Investigating the impact of charge and hydrophilicity on peptide-mucin interactions using a simple mucin model

Electrostatic driven transport enhances penetration of positively charged peptide surfaces through tumor extracellular matrix

In this work, the behaviors diffusion of sweeteners (sucrose, erythritol and stevioside) through the mucin layer were investigated based on the mucin-sweeteners interactions. Several techniques including fluorescence spectroscopy, rheological measurement, QCM-D and TEM were applied to investigate the interactions between the sweeteners and mucin at different pH values. The results point to the relatively fast diffusion behavior of sweeteners at pH 5.0 or pH 7.0, which was associated with the molecular weight of sweeteners and mucin-sweeteners interactions. The rheological measurement and QCM-D results indicated that hydrogen bonding and hydrophobic interactions may play an important role in mucin-sweeteners interactions. A homogeneous mucin network with large network pore size at pH 5.0 or pH 7.0 favored the diffusion of sweeteners through the mucin layer. Additionally, the relatively weak interactions between mucin layer and sweeteners at pH 5.0 or pH 7.0 contributes to the rapid penetration of sweeteners, enhancing the sweetness perception.

The orientation of most single-spanning membrane proteins obeys the "positive-inside rule", i.e. the flanking region of the transmembrane segment that is more positively charged remains in the cytosol. These membrane proteins are integrated by the Sec61/SecY translocon, but how their orientation is achieved is unknown. We have screened for mutations in yeast Sec61p that alter the orientation of single-spanning membrane proteins. We identified a class of mutants that are less efficient in retaining the positively charged flanking region in the cytosol. Surprisingly, these mutations are located at many different sites in the Sec61/SecY molecule, and they do not only involve charged amino acid residues. All these mutants have a prl phenotype that so far have only been seen in bacteria; they allow proteins with defective signal sequences to be translocated, likely because the Sec61p channel opens more easily. A similar correlation between topology defects and prl phenotype was also seen with previously identified yeast Sec61 mutants. Our results suggest a model in which the regulated opening of the translocon is required for the faithful orientation of membrane proteins.

Interfacial adsorption and lubrication dynamics of β-lactoglobulin and MCT on the mucin layer.

the role of mitochondria as the foci of apoptotic signaling events in physiological and pathological conditions is now well-established (12). Multiple studies have demonstrated the recruitment of various cytosolic effector proteins, such as caspase-8, Bax, and Bak, to the outer membrane surface of mitochondria in apoptotic cells. Other reports have documented the redistribution of pro-apoptotic proteins, such as cytochrome c and Smac/Diablo, from the intermembrane space to the mitochondrial outer membrane (MOM) or the cytosol. The assembly of these newly recruited or transferred proteins into apoptotic signaling complexes, such as the Bax/Bak-based pores that mediate MOM permeabilization, has also been extensively characterized. Although the mechanisms by which apoptotic effector proteins recognize the MOM as a specific subcellular platform remain incompletely understood, recent studies have identified cardiolipin (CL), a predominantly mitochondrial phospholipid, as a key “targeting” factor (4, 6, 13). Both tBid (the pro-apoptotic truncated form of Bid) and caspase-8 bind to CL and disruption of these CL-effector protein interactions markedly attenuates the progression of apoptosis. Notably, association of these pro-apoptotic proteins with CL can involve stereospecific sites such as the hairpin motif within tBid formed by its α-6 helix (αH6) domain but independent of its BH3 domain (5, 11). However, CL is also dianionic at physiological pH and thus contributes to the overall surface charge of membranes in which it is abundant. In this issue of AJP-Cell Physiol, Heit et al. (8) report that mitochondria rapidly gain negative surface charge, likely due to increased CL in the outer membrane, during the early phases of apoptosis. Based on CL neutralization approaches, this study from Grinstein's group supports a novel and mechanistically significant model wherein mitochondrial exposure of negatively charged CL facilitates the electrostatic recruitment of multiple cationic proteins required for the progression of critical apoptotic signaling events.

The GYF domain of CD2BP2 serves as an adapter that recognizes proline-rich sequences in intracellular proteins. Although the T cell adhesion molecule CD2 and the core splicing protein SmB/B' were previously shown to interact with CD2BP2-GYF, we are now using a general approach to identify putative GYF domain target sites within the human proteome. The phage display-derived recognition motif for CD2BP2-GYF is PPG(W/F/Y/M/L). SPOT analysis confirmed that the GYF domain interacts with peptides from human proteins containing the consensus site. Epitope mapping by NMR spectroscopy performed for several peptides revealed a conserved binding surface. A direct interaction of the CD2BP2-GYF domain with the novel protein interaction partners PI31 and NPWBP was verified by yeast two-hybrid analysis.

