Membrane shape as determinant of protein properties

"Living Detergents": An in Situ Detergent Tailoring Strategy for Efficient Membrane Protein Stabilization and Analysis

Detergents are essential molecular tools for membrane protein (MP) research, yet traditional detergents with static properties often fail to address the diverse and evolving needs of MP studies. To this end, this study introduces "living detergents", an innovative class of detergents equipped with functional tags that enable bioorthogonal modifications with externally introduced structural elements. This approach allows for not only the parallel generation of new detergents, but also in situ tuning of MP samples within freshly formed detergents. The efficacy of this strategy was demonstrated through the rapid identification of optimal detergents for high-quality electron microscopy studies of A2AAR. Overall, this flexible and robust platform enables efficient tailoring of detergents, advancing the exploration of detergent structure-function relationships in MP research and opening pathways for more specialized solutions for diverse experimental demands.

Engineering cardiolipin binding to an artificial membrane protein reveals determinants for lipid-mediated stabilization

Integral membrane proteins carry out essential functions in the cell, and their activities are often modulated by specific protein-lipid interactions in the membrane. Here, we elucidate the intricate role of cardiolipin (CDL), a regulatory lipid, as a stabilizer of membrane proteins and their complexes. Using the in silico-designed model protein TMHC4_R (ROCKET) as a scaffold, we employ a combination of molecular dynamics simulations and native mass spectrometry to explore the protein features that facilitate preferential lipid interactions and mediate stabilization. We find that the spatial arrangement of positively charged residues as well as local conformational flexibility are factors that distinguish stabilizing from non-stabilizing CDL interactions. However, we also find that even in this controlled, artificial system, a clear-cut distinction between binding and stabilization is difficult to attain, revealing that overlapping lipid contacts can partially compensate for the effects of binding site mutations. Extending our insights to naturally occurring proteins, we identify a stabilizing CDL site within the E. coli rhomboid intramembrane protease GlpG and uncover its regulatory influence on enzyme substrate preference. In this work, we establish a framework for engineering functional lipid interactions, paving the way for the design of proteins with membrane-specific properties or functions.

Exploring the Physical Properties of Lipid Membranes with Polyhydroxy Oxanorbornane Head Group Using NBD-Conjugated and DPH Fluorescent Probes

The present study focuses on exploring the physical properties of lipid membranes based on the polyhydroxy oxanorbornane (PH-ONB) headgroup, designed as synthetic analogues of naturally occurring archaeal lipid membranes. Specifically, we study two variants of PH-ONB headgroup-based lipids differing in the number of hydroxy groups present in the headgroup, with one having two hydroxy groups (ONB-2OH) and the other having three (ONB-3OH). These lipids form stable bilayer membranes. The study begins with a comprehensive analysis of the fluorescence characteristics of nitrobenzoxadiazole (NBD)-tagged ONB-based lipids in different solvent environments and within a model lipid membrane 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Subsequently, the physical properties of the ONB-based membranes were examined by using an NBD-tagged ONB-based probe and a commonly used extrinsic 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescent probe. The steady-state and time-resolved fluorescence properties of the NBD-tagged ONB-based probe and DPH were used to compare the physical properties of the ONB-based membranes, including polarity, fluidity, phase transition, order, hydration, location, heterogeneity, and rotational diffusion. The solid gel to liquid crystalline phase transition temperatures of ONB-2OH and ONB-3OH lipid membranes are found to be (68 ± 1) °C and (74 ± 1) °C, respectively. The variation in organization (size), fluidity, and phase transition temperature of ONB-based lipid membranes is explained by the extent of hydrogen bonding interactions between lipid head groups. ONB-based membranes exhibit characteristics similar to those of phospholipid membranes and possess a notably high phase transition temperature. These properties make them a promising and cost-effective synthetic alternative to archaeal lipid membranes with a wide range of potential applications.

