membrane fusion. We show that membrane remodeling by Synechocystis IM30 is slightly affected by nucleotides. Yet, despite IM30 clearly catalyzing GTP hydrolysis, this does not seem to be vital for its membrane remodeling function.

VIPP2 interacts with VIPP1 and HSP22E/F at chloroplast membranes and modulates a retrograde signal for HSP22E/F gene expression

VIPP proteins aid thylakoid biogenesis and membrane maintenance in cyanobacteria, algae, and plants. Some members of the Chlorophyceae contain two VIPP paralogs termed VIPP1 and VIPP2, which originate from an early gene duplication event during the evolution of green algae. VIPP2 is barely expressed under nonstress conditions but accumulates in cells exposed to high light intensities or H₂ O₂ , during recovery from heat stress, and in mutants with defective integration (alb3.1) or translocation (secA) of thylakoid membrane proteins. Recombinant VIPP2 forms rod-like structures in vitro and shows a strong affinity for phosphatidylinositol phosphate. Under stress conditions, >70% of VIPP2 is present in membrane fractions and localizes to chloroplast membranes. A vipp2 knock-out mutant displays no growth phenotypes and no defects in the biogenesis or repair of photosystem II. However, after exposure to high light intensities, the vipp2 mutant accumulates less HSP22E/F and more LHCSR3 protein and transcript. This suggests that VIPP2 modulates a retrograde signal for the expression of nuclear genes HSP22E/F and LHCSR3. Immunoprecipitation of VIPP2 from solubilized cells and membrane-enriched fractions revealed major interactions with VIPP1 and minor interactions with HSP22E/F. Our data support a distinct role of VIPP2 in sensing and coping with chloroplast membrane stress.

Lipid Organization in Mixed Lipid Membranes Driven by Intrinsic Curvature Difference

Laurdan fluorescence, novel spectral fitting, and dynamic light scattering were combined to determine lateral lipid organization in mixed lipid membranes of the oxidized lipid, 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine (PGPC), and each of the three bilayer lipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC). Second harmonic spectra were computed to determine the number of elementary emissions present. All mixtures indicated two emissions. Accordingly, spectra were fit to two log-normal distributions. Changes with PGPC mole fraction, X_PGPC, of the area of the shorter wavelength line and of dynamic light scattering-derived aggregate sizes show that: DPPC and PGPC form component-separated mixed vesicles for X_PGPC ≤ 0.2 and coexisting vesicles and micelles for X_PGPC > 0.2 in gel and liquid-ordered phases and for all X_PGPC in the liquid-disordered phase; POPC and PGPC form randomly mixed vesicles for X_PGPC ≤ 0.2 and component-separated mixed vesicles for X_PGPC > 0.2. DOPC and PGPC separate into vesicles and micelles. Component segregation is due to unstable inhomogeneous membrane curvature stemming from lipid-specific intrinsic curvature differences between mixing molecules. PGPC is inverse cone-shaped because its truncated tail with a terminal polar group points into the interface. It is similar to and mixes with POPC, also an inverse cone because of mobility of its unsaturated tail. PGPC is least similar to DOPC because mobilities of both unsaturated tails confer a cone shape to DOPC, and PGPC separates form DOPC. DPPC and PGPC do not mix in the liquid-disordered phase because mobility of both tails in this phase renders DPPC a cone. DPPC is a cylinder in the gel phase and of moderate similarity to PGPC and mixes moderately with PGPC.

Envelope stress responses: balancing damage repair and toxicity

The Gram-negative envelope is a complex structure that consists of the inner membrane, the periplasm, peptidoglycan and the outer membrane, and protects the bacterial cell from the environment. Changing environmental conditions can cause damage, which triggers the envelope stress responses to maintain cellular homeostasis. In this Review, we explore the causes, both environmental and intrinsic, of envelope stress, as well as the cellular stress response pathways that counter these stresses. Furthermore, we discuss the damage to the cell that occurs when these pathways are aberrantly activated either in the absence of stress or to an excessive degree. Finally, we review the mechanisms whereby the σ^E response constantly acts to prevent cell death caused by highly toxic unfolded outer membrane proteins. Together, the recent work that we discuss has provided insights that emphasize the necessity for proper levels of stress response activation and the detrimental consequences that can occur in the absence of proper regulation.

