Modulating the influenza A virus – target membrane fusion interface with synthetic DNA-lipid receptors

Influenza A virus (IAV) binds to sialylated glycans on the cell membrane before endocytosis and fusion. Cell surface glycans are highly heterogenous in length and glycosylation density, which leads to variation in the distance and rigidity with which IAV is held away from the cell membrane. To gain mechanistic insight into how receptor length and rigidity impact the mechanism of IAV entry, we employed synthetic DNA-lipids as highly tunable surrogate receptors. We tethered IAV to target membranes with a panel of DNA-lipids to investigate the effects of the distance and tether flexibility between virions and target membranes on the kinetics of IAV binding and fusion. Tether length and the presence of a flexible linker led to higher rates of IAV binding, while the efficiencies of lipid and content mixing were typically lower for longer and more rigid DNA tethers. For all DNA tether modifications, we found that the rates of IAV lipid and content mixing were unchanged. These results suggest that variations in the interface between IAV and a target membrane do not significantly impact the rate-limiting step of fusion, or the low-pH triggered engagement of viral fusion peptides with the target membrane. However, our results imply that the flexibility of the viral receptor is important for ensuring that hemifusion events are able to successfully proceed to pore formation.

Modulating the influenza A virus – target membrane fusion interface with synthetic DNA-lipid receptors

Influenza A virus (IAV) binds to sialylated glycans on the cell membrane before endocytosis and fusion. Cell surface glycans are highly heterogenous in length and glycosylation density, which leads to variation in the distance and rigidity with which IAV is held away from the cell membrane. To gain mechanistic insight into how receptor length and rigidity impact the mechanism of IAV entry, we employed synthetic DNA-lipids as highly tunable surrogate receptors. We tethered IAV to target membranes with a panel of DNA-lipids to investigate the effects of the distance and tether flexibility between virions and target membranes on the kinetics of IAV binding and fusion. Tether length and the presence of a flexible linker led to higher rates of IAV binding, while the efficiencies of lipid and content mixing were typically lower for longer and more rigid DNA tethers. For all DNA tether modifications, we found that the rates of IAV lipid and content mixing were unchanged. These results suggest that variations in the interface between IAV and a target membrane do not significantly impact the rate-limiting step of fusion, or the low-pH triggered engagement of viral fusion peptides with the target membrane. However, our results imply that the flexibility of the viral receptor is important for ensuring that hemifusion events are able to successfully proceed to pore formation.

Peptide-conjugated nanoparticles for targeted photodynamic therapy

Cancer is the second leading cause of death worldwide after cardiovascular disease. Depending on the type and the location of the tumor, several cancer treatments are implemented. Among these, the three most conventional therapies are surgery, radiotherapy and chemotherapy. However, there are other therapeutic approaches such as photodynamic therapy (PDT). PDT relies on the combined action of light, a photoactivable molecule called photosensitizer (PS) and molecular oxygen. Most of the PSs used for clinical applications are not cancer-cell specific. One of the solutions to overcome this problem is the use of nanoparticles (NPs) to induce a passive targeting. It is also possible to graft a vector onto the NPs to specifically target membrane receptors overexpressed in the tumor cells or neovessels surrounding the tumor. In this review, we focus on the NPs loaded with PSs and coupled to peptides for targeted PDT. We described nanosystems that targeted Neuropilin-1 (NRP-1), αvβ3 integrins, nucleolin membrane receptor, epidermal growth factor (EGF) receptor, protein-glutamine-gamma-glutamyltransferase (TGM2), p32, transferrin, PD-1, and mitochondrial membrane. The use of a cell absorbing-peptide is also described.

Plasma membrane phospholipid signature recruits the plant exocyst complex via the EXO70A1 subunit

Polarized exocytosis is essential for many vital processes in eukaryotic cells, where secretory vesicles are targeted to distinct plasma membrane domains characterized by their specific lipid–protein composition. Heterooctameric protein complex exocyst facilitates the vesicle tethering to a target membrane and is a principal cell polarity regulator in eukaryotes. The architecture and molecular details of plant exocyst and its membrane recruitment have remained elusive. Here, we show that the plant exocyst consists of two modules formed by SEC3–SEC5–SEC6–SEC8 and SEC10–SEC15–EXO70–EXO84 subunits, respectively, documenting the evolutionarily conserved architecture within eukaryotes. In contrast to yeast and mammals, the two modules are linked by a plant-specific SEC3–EXO70 interaction, and plant EXO70 functionally dominates over SEC3 in the exocyst recruitment to the plasma membrane. Using an interdisciplinary approach, we found that the C-terminal part of EXO70A1, the canonical EXO70 isoform in Arabidopsis, is critical for this process. In contrast to yeast and animal cells, the EXO70A1 interaction with the plasma membrane is mediated by multiple anionic phospholipids uniquely contributing to the plant plasma membrane identity. We identified several evolutionary conserved EXO70 lysine residues and experimentally proved their importance for the EXO70A1–phospholipid interactions. Collectively, our work has uncovered plant-specific features of the exocyst complex and emphasized the importance of the specific protein–lipid code for the recruitment of peripheral membrane proteins.