Solid-state NMR spectroscopy for structural studies of polypeptides and lipids in extended physiological membranes

Solid-state NMR is a quickly developing technique that allows one to obtain structural information at atomic resolution in extended lipid bilayers in a rather unique manner. Two approaches have been developed for membrane proteins and peptides namely magic angle sample spinning and the use of uniaxially oriented membrane samples. The state-of-the-art of both approaches will be introduced and the perspectives of solid-state NMR spectroscopy in the context of other structural biology techniques, pressing biomedical questions and membrane biophysics will be discussed.

Cardiolipin binding enhances KcsA channel gating via both its specific and dianion-monoanion interchangeable sites

KcsA is a potassium channel with a plethora of structural and functional information, but its activity in the KcsA-producing actinomycete membranes remains elusive. To determine lipid species involved in channel-modulation, a surface plasmon resonance (SPR)-based methodology, characterized by immobilization of membrane proteins under a membrane environment, was applied. Dianionic cardiolipin (CL) showed extremely higher affinity for KcsA than monoanionic lipids. The SPR experiments further demonstrated that CL bound not only to the N-terminal M0 helix, a lipid-sensor domain, but to the M0 helix-deleted mutant. In contrast, monoanionic lipids interacted primarily with the M0 helix. This indicates the presence of an alternative CL-binding site, plausibly in the transmembrane domain. Single-channel recordings demonstrated that CL enhanced channel opening in an M0-independent manner. Taken together, the action of monoanionic lipids is exclusively mediated by the M0 helix, while CL binds both the M0 helix and its specific site, further enhancing the channel activity.

Conformational analysis of membrane-proximal segments of GDAP1 in a lipidic environment using synchrotron radiation suggests a mode of assembly at the mitochondrial outer membrane

The mitochondrial outer membrane creates a diffusion barrier between the cytosol and the mitochondrial intermembrane space, allowing the exchange of metabolic products, important for efficient mitochondrial function in neurons. The ganglioside-induced differentiation-associated protein 1 (GDAP1) is a mitochondrial outer membrane protein with a critical role in mitochondrial dynamics and metabolic balance in neurons. Missense mutations in the GDAP1 gene are linked to the most common human peripheral neuropathy, Charcot-Marie-Tooth disease (CMT). GDAP1 is a distant member of the glutathione-S-transferase (GST) superfamily, with unknown enzymatic properties or functions at the molecular level. The structure of the cytosol-facing GST-like domain has been described, but there is no consensus on how the protein interacts with the mitochondrial outer membrane. Here, we describe a model for GDAP1 assembly on the membrane using peptides vicinal to the GDAP1 transmembrane domain. We used oriented circular dichroism spectroscopy (OCD) with synchrotron radiation to study the secondary structure and orientation of GDAP1 segments at the outer and inner surfaces of the outer mitochondrial membrane. These experiments were complemented by small-angle X-ray scattering, providing the first experimental structural models for full-length human GDAP1. The results indicate that GDAP1 is bound into the membrane via a single transmembrane helix, flanked by two peripheral helices interacting with the outer and inner leaflets of the mitochondrial outer membrane in different orientations. Impairment of these interactions could be a mechanism for CMT in the case of missense mutations affecting these segments instead of the GST-like domain.

Application of synthetic lipid droplets in metabolic diseases

BACKGROUND

The study and synthesis of membrane organelles are becoming increasingly important, not only as simplified cellular models for corresponding molecular and metabolic studies but also for applications in synthetic biology of artificial cells and drug delivery vehicles. Lipid droplets (LDs) are central organelles in cellular lipid metabolism and are involved in almost all metabolic processes. Multiple studies have also demonstrated a high correlation between LDs and metabolic diseases. During these processes, LDs reveal a highly dynamic character, with their lipid fraction, protein composition and subcellular localisation constantly changing in response to metabolic demands. However, the molecular mechanisms underlying these functions have not been fully understood due to the limitations of cell biology approaches. Fortunately, developments in synthetic biology have provided a huge breakthrough for metabolism research, and methods for in vitro synthesis of LDs have been successfully established, with great advances in protein binding, lipid function, membrane dynamics and enzymatic reactions.