The Chlamydomonas deg1c Mutant Accumulates Proteins Involved in High Light Acclimation

Degradation of periplasmic proteins (Deg)/high temperature requirement A (HtrA) proteases are ATP-independent Ser endopeptidases that perform key aspects of protein quality control in all domains of life. Here, we characterized Chlamydomonas reinhardtii DEG1C, which together with DEG1A and DEG1B is orthologous to Arabidopsis (Arabidopsis thaliana) Deg1 in the thylakoid lumen. We show that DEG1C is localized to the stroma and the periphery of thylakoid membranes. Purified DEG1C exhibited high proteolytic activity against unfolded model substrates and its activity increased with temperature and pH. DEG1C forms monomers, trimers, and hexamers that are in dynamic equilibrium. DEG1C protein levels increased upon nitrogen, sulfur, and phosphorus starvation; under heat, oxidative, and high light stress; and when Sec-mediated protein translocation was impaired. DEG1C depletion was not associated with any obvious aberrant phenotypes under nonstress conditions, high light exposure, or heat stress. However, quantitative shotgun proteomics revealed differences in the abundance of 307 proteins between a deg1c knock-out mutant and the wild type under nonstress conditions. Among the 115 upregulated proteins are PSII biogenesis factors, FtsH proteases, and proteins normally involved in high light responses, including the carbon dioxide concentrating mechanism, photorespiration, antioxidant defense, and photoprotection. We propose that the lack of DEG1C activity leads to a physiological state of the cells resembling that induced by high light intensities and therefore triggers high light protection responses.

Functional Implications of Multiple IM30 Oligomeric States

The inner membrane-associated protein of 30 kDa (IM30), also known as the vesicle-inducing protein in plastids 1 (Vipp1), is essential for photo-autotrophic growth of cyanobacteria, algae and higher plants. While its exact function still remains largely elusive, it is commonly accepted that IM30 is crucially involved in thylakoid membrane biogenesis, stabilization and/or maintenance. A characteristic feature of IM30 is its intrinsic propensity to form large homo-oligomeric protein complexes. 15 years ago, it has been reported that these supercomplexes have a ring-shaped structure. However, the in vivo significance of these ring structures is not finally resolved yet and the formation of more complex assemblies has been reported. We here present and discuss research on IM30 conducted within the past 25 years with a special emphasis on the question of why we potentially need IM30 supercomplexes in vivo.

Characterization of the Vibrio cholerae Phage Shock Protein Response

The phage shock protein (Psp) system is a stress response pathway that senses and responds to inner membrane damage. The genetic components of the Psp system are present in several clinically relevant Gram-negative bacteria, including Vibrio cholerae However, most of the current knowledge about the Psp response stems from in vitro studies in Escherichia coli and Yersinia enterocolitica In fact, the Psp response in V. cholerae has remained completely uncharacterized. In this study, we demonstrate that V. cholerae does have a functional Psp response system. We found that overexpression of GspD (EpsD), the type II secretion system secretin, induces the Psp response, whereas other V. cholerae secretins do not. In addition, we have identified several environmental conditions that induce this stress response. Our studies on the genetic regulation and induction of the Psp system in V. cholerae suggest that the key regulatory elements are conserved with those of other Gram-negative bacteria. While a psp null strain is fully capable of colonizing the infant mouse intestine, it exhibits a colonization defect in a zebrafish model, indicating that this response may be important for disease transmission in the environment. Overall, these studies provide an initial understanding of a stress response pathway that has not been previously investigated in V. choleraeIMPORTANCEVibrio cholerae leads a dual life cycle, as it can exist in the aquatic environment and colonize the human small intestine. In both life cycles, V. cholerae encounters a variety of stressful conditions, including fluctuating pH and temperature and exposure to other agents that may negatively affect cell envelope homeostasis. The phage shock protein (Psp) response is required to sense and respond to such insults in other bacteria but has remained unstudied in V. cholerae Interestingly, the Psp system has protein homologs, principally, PspA, in a number of bacterial clades as well as in archaea and plants. Therefore, our findings not only fill a gap in knowledge about an unstudied extracytoplasmic stress response in V. cholerae, but also may have far-reaching implications.