Hormonal and environmental signaling pathways target membrane water transport

Plant water transport and its molecular components including aquaporins are responsive, across diverse time scales, to an extremely wide array of environmental and hormonal signals. These include water deficit and abscisic acid (ABA) but also more recently identified stimuli such as peptide hormones or bacterial elicitors. The present review makes an inventory of corresponding signalling pathways. It identifies some main principles, such as the central signalling role of ROS, with a dual function of aquaporins in water and hydrogen peroxide transport, the importance of aquaporin phosphorylation that is targeted by multiple classes of protein kinases, and the emerging role of lipid signalling. More studies including systems biology approaches are now needed to comprehend how plant water transport can be adjusted in response to combined stresses.

Functional Specialization within the EXO70 Gene Family in Arabidopsis

Localized delivery of plasma-membrane and cell-wall components is a crucial process for plant cell growth. One of the regulators of secretory-vesicle targeting is the exocyst tethering complex. The exocyst mediates first interaction between transport vesicles and the target membrane before their fusion is performed by SNARE proteins. In land plants, genes encoding the EXO70 exocyst subunit underwent an extreme proliferation with 23 paralogs present in the Arabidopsis (Arabidopsis thaliana) genome. These paralogs often acquired specialized functions during evolution. Here, we analyzed functional divergence of selected EXO70 paralogs in Arabidopsis. Performing a systematic cross-complementation analysis of exo70a1 and exo70b1 mutants, we found that EXO70A1 was functionally substituted only by its closest paralog, EXO70A2. In contrast, none of the EXO70 isoforms tested were able to substitute EXO70B1, including its closest relative, EXO70B2, pointing to a unique function of this isoform. The presented results document a high degree of functional specialization within the EXO70 gene family in land plants.

Cloning, expression, and purification of chaperone AcrH from Aeromonas hydrophilausing Escherichia coli host cells

Aeromonas hydrophila (A. hydrophila) is a gram-negativebacterium, using the type III secretion system (T3SS). In the T3SS, a key structure is a translocon that inserts into the target membrane and forms a channel for bacterial toxins into the host cell. A. hydrophila is pathogenic to different organisms, including humans and aquatic animals (especially domestic animals with high economic value in Vietnam and the world, such as fishes, shrimps, amphibians). The pore completes the channel from bacteria to host, is composed of a major translocator (AopB) and minor translocator (AopD). These translocators are bound by a small chaperone (AcrH) in bacterial cytosol. AcrH chaperone plays an important role in keeping the high stability of translocators and prevents nonspecific interactions of hydrophobic domains before the pore formed in the host cell membrane. Previous studies only analysed the structure of the AcrH in combination with the AopB, but in a non-binding form with the AopB has not been elucidated. That limits the understanding of the formation mechanism of T3SS. Therefore, the authors aimed to clone, express, and purify the AcrH recombinant protein which can be used for the structural study and elucidation of T3SS pore formation. In this study, the authors cloned a fragment of the gene encoding for AcrH chaperone from A. hydrophila and inserted the gene into the pET-28a expression vector. AcrH protein from amino acids 21 to 158 was expressed in E. coli BL21 (DE3) and purified using a nickel bead column with high purity (over 99%). As a result, the obtained AcrH protein can be used for studies of structure and function that contribute to perfecting the pathogenesis of gram-negative bacteria and developing research on the treatment mechanism caused by these bacteria.