AIMS AND METHODS

In this review, we provide a comprehensive overview of the assembly and function of endogenous LDs, from the generation of lipid molecules to how they are assembled into LDs in the endoplasmic reticulum. In particular, we highlight two major classes of synthetic LD models for fabrication techniques and their recent advances in biology and explore their roles and challenges in achieving real applications of artificial LDs in the future.

Detection of bacteriorhodopsin trimeric rotation at thermal phase transitions of purple membrane in suspension

Bacteriorhodopsin (bR) of purple membrane (PM) is a retinal protein that forms aggregates in the form of trimers constituting, together with archaeal lipids, the crystalline structure of PM. The rotary motion of bR inside PM may be pertinent in understanding the essence of the crystalline lattice. An attempt has been made to determine the rotation of bR trimers which has been found to be detected solely at thermal phase transitions of PM, namely lipid, crystalline lattice and protein melting phase transitions. The temperature dependences of dielectric versus electronic absorption spectra of bR have been determined. The results suggest that the rotation of bR trimers, together with concomitant bending of PM, are most likely brought by structural changes in bR which might be driven by retinal isomerization and mediated by lipid. The rupturing of the lipid-protein contact might consequently lead to rotation of trimers associated with bending, curling or vesicle formation of PM. So the retinal reorientation may underlie the concomitant rotation of trimers. Most importantly, rotation of trimers might play a role, in terms of the essence of the crystalline lattice, in the functional activity of bR and may serve physiological relevance.

Different membrane order measurement techniques are not mutually consistent

"Membrane order" is a term commonly used to describe the elastic and mechanical properties of the lipid bilayer, though its exact meaning is somewhat context- and method dependent. These mechanical properties of the membrane control many cellular functions and are measured using various biophysical techniques. Here, we ask if the results obtained from various techniques are mutually consistent. Such consistency cannot be assumed a priori because these techniques probe different spatial locations and different spatial and temporal scales. We evaluate the change of membrane order induced by serotonin using nine different techniques in lipid bilayers of three different compositions. Serotonin is an important neurotransmitter present at 100s of mM concentrations in neurotransmitter vesicles, and therefore its interaction with the lipid bilayer is biologically relevant. Our measurement tools include fluorescence of lipophilic dyes (Nile Red, Laurdan, TMA-DPH, DPH), whose properties are a function of membrane order; atomic force spectroscopy, which provides a measure of the force required to indent the lipid bilayer; 2H solid-state NMR spectroscopy, which measures the molecular order of the lipid acyl chain segments; fluorescence correlation spectroscopy, which provides a measure of the diffusivity of the probe in the membrane; and Raman spectroscopy, where spectral intensity ratios are affected by acyl chain order. We find that different measures often do not correlate with each other and sometimes even yield conflicting results. We conclude that no probe provides a general measure of membrane order and that any inference based on the change of membrane order measured by a particular probe may be unreliable.

Characterization of retinal chromophore and protonated Schiff base in Thermoplasmatales archaeon heliorhodopsin using solid-state NMR spectroscopy

Heliorhodopsin (HeR) is a seven-helical transmembrane protein with a retinal chromophore that corresponds to a new rhodopsin family. HeR from the archaebacterium Thermoplasmatales archaeon (TaHeR) exhibits unique features, such as the inverted protein orientation in the membrane compared to other rhodopsins and a long photocycle. Here, we used solid-state nuclear magnetic resonance (NMR) spectroscopy to investigate the ¹³C and ¹⁵N NMR signals of the retinal chromophore and protonated Schiff base (RPSB) in TaHeR embedded in POPE/POPG membrane. Although the 14- and 20-¹³C retinal signals indicated 13-trans/15-anti (all-trans) configurations, the 20-¹³C chemical shift value was different from that of other microbial rhodopsins, indicating weakly steric hinderance between Phe203 and the C20 methyl group. ¹⁵N RPSB/λ_max plot deviated from the linear correlation based on retinylidene-halide model compounds. Furthermore, ¹⁵N chemical shift anisotropy (CSA) suggested that Ser112 and Ser234 polar residues distinguish the electronic environment tendencies of RPSB from those of other microbial rhodopsins. Our NMR results revealed that the retinal chromophore and the RPSB in TaHeR exhibit unique electronic environments.