VIPP1 rods engulf membranes containing phosphatidylinositol phosphates

In cyanobacteria and plants, VIPP1 plays crucial roles in the biogenesis and repair of thylakoid membrane protein complexes and in coping with chloroplast membrane stress. In chloroplasts, VIPP1 localizes in distinct patterns at or close to envelope and thylakoid membranes. In vitro, VIPP1 forms higher-order oligomers of >1 MDa that organize into rings and rods. However, it remains unknown how VIPP1 oligomerization is related to function. Using time-resolved fluorescence anisotropy and sucrose density gradient centrifugation, we show here that Chlamydomonas reinhardtii VIPP1 binds strongly to liposomal membranes containing phosphatidylinositol-4-phosphate (PI4P). Cryo-electron tomography reveals that VIPP1 oligomerizes into rods that can engulf liposomal membranes containing PI4P. These findings place VIPP1 into a group of membrane-shaping proteins including epsin and BAR domain proteins. Moreover, they point to a potential role of phosphatidylinositols in directing the shaping of chloroplast membranes.

Phage shock protein and gene responses of Escherichia coli exposed to carbon nanotubes

Two-dimensional electrophoretic, western blotting, and quantitative polymerase chain reaction analyses of Escherichia coli cells exposed to pristine single walled carbon nanotubes (SWCNTs), and hydroxyl and carboxylic functionalized SWCNTs (SWCNT-OHs and SWCNT-COOHs) were conducted. SWCNT concentration and length were experimental variables. Exposing E. coli cells to SWCNTs led to changes in protein and gene expressions. Several proteins altered their regulations at a low SWCNT concentration (10 μg/ml) and were shut down at a high SWCNT concentration (100 μg/ml). The expressions of the phage shock protein (psp) operon including pspA, pspB, and pspC genes responded to the membrane stressors, SWCNTs, were also examined. While pspA and pspC expressions were influenced by the length, concentration, and functional groups of SWCNTs, pspB expression was not induced by SWCNTs. The alterations in phage shock protein and gene expressions indicated that SWCNTs caused cell membrane perturbation.

Regulation and overexpression studies of YidC in Mycobacterium tuberculosis

The preprotein translocase, YidC is an envelope protein which controls respiratory metabolism in Mycobacterium tuberculosis. Previously, we have established that depletion of yidC is deleterious for both extra- and intracellular proliferation of M. tuberculosis; however, it remains unclear how YidC expression is regulated under different growth conditions and whether its altered expression impact mycobacterial physiology. Herein, we show that yidC is expressed as an operon with upstream genes. Interestingly, expression analysis under various stress conditions reveals a distinct paradox in the profile of the yidC mRNA transcripts and the YidC protein. While YidC protein level is moderately elevated upon bacterial exposure to cell surface stresses, the corresponding mRNA transcript levels are significantly repressed under these conditions. In contrast, overexpression of M. tuberculosis yidC under a strong anhydrotetracycline-inducible promoter results in significant induction of YidC protein. Additionally, we also observe that overexpression of M. tuberculosis yidC, and not of its counterpart from fast-growing M. smegmatis, results in altered in vitro growth of bacteria, compromised integrity of bacterial cell envelope and differential expression of a small set of genes including those which are regulated under detergent stress. Overall findings of our study suggest that YidC proteins of slow- and fast-growing mycobacteria are functionally distinct despite exhibiting a great deal of identity.