Single Molecule Studies Reveal Regulatory Interactions between Master Kinases PDK1, AKT1 and PKCalpha

Leukocyte migration is controlled by a leading edge chemosensory pathway that generates the regulatory lipid PIP3, a growth signal, thereby driving leading edge expansion up attractant gradients toward sites of infection, inflammation, or tissue damage. PIP3 also serves as an important growth signal in growing cells and oncogenesis. The kinases PDK1, AKT1/PKB and PKCα are key components of a plasma membrane-based PIP3 and Ca2+ signaling circuit that regulates these processes. PDK1 and AKT1 are recruited to the membrane by PIP3, while PKCα is recruited to the membrane by Ca2+. All three of these master kinases phosphoregulate an array of protein targets. For example, PDK1 activates AKT1, PKCα and other AGC kinases by phosphorylation at key sites. PDK1 is known to form PDK1:AKT1 and PDK1:PKCα heterodimers stabilized by a PIF interaction between the PDK1 PIF pocket and the PIF motif of the AGC binding partner. Here we present the first, to our knowledge, single molecule studies of full length PDK1 and AKT1 on target membrane surfaces, as well as their interaction with full length PKCα. The findings show that membrane-bound PDK1:AKT1 and PDK1:PKCα heterodimers form under physiological conditions, and are stabilized by PIF interaction. PKCα exhibits 8-fold higher PDK1 affinity than AKT1, thus PKCα competitively displaces AKT1 from PDK1:AKT1 heterodimers. Ensemble activity measurements under matched conditions reveal that PDK1 activates AKT1 via a cis mechanism by phosphorylating an AKT1 molecule in the same PDK1:AKT1 heterodimer, while PKCα acts as a competitive inhibitor of this phosphoactivation reaction by displacing AKT1 from PDK1. Overall, the findings provide new insights into molecular and regulatory interactions of the three master kinases on their target membrane, and suggest that the recently described tumor suppressor activity of PKC may arise from its ability to downregulate PDK1-AKT1 phosphoactivation in the PIP3-PDK1-AKT1-mTOR pathway linked to cell growth and oncogenesis.

AtSEC22 Regulates Cell Morphogenesis via Affecting Cytoskeleton Organization and Stabilities

The plant cytoskeleton forms a stereoscopic network that regulates cell morphogenesis. The cytoskeleton also provides tracks for trafficking of vesicles to the target membrane. Fusion of vesicles with the target membrane is promoted by SNARE proteins, etc. The vesicle-SNARE, Sec22, regulates membrane trafficking between the ER and Golgi in yeast and mammals. Arabidopsis AtSEC22 might also regulate early secretion and is essential for gametophyte development. However, the role of AtSEC22 in plant development is unclear. To clarify the role of AtSEC22 in the regulation of plant development, we isolated an AtSEC22 knock-down mutant, atsec22-4, and found that cell morphogenesis and development were seriously disturbed. atsec22-4 exhibited shorter primary roots (PRs), dwarf plants, and partial abortion. More interestingly, the atsec22-4 mutant had less trichomes with altered morphology, irregular stomata, and pavement cells, suggesting that cell morphogenesis was perturbed. Further analyses revealed that in atsec22-4, vesicle trafficking was blocked, resulting in the trapping of proteins in the ER and collapse of structures of the ER and Golgi apparatus. Furthermore, AtSEC22 defects resulted in impaired organization and stability of the cytoskeleton in atsec22-4. Our findings revealed essential roles of AtSEC22 in membrane trafficking and cytoskeleton dynamics during plant development.

Transient Excursions to Membrane Core as Determinants of Influenza Virus Fusion Peptide Activity

Fusion of viral and host cell membranes is a critical step in the life cycle of enveloped viruses. In the case of influenza virus, it is mediated by subunit 2 of hemagglutinin (HA) glycoprotein whose N-terminal fragments insert into the target membrane and initiate lipid exchange. These isolated fragments, known as fusion peptides (HAfp), already possess own fusogenic activity towards liposomes. Although they have long been studied with the hope to uncover the details of HA-mediated fusion, their actual mechanism of action remains elusive. Here, we use extensive molecular dynamics simulations combined with experimental studies of three HAfp variants to fully characterize their free energy landscape and interaction with lipid bilayer. In addition to customary assumed peptides localization at lipid–water interface, we characterize membrane-spanning configurations, which turn out to be metastable for active HAfps and unstable for the fusion inactive W14A mutant. We show that, while the degree of membrane perturbation by surface peptide configurations is relatively low and does not show any mutation-related differences, the effect of deeply inserted configurations is significant and correlates with insertion depth of the N-terminal amino group which is the highest for the wild type HAfp. Finally, we demonstrate the feasibility of spontaneous peptide transition to intramembrane location and the critical role of strictly conserved tryptofan residue 14 in this process.