Orexin A, an amphipathic α-helical neuropeptide involved in pleiotropic functions in the nervous and immune systems: Synthetic approach and biophysical studies of the membrane-bound state

This research reports on the membrane interactions of orexin A (OXA), an α-helical and amphipathic neuropeptide that contains 33 residues and two disulfide bonds in the N-terminal region. OXA, which activates the orexins 1 and 2 receptors in neural and immune cell membranes, has essential pleiotropic physiological effects, including at the levels of arousal, sleep/wakefulness, energy balance, neuroprotection, lipid signaling, the inflammatory response, and pain. As a result, the orexin system has become a prominent target to treat diseases such as sleep disorders, drug addiction, and inflammation. While the high-resolution structure of OXA has been investigated in water and bound to micelles, there is a lack of information about its conformation bound to phospholipid membranes and its receptors. NMR is a powerful method to investigate peptide structures in a membrane environment. To facilitate the NMR structural studies of OXA exposed to membranes, we present a novel synthetic scheme, leading to the production of isotopically-labeled material at high purity. A receptor activation assay shows that the ¹⁵N-labeled peptide is biologically active. Biophysical studies are performed using surface plasmon resonance, circular dichroism, and NMR to investigate the interactions of OXA with phospholipid bilayers. The results demonstrate a strong interaction between the peptide and phospholipids, an increase in α-helical content upon membrane binding, and an in-plane orientation of the C-terminal region critical to function. This new knowledge about structure-activity relationships in OXA could inspire the design of novel therapeutics that leverage the anti-inflammatory and neuro-protective functions of OXA, and therefore could help address neuroinflammation, a major issue associated with neurological disorders such as Alzheimer's disease.

The role of hydrophobic patches of de novo designed MSI-78 and VG16KRKP antimicrobial peptides on fragmenting model bilayer membranes

Antimicrobial peptides (AMPs) with cell membrane lysing capability are considered potential candidates for the development of the next generation of antibiotics. Designing novel AMPs requires an in-depth understanding of the mechanism of action of the peptides. In this work, we used various biophysical techniques including ³¹P solid-state NMR to examine the interaction of model membranes with amphipathic de novo-designed peptides. Two such peptides, MSI-78 and VG16KRKP, were designed with different hydrophobicity and positive charges. The model lipid membranes were constituted by mixing lipids of varying degrees of 'area per lipid' (APL), which directly affected the packing properties of the membrane. The observed emergence of the isotropic peak in ³¹P NMR spectra as a function of time is a consequence of the fragmentation of the membrane mediated by the peptide interaction. The factors such as the charges, overall hydrophilicity of the AMPs, as well as lipid membrane packing, contributed to the kinetics of membrane fragmentation. Furthermore, we anticipate the designed AMPs follow the carpet and toroidal pore mechanisms when lysing the cell membrane. This study highlights the significance of the effect of the overall charges and the hydrophobicity of the novel AMPs designed for antimicrobial activity.

Dynamic Interactions Between Brilliant Green and MscL Investigated by Solid-State NMR Spectroscopy and Molecular Dynamics Simulations

The mechanosensitive ion channel of large conductance (MscL) is a promising template for the development of new antibiotics due to its high conservation and uniqueness to microbes. Brilliant green (BG), a triarylmethane dye, has been identified as a new antibiotic targeted MscL. However, the detailed binding sites to MscL and the dynamic pathway of BG through the MscL channel remain unknown. Here, the dynamic interactions between BG and MscL were investigated using solid-state NMR spectroscopy and molecule dynamics (MD) simulations. Residue site-specific binding sites of BG to the MscL channel were identified by solid-state NMR. In addition, MD simulations revealed that BG conducts through the MscL channel via residues along the inner surface of the pore sequentially, in which the strong hydrophobic interactions between BG and hydrophobic residues F23 and I27 in the hydrophobic gate region of the MscL channel are major restrictions. Particularly, it was demonstrated that BG activates the MscL channel by reducing the hydrophobicity of the F23 in the gate region by water molecules that are bound to BG. Taken together, these simulations and experimental data provide novel insights into the dynamic interactions between BG and MscL, based on which new hydrophobic antibiotics and adjuvants targeting MscL can be developed.