Stress Resistance Development and Genome-Wide Transcriptional Response of Escherichia coli O157:H7 Adapted to Sublethal Thymol, Carvacrol, and trans-Cinnamaldehyde

Thymol, carvacrol, and trans-cinnamaldehyde are essential oil (EO) compounds with broad-spectrum antimicrobial activities against foodborne pathogens, including Escherichia coli O157:H7. However, little is known regarding direct resistance and cross-resistance development in E. coli O157:H7 after adaptation to sublethal levels of these compounds, and information is scarce on microbial adaptive responses at a molecular level. The present study demonstrated that E. coli O157:H7 was able to grow in the presence of sublethal thymol (1/2T), carvacrol (1/2C), or trans-cinnamaldehyde (1/2TC), displaying an extended lag phase duration and a lower maximum growth rate. EO-adapted cells developed direct resistance against lethal EO treatments and cross-resistance against heat (58°C) and oxidative (50 mM H₂O₂) stresses. However, no induction of acid resistance (simulated gastric fluid, pH 1.5) was observed. RNA sequencing revealed a large number (310 to 338) of differentially expressed (adjusted P value [P_adj ], <0.05; fold change, ≥5) genes in 1/2T and 1/2C cells, while 1/2TC cells only showed 27 genes with altered expression. In accordance with resistance phenotypes, the genes related to membrane, heat, and oxidative stress responses and genes related to iron uptake and metabolism were upregulated. Conversely, virulence genes associated with motility, biofilm formation, and efflux pumps were repressed. This study demonstrated the development of direct resistance and cross-resistance and characterized whole-genome transcriptional responses in E. coli O157:H7 adapted to sublethal thymol, carvacrol, or trans-cinnamaldehyde. The data suggested that caution should be exercised when using EO compounds as food antimicrobials, due to the potential stress resistance development in E. coli O157:H7.IMPORTANCE The present study was designed to understand transcriptomic changes and the potential development of direct and cross-resistance in essential oil (EO)-adapted Escherichia coli O157:H7. The results demonstrated altered growth behaviors of E. coli O157:H7 during adaptation in sublethal thymol, carvacrol, and trans-cinnamaldehyde. Generally, EO-adapted bacteria showed enhanced resistance against subsequent lethal EO, heat, and oxidative stresses, with no induction of acid resistance in simulated gastric fluid. A transcriptomic analysis revealed the upregulation of related stress resistance genes and a downregulation of various virulence genes in EO-adapted cells. This study provides new insights into microbial EO adaptation behaviors and highlights the risk of resistance development in adapted bacteria.

Association of Mycobacterium Proteins with Lipid Droplets

Mycobacterium tuberculosis is a global pathogen of significant medical importance. A key aspect of its life cycle is the ability to enter into an altered physiological state of nonreplicating persistence during latency and resist elimination by the host immune system. One mechanism by which M. tuberculosis facilitates its survival during latency is by producing and metabolizing intracytoplasmic lipid droplets (LDs). LDs are quasi-organelles consisting of a neutral lipid core such as triacylglycerol surrounded by a phospholipid monolayer and proteins. We previously reported that PspA (phage shock protein A) associates with LDs produced in Mycobacterium In particular, the loss or overproduction of PspA alters LD homeostasis in Mycobacterium smegmatis and attenuates the survival of M. tuberculosis during nonreplicating persistence. Here, M. tuberculosis PspA (PspA_Mtb) and a ΔpspA M. smegmatis mutant were used as model systems to investigate the mechanism by which PspA associates with LDs and determine if other Mycobacterium proteins associate with LDs using a mechanism similar to that for PspA. Through this work, we established that the amphipathic helix present in the first α-helical domain (H1) of PspA is both necessary and sufficient for the targeting of this protein to LDs. Furthermore, we identified other Mycobacterium proteins that also possess amphipathic helices similar to PspA H1, including a subset that localize to LDs. Altogether, our results indicate that amphipathic helices may be an important mechanism by which proteins target LDs in prokaryotes.IMPORTANCEMycobacterium spp. are one of the few prokaryotes known to produce lipid droplets (LDs), and their production has been linked to aspects of persistent infection by M. tuberculosis Unfortunately, little is known about LD production in these organisms, including how LDs are formed, their function, or the identity of proteins that associate with them. In this study, an established M. tuberculosis LD protein and a surrogate Mycobacterium host were used as model systems to study the interactions between proteins and LDs in bacteria. Through these studies, we identified a commonly occurring protein motif that is able to facilitate the association of proteins to LDs in prokaryotes.