Interaction of IAPP and Insulin with Model Interfaces Studied Using Neutron Reflectometry

Magnetic ion-exchange (MIEX) resin for perfluorinated alkylsubstance (PFAS) removal in groundwater: Roles of atomic charges for adsorption

The physicochemical forces that mediate attachment of yeasts to the phylloplane are unknown. Cell surface charge and hydrophobicity and adhesion to polystyrene, glass, and barley were assessed for wild-type Rhodosporidium toruloides and attachment-minus (Att-) mutants. Cells were grown under conditions promoting (excess carbon) or not promoting (excess nitrogen) capsule production. Hydrophobicity was measured by adhesion to xylenes, and surface charge characteristics were assessed by attachment to either DEAE (positive)- or carboxymethyl (CM) (negative)-Sephadex ion-exchange beads. Hydrophobicity and adhesiveness of nonencapsulated, wild-type R. toruloides decreased from mid-log to late stationary phase. Encapsulated wild-type R. toruloides cells were more hydrophobic and more adhesive than nonencapsulated cells. However, two encapsulated Att- mutants were more hydrophobic than the wild type and levels of adhesion of R. toruloides were similar on polystyrene and less hydrophobic glass surfaces. Adhesion of wild-type yeast to barley and polystyrene was correlated with attachment to CM-Sephadex beads, indicating a positive cell surface charge. Sixteen Att- mutants did not exhibit a positive cell surface charge, and wild-type yeast cells that did not attach to CM-Sephadex did not adhere to either polystyrene or barley. Wild-type R. toruloides attached to CM-Sephadex beads by the poles of the cells, indicating a localization of positive charge which was also visualized with India ink. We conclude that localized, positive charge, and not hydrophobic interactions, mediates attachment of R. toruloides to barley leaves.

Simultaneous N doping and reduction of GO: Compositional, structural characterization and its effects in negative electrostatic charges repulsion

Pluronic-PAA, a thermogelling copolymer composed of side chains of poly(acrylic acid) (PAA) grafted onto a backbone of Pluronic copolymer, is of interest as a vehicle for the controlled release of compounds. An important feature of such a vehicle is its bioadhesive/mucoadhesive properties, which in the case of Pluronic-PAA are significant due to the presence of the PAA side chains. An atomic force microscopy (AFM) method has been developed and utilized to investigate the interactions between a Pluronic-PAA-modified microsphere and mucous substrates. The bioadhesive force was successfully measured, and trends were observed under conditions of varying pH and ionic strength. Pluronic-PAA exhibits significant mucoadhesion over a range of pH values, with mucoadhesion being optimal at pH 4-5 (adhesive force approximately 80 mN/cm(2)) and dropping sharply at higher pH, to a value of approximately 20 mN/cm(2) at pH 8. The mucoadhesive force decreased with increasing ionic strength, from a value of approximately 80 mN/cm(2) in 0.025 M NaCl to approximately 25 mN/cm(2) in 1.0 M NaCl. These results have been interpreted in terms of the effect of changing pH and ionic strength on electrostatic interactions and swelling of the polymer and mucin layers. Tensiometric force measurements indicated that hydrophobic interactions, as well as hydrogen bonding and electrostatic interactions, were significant in the mucoadhesion of Pluronic-PAA copolymers. Experiments with a range of Pluronic-PAA copolymers with varying PPO contents in the Pluronic segments showed that increasing the overall PPO content increased the hydrophobicity of the polymer solutions. This was reflected in the increases in the advancing contact angles with the mucin layer, indicating that hydrophobic interactions play a role in the adhesion of Pluronic-PAA to mucin.