The Advanced Properties of Circularized MSP Nanodiscs Facilitate High-resolution NMR Studies of Membrane Proteins

Membrane mimetics are essential for structural and functional studies of membrane proteins. A promising lipid-based system are phospholipid nanodiscs, where two copies of a so-called membrane scaffold protein (MSP) wrap around a patch of lipid bilayer. Consequently, the size of a nanodisc is determined by the length of the MSP. Furthermore, covalent MSP circularization was reported to improve nanodisc stability. However, a more detailed comparative analysis of the biophysical properties of circularized and linear MSP nanodiscs for their use in high-resolution NMR has not been conducted so far. Here, we analyze the membrane fluidity and temperature-dependent size variability of circularized and linear nanodiscs using a large set of analytical methods. We show that MSP circularization does not alter the membrane fluidity in nanodiscs. Further, we show that the phase transition temperature increases for circularized versions, while the cooperativity decreases. We demonstrate that circularized nanodiscs keep a constant size over a large temperature range, in contrast to their linear MSP counterparts. Due to this size stability, circularized nanodiscs are beneficial for high-resolution NMR studies of membrane proteins at elevated temperatures. Despite their slightly larger size as compared to linear nanodiscs, 3D NMR experiments of the voltage-dependent anion channel 1 (VDAC1) in circularized nanodiscs have a markedly improved spectral quality in comparison to VDAC1 incorporated into linear nanodiscs of a similar size. This study provides evidence that circularized MSP nanodiscs are a promising tool to facilitate high-resolution NMR studies of larger and challenging membrane proteins in a native lipid environment.

Phospholipid headgroups govern area per lipid and emergent elastic properties of bilayers

Phospholipid bilayers are liquid-crystalline materials whose intermolecular interactions at mesoscopic length scales have key roles in the emergence of membrane physical properties. Here we investigated the combined effects of phospholipid polar headgroups and acyl chains on biophysical functions of membranes with solid-state 2H NMR spectroscopy. We compared the structural and dynamic properties of phosphatidylethanolamine and phosphatidylcholine with perdeuterated acyl chains in the solid-ordered (so) and liquid-disordered (ld) phases. Our analysis of spectral lineshapes of 1,2-diperdeuteriopalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE-d62) and 1,2-diperdeuteriopalmitoyl-sn-glycero-3-phosphocholine (DPPC-d62) in the so (gel) phase indicated an all-trans rotating chain structure for both lipids. Greater segmental order parameters (SCD) were observed in the ld (liquid-crystalline) phase for DPPE-d62 than for DPPC-d62 membranes, while their mixtures had intermediate values irrespective of the deuterated lipid type. Our results suggest the SCD profiles of the acyl chains are governed by methylation of the headgroups and are averaged over the entire system. Variations in the acyl chain molecular dynamics were further investigated by spin-lattice (R1Z) and quadrupolar-order relaxation (R1Q) measurements. The two acyl-perdeuterated lipids showed distinct differences in relaxation behavior as a function of the order parameter. The R1Z rates had a square-law dependence on SCD, implying collective mesoscopic dynamics, with a higher bending rigidity for DPPE-d62 than for DPPC-d62 lipids. Remodeling of lipid average and dynamic properties by methylation of the headgroups thus provides a mechanism to control the actions of peptides and proteins in biomembranes.