Evidence for a second regulatory binding site on PspF that is occupied by the C-terminal domain of PspA

PspA is a key component of the bacterial Psp membrane-stress response system. The biochemical and functional characterization of PspA is impeded by its oligomerization and aggregation properties. It was recently possible to solve the coiled coil structure of a completely soluble PspA fragment, PspA(1–144), that associates with the σ⁵⁴ enhancer binding protein PspF at its W56-loop and thereby down-regulates the Psp response. We now found that the C-terminal part of PspA, PspA(145–222), also interacts with PspF and inhibits its activity in the absence of full-length PspA. Surprisingly, PspA(145–222) effects changed completely in the presence of full-length PspA, as promoter activity was triggered instead of being inhibited under this condition. PspA(145–222) thus interfered with the inhibitory effect of full-length PspA on PspF, most likely by interacting with full-length PspA that remained bound to PspF. In support of this view, a comprehensive bacterial-2-hybrid screen as well as co-purification analyses indicated a self-interaction of PspA(145–222) and an interaction with full-length PspA. This is the first direct demonstration of PspA/PspA and PspA/PspF interactions in vivo that are mediated by the C-terminus of PspA. The data indicate that regulatory binding sites on PspF do not only exist for the N-terminal coiled coil domain but also for the C-terminal domain of PspA. The inhibition of PspF by PspA-(145–222) was reduced upon membrane stress, whereas the inhibition of PspF by PspA(1–144) did not respond to membrane stress. We therefore propose that the C-terminal domain of PspA is crucial for the regulation of PspF in response to Psp system stimuli.

Synthetic antimicrobial peptides delocalize membrane bound proteins thereby inducing a cell envelope stress response

BACKGROUND

Three amphipathic cationic antimicrobial peptides (AMPs) were characterized by determining their effect on Gram-positive bacteria using Bacillus subtilis strain 168 as a model organism. These peptides were TC19 and TC84, derivatives of thrombocidin-1 (TC-1), the major AMPs of human blood platelets, and Bactericidal Peptide 2 (BP2), a synthetic designer peptide based on human bactericidal permeability increasing protein (BPI).

METHODS

To elucidate the possible mode of action of the AMPs we performed a transcriptomic analysis using microarrays. Physiological analyses were performed using transmission electron microscopy (TEM), fluorescence microscopy and various B. subtilis mutants that produce essential membrane bound proteins fused to green fluorescent protein (GFP).

RESULTS

The transcriptome analysis showed that the AMPs induced a cell envelope stress response (cell membrane and cell wall). The cell membrane stress response was confirmed with the physiological observations that TC19, TC84 and BP2 perturb the membrane of B. subtilis. Using B. subtilis mutants, we established that the cell wall stress response is due to the delocalization of essential membrane bound proteins involved in cell wall synthesis. Other essential membrane proteins, involved in cell membrane synthesis and metabolism, were also delocalized due to alterations caused by the AMPs.

CONCLUSIONS

We showed that peptides TC19, TC84 and BP2 perturb the membrane causing essential proteins to delocalize, thus preventing the possible repair of the cell envelope after the initial interference with the membrane.

GENERAL SIGNIFICANCE

These AMPs show potential for eventual clinical application against Gram-positive bacterial cells and merit further application-oriented investigation.