Solid particle-stabilized Pickering emulsions can be used as alternative reaction systems, for example, for the homogeneously catalyzed hydroformylation reaction. This study addresses the understanding of the physicochemical behavior of Pickering emulsions in terms of the hydroformylation in a recycable process. In the first part (see chapter 4), fumed silica with different hydrophobicities were used to stabilize the emulsions. Hence, the droplets with W/O Pickering emulsions exhibit a size that is a function of particle concentration and energy input during the preparation. Furthermore, adsorption of interface impurities on the particles is observed, resulting in an increase of the interfacial tension. In addition, the Pickering emulsions are highly stable in a batch reactor. Hence, the hydroformylation reaction in Pickering emulsions was optimized and a complete recycling cycle with a membrane filtration was successfully demonstrated. In chapter 5, hydrophilic particles with different particle shapes, so-called Halloysite nanotubes and fumed silica, which stabilize an O/W Pickering emulsion were used due to higher conversions. The larger Halloysite nanotubes initially exhibit an isotropic interface orientation that converts to a radial configuration by increasing the particle concentration. It was possible to modify the Halloysite nanotubes, but the change in wettability was not strongly pronounced. Furthermore, emulsions stabilized by pristine Halloysite nanotubes or by silica show a dependency on the particle concentration, hence, in the case of Halloysite nanotubes, the droplet size does not decrease monotonically. The addition of the interface-active Rh-catalyst leads to a droplet size in the order of nanometers, resulting in droplets without adherent particles. The increase in the droplet size to the micrometer scale leads to an adherence of the particles. Thus, a corresponding model of Pickering emulsions is postulated in a batch reactor, with intermediate emulsion stability promoting the reaction. The last chapter (see chapter 6) investigates the interaction between positively charged particles and the negatively charged rhodium (Rh-) catalyst in terms of emulsion structure and hydroformylation. The positively charged polystyrene particles used stabilize a W/O emulsion while the modified positively charged Halloysite nanotubes stabilize an O/W emulsion. It is shown that the Rh-catalyst adsorbs at the particle surface, which does not change the emulsion type. Further, in the case of polystyrene-stabilized Pickering emulsions, the particle density at the interface is also not affected by the adsorption of the Rh-catalyst. However, the diffusion behavior of the polystyrene particles at the interface is influenced by the adsorption of the Rh-catalyst on the particle surface. In general, it is demonstrated that the positive surface charge for both particle types leads to a higher conversion and selectivity in comparison to their negatively charged analogous.

A growing body of evidence suggests that the extracellular domain of the epithelial Na(+) channel (ENaC) functions as a sensor that fine tunes channel activity in response to changes in the extracellular environment. We previously found that acidic pH increases the activity of human ENaC, which results from a decrease in Na(+) self-inhibition. In the current work, we identified extracellular domain residues responsible for this regulation. We found that rat ENaC is less sensitive to pH than human ENaC, an effect mediated in part by the γ subunit. We identified a group of seven residues in the extracellular domain of γENaC (Asp-164, Gln-165, Asp-166, Glu-292, Asp-335, His-439, and Glu-455) that, when individually mutated to Ala, decreased proton activation of ENaC. γ(E455) is conserved in βENaC (Glu-446); mutation of this residue to neutral amino acids (Ala, Cys) reduced ENaC stimulation by acidic pH, whereas reintroduction of a negative charge (by MTSES modification of Cys) restored pH regulation. Combination of the seven γENaC mutations with β(E446A) generated a channel that was not activated by acidic pH, but inhibition by alkaline pH was intact. Moreover, these mutations reduced the effect of pH on Na(+) self-inhibition. Together, the data identify eight extracellular domain residues in human β- and γENaC that are required for regulation by acidic pH.

Lipid bilayer disruption by oligomeric α-synuclein depends on bilayer charge and accessibility of the hydrophobic core

In Alzheimer disease, amyloid beta, a 39-43-residue peptide produced by cleavage from a large amyloid precursor protein, undergoes conformational change to form amyloid fibrils and deposits as senile amyloid plaques in the extracellular cerebral cortices of the brain. However, the mechanism of how the intrinsically linear amyloid fibrils form spherical senile plaques is unknown. With total internal reflection fluorescence microscopy combined with the use of thioflavin T, an amyloid-specific fluorescence dye, we succeeded in observing the formation of the senile plaque-like spherulitic structures with diameters of around 15 microm on the chemically modified quartz surface. Real-time observation at a single fibrillar level revealed that, in the absence of tight contact with the surface, the cooperative and radial growth of amyloid fibrils from the core leads to a huge spherulitic structure. The results suggest the underlying physicochemical mechanism of senile plaque formation, essential for obtaining insight into prevention of Alzheimer disease.