All-atom molecular dynamics simulations of the combined effects of different phospholipids and cholesterol content on electroporation

The electroporation mechanism could be related to the composition of the plasma membrane, and the combined effect of different phospholipid molecules and cholesterol content on electroporation has rarely been studied nor conclusions drawn. In this paper, we applied all-atom molecular dynamics (MD) simulations to study the effects of phospholipids and cholesterol content on bilayer membrane electroporation. The palmitoyloleoylphosphatidylcholine (POPC) model, palmitoyloleoylphosphatidylethanolamine (POPE) model, and a 1 : 1 mixed model of POPC and POPE called PEPC, were the three basic models used. An electric field of 0.45 V nm-1 was applied to nine models, which were the three basic models, each with three different cholesterol content values of 0%, 24%, and 40%. The interfacial water molecules moved under the electric field and, once the first water bridge formed, the rest of the water molecules would dramatically flood into the membrane. The simulation showed that a rapid rise in the Z-component of the average dipole moment of the interfacial water molecules (Z-DM) indicated the occurrence of electroporation, and the same increment of Z-DM represented a similar change in the size of the water bridge. With the same cholesterol content, the formation of the first water bridge was the most rapid in the POPC model, regarding the average electroporation time (t ep), and the average t ep of the PEPC model was close to that of the POPE model. We speculate that the differences in membrane thickness and initial number of hydrogen bonds of the interfacial water molecules affect the average t ep for different membrane compositions. Our results reveal the influence of membrane composition on the electroporation mechanism at the molecular level.

Structural Entities Associated with Different Lipid Phases of Plant Thylakoid Membranes—Selective Susceptibilities to Different Lipases and Proteases

It is well established that plant thylakoid membranes (TMs), in addition to a bilayer, contain two isotropic lipid phases and an inverted hexagonal (H_II) phase. To elucidate the origin of non-bilayer lipid phases, we recorded the ³¹P-NMR spectra of isolated spinach plastoglobuli and TMs and tested their susceptibilities to lipases and proteases; the structural and functional characteristics of TMs were monitored using biophysical techniques and CN-PAGE. Phospholipase-A1 gradually destroyed all ³¹P-NMR-detectable lipid phases of isolated TMs, but the weak signal of isolated plastoglobuli was not affected. Parallel with the destabilization of their lamellar phase, TMs lost their impermeability; other effects, mainly on Photosystem-II, lagged behind the destruction of the original phases. Wheat-germ lipase selectively eliminated the isotropic phases but exerted little or no effect on the structural and functional parameters of TMs—indicating that the isotropic phases are located outside the protein-rich regions and might be involved in membrane fusion. Trypsin and Proteinase K selectively suppressed the H_II phase—suggesting that a large fraction of TM lipids encapsulate stroma-side proteins or polypeptides. We conclude that—in line with the Dynamic Exchange Model—the non-bilayer lipid phases of TMs are found in subdomains separated from but interconnected with the bilayer accommodating the main components of the photosynthetic machinery.

Detergent-Free Isolation of Membrane Proteins and Strategies to Study Them in a Near-Native Membrane Environment

Atomic-resolution structural studies of membrane-associated proteins and peptides in a membrane environment are important to fully understand their biological function and the roles played by them in the pathology of many diseases. However, the complexity of the cell membrane has severely limited the application of commonly used biophysical and biochemical techniques. Recent advancements in NMR spectroscopy and cryoEM approaches and the development of novel membrane mimetics have overcome some of the major challenges in this area. For example, the development of a variety of lipid-nanodiscs has enabled stable reconstitution and structural and functional studies of membrane proteins. In particular, the ability of synthetic amphipathic polymers to isolate membrane proteins directly from the cell membrane, along with the associated membrane components such as lipids, without the use of a detergent, has opened new avenues to study the structure and function of membrane proteins using a variety of biophysical and biological approaches. This review article is focused on covering the various polymers and approaches developed and their applications for the functional reconstitution and structural investigation of membrane proteins. The unique advantages and limitations of the use of synthetic polymers are also discussed.