PCYT1A Regulates Phosphatidylcholine Homeostasis from the Inner Nuclear Membrane in Response to Membrane Stored Curvature Elastic Stress

Cell and organelle membranes consist of a complex mixture of phospholipids (PLs) that determine their size, shape, and function. Phosphatidylcholine (PC) is the most abundant phospholipid in eukaryotic membranes, yet how cells sense and regulate its levels in vivo remains unclear. Here we show that PCYT1A, the rate-limiting enzyme of PC synthesis, is intranuclear and re-locates to the nuclear membrane in response to the need for membrane PL synthesis in yeast, fly, and mammalian cells. By aligning imaging with lipidomic analysis and data-driven modeling, we demonstrate that yeast PCYT1A membrane association correlates with membrane stored curvature elastic stress estimates. Furthermore, this process occurs inside the nucleus, although nuclear localization signal mutants can compensate for the loss of endogenous PCYT1A in yeast and in fly photoreceptors. These data suggest an ancient mechanism by which nucleoplasmic PCYT1A senses surface PL packing defects on the inner nuclear membrane to control PC homeostasis.

VIPP1 Involved in Chloroplast Membrane Integrity Has GTPase Activity in Vitro

VESICLE-INDUCING PROTEIN IN PLASTID1 (VIPP1) is conserved among oxygenic photosynthetic organisms and appears to have diverged from the bacterial PspA protein. VIPP1 localizes to the chloroplast envelope and thylakoid membrane, where it forms homooligomers of high molecular mass. Although multiple roles of VIPP1 have been inferred, including thylakoid membrane formation, envelope maintenance, membrane fusion, and regulation of photosynthetic activity, its precise role in chloroplast membrane quality control remains unknown. VIPP1 forms an oligomer through its amino-terminal domain and triggers membrane fusion in an Mg²⁺-dependent manner. We previously demonstrated that Arabidopsis (Arabidopsis thaliana) VIPP1 also exhibits dynamic complex disassembly in response to osmotic and heat stresses in vivo. These results suggest that VIPP1 mediates membrane fusion/remodeling in chloroplasts. Considering that protein machines that regulate intracellular membrane fusion/remodeling events often require a capacity for GTP binding and/or hydrolysis, we questioned whether VIPP1 has similar properties. We conducted an in vitro assay using a purified VIPP1-His fusion protein expressed in Escherichia coli cells. VIPP1-His showed GTP hydrolysis activity that was inhibited competitively by an unhydrolyzable GTP analog, GTPγS, and that depends on GTP binding. It is particularly interesting that the ancestral PspA from E. coli also possesses GTP hydrolysis activity. Although VIPP1 does not contain a canonical G domain, the amino-terminal α-helix was found to be important for both GTP binding and GTP hydrolysis as well as for oligomer formation. Collectively, our results reveal that the properties of VIPP1/PspA are similar to those of GTPases.

Dynamical localization of a thylakoid membrane binding protein is required for acquisition of photosynthetic competency

Vipp1 is highly conserved and essential for photosynthesis, but its function is unclear as it does not participate directly in light-dependent reactions. We analyzed Vipp1 localization in live cyanobacterial cells and show that Vipp1 is highly dynamic, continuously exchanging between a diffuse fraction that is uniformly distributed throughout the cell and a punctate fraction that is concentrated at high curvature regions of the thylakoid located at the cell periphery. Experimentally perturbing the spatial distribution of Vipp1 by relocalizing it to the nucleoid causes a severe growth defect during the transition from non-photosynthetic (dark) to photosynthetic (light) growth. However, the same perturbation of Vipp1 in dark alone or light alone growth conditions causes no growth or thylakoid morphology defects. We propose that the punctuated dynamics of Vipp1 at the cell periphery in regions of high thylakoid curvature enable acquisition of photosynthetic competency, perhaps by facilitating biogenesis of photosynthetic complexes involved in light-dependent reactions of photosynthesis.

Psp Stress Response Proteins Form a Complex with Mislocalized Secretins in the Yersinia enterocolitica Cytoplasmic Membrane

The bacterial phage shock protein system (Psp) is a conserved extracytoplasmic stress response that is essential for the virulence of some pathogens, including Yersinia enterocolitica It is induced by events that can compromise inner membrane (IM) integrity, including the mislocalization of outer membrane pore-forming proteins called secretins. In the absence of the Psp system, secretin mislocalization permeabilizes the IM and causes rapid cell death. The Psp proteins PspB and PspC form an integral IM complex with two independent roles. First, the PspBC complex is required to activate the Psp response in response to some inducing triggers, including a mislocalized secretin. Second, PspBC are sufficient to counteract mislocalized secretin toxicity. Remarkably, secretin mislocalization into the IM induces psp gene expression without significantly affecting the expression of any other genes. Furthermore, psp null strains are killed by mislocalized secretins, whereas no other null mutants have been found to share this specific secretin sensitivity. This suggests an exquisitely specific relationship between secretins and the Psp system, but there has been no mechanism described to explain this. In this study, we addressed this deficiency by using a coimmunoprecipitation approach to show that the Psp proteins form a specific complex with mislocalized secretins in the Y. enterocolitica IM. Importantly, analysis of different secretin mutant proteins also revealed that this interaction is absolutely dependent on a secretin adopting a multimeric state. Therefore, the Psp system has evolved with the ability to detect and detoxify dangerous secretin multimers while ignoring the presence of innocuous monomers.IMPORTANCE The phage shock protein (Psp) response has been linked to important phenotypes in diverse bacteria, including those related to antibiotic resistance, biofilm formation, and virulence. This has generated widespread interest in understanding various aspects of its function. Outer membrane secretin proteins are essential components of export systems required for the virulence of many bacterial pathogens. However, secretins can mislocalize into the inner membrane, and this induces the Psp response in a highly specific manner and kills Psp-defective strains with similar specificity. There has been no mechanism described to explain this exquisitely specific relationship between secretins and the Psp system. Therefore, this study provides a critical advance by discovering that Psp effector proteins form a complex with secretins in the Yersinia enterocolitica inner membrane. Remarkably, this interaction is absolutely dependent on a secretin adopting its multimeric state. Therefore, the Psp system detects and detoxifies dangerous secretin multimers, while ignoring the presence of innocuous secretin monomers.

Structural disorder in plant proteins: where plasticity meets sessility

Plants are sessile organisms. This intriguing nature provokes the question of how they survive despite the continual perturbations caused by their constantly changing environment. The large amount of knowledge accumulated to date demonstrates the fascinating dynamic and plastic mechanisms, which underpin the diverse strategies selected in plants in response to the fluctuating environment. This phenotypic plasticity requires an efficient integration of external cues to their growth and developmental programs that can only be achieved through the dynamic and interactive coordination of various signaling networks. Given the versatility of intrinsic structural disorder within proteins, this feature appears as one of the leading characters of such complex functional circuits, critical for plant adaptation and survival in their wild habitats. In this review, we present information of those intrinsically disordered proteins (IDPs) from plants for which their high level of predicted structural disorder has been correlated with a particular function, or where there is experimental evidence linking this structural feature with its protein function. Using examples of plant IDPs involved in the control of cell cycle, metabolism, hormonal signaling and regulation of gene expression, development and responses to stress, we demonstrate the critical importance of IDPs throughout the life of the plant.

Evidence that Listeria innocua modulates its membrane's stored curvature elastic stress, but not fluidity, through the cell cycle

This paper reports that the abundances of endogenous cardiolipin and phosphatidylethanolamine halve during elongation of the Gram-positive bacterium Listeria innocua. The lyotropic phase behaviour of model lipid systems that describe these modulations in lipid composition indicate that the average stored curvature elastic stress of the membrane is reduced on elongation of the cell, while the fluidity appears to be maintained. These findings suggest that phospholipid metabolism is linked to the cell cycle and that changes in membrane composition can facilitate passage to the succeding stage of the cell cycle. This therefore suggests a means by which bacteria can manage the physical properties of their membranes through the cell cycle.

Membrane chaperoning by members of the PspA/IM30 protein family

PspA, IM30 (Vipp1) and LiaH, which all belong to the PspA/IM30 protein family, form high molecular weight oligomeric structures. For all proteins membrane binding and protection of the membrane structure and integrity has been shown or postulated. Here we discuss the possible membrane chaperoning activity of PspA, IM30 and LiaH and propose that larger oligomeric structures bind to stressed membrane regions, followed by oligomer disassembly and membrane stabilization by protein monomers or smaller/different oligomeric scaffolds.