Depth-Dependent Segmental Melting of the Sphingomyelin Alkyl Chain in Lipid Bilayers

The chain melting of lipid bilayers has often been investigated in detail using calorimetric methods, such as differential scanning calorimetry (DSC), and the resultant main transition temperature is regarded as one of the most important parameters in model membrane experiments. However, it is not always clear whether the hydrocarbon chains of lipids are gradually melting along the depth of the lipid bilayer or whether they all melt concurrently in a very narrow temperature range, as implied by DSC. In this study, we focused on stearoyl-d-sphingomyelin (SSM) as an example of raft-forming lipids. We synthesized deuterium-labeled SSMs at the 4', 10', and 16' positions, and their depth-dependent melting was measured using solid-state deuterium NMR by changing the temperature by 1.0 °C, and comparing with that observed from a saturated lipid, palmitoylstearoylphosphatidylcholine (PSPC). The results showed that SSM exhibited a characteristic depth-dependent melting, which was not observed for PSPC. The strong intermolecular hydrogen bonds between the sphingomyelin amide moiety probably caused the chain melting to start from the chain terminus through the middle part and end in the upper part. This depth-dependent melting implies that the small gel-like domains of SSM remain at temperatures slightly above the main transition temperature. These sphingomyelin features may be responsible for the biological properties of SM-based lipid rafts.

Effect of cholesterol on the lactosylceramide domains in phospholipid bilayers

Lactosylceramide (LacCer) in the plasma membranes of immune cells is an important lipid for signaling in innate immunity through the formation of LacCer-rich domains together with cholesterol (Cho). However, the properties of the LacCer domains formed in multicomponent membranes remain unclear. In this study, we examined the properties of the LacCer domains formed in Cho containing 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) membranes by deuterium solid-state NMR and fluorescence lifetimes. The potent affinity of LacCer-LacCer (homophilic interaction) is known to induce a thermally stable gel phase in the unitary LacCer bilayer. In LacCer/Cho binary membranes, Cho gradually destabilized the LacCer gel phase to form the liquid-ordered (Lo) phase by its potent order effect. In the LacCer/POPC binary systems without Cho, the ²H NMR spectra of 10',10'-d₂-LacCer and 18',18',18'-d₃-LacCer probes revealed that LacCer was poorly miscible with POPC in the membranes and formed stable gel phases without being distributed in the liquid crystalline (Ld) domain. The lamellar structure of the LacCer/POPC membrane was gradually disrupted at around 60 °C, while the addition of Cho increased the thermal stability of the lamellarity. Furthermore, the area of the LacCer gel phase and its chain order were decreased in the LacCer/POPC/Cho ternary membranes, while the Lo domain, which was observed in the LacCer/Cho binary membrane, was not observed. Cho surrounding the LacCer gel domain liberated LacCer and facilitated forming the submicron- to nano-scale small domains in the Ld domain of the LacCer/POPC/Cho membranes, as revealed by the fluorescence lifetimes of trans-parinaric acid (tPA) and tPA-LacCer. Our findings on the membrane properties of the LacCer domains, particularly in the presence of Cho, would help elucidate the properties of the LacCer domains in biological membranes.

In vitro evaluation of the protective effect of crocin on human erythrocytes

The interactions and the protective effect of the carotenoid crocin (CRO) on human erythrocytes (RBC) and molecular models of its membrane were investigated. The latter consisted of bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE), representative of phospholipid classes located in the outer and inner monolayers of the RBC membrane, respectively. X-ray diffraction, differential scanning calorimetry (DSC) and electronic paramagnetic resonance spectroscopy (EPR) showed that CRO produced structural perturbations in DMPC bilayers and in isolated unsealed human erythrocyte membranes. On the other hand, scanning electron microscopy (SEM) showed that CRO induced shape changes in the RBC from their normal discoid form to echinocytes. This result indicates that the CRO molecules were mainly localized in the outer monolayer of the RBC membrane. The assessment of the protective capacity of CRO was revealed by the carotenoid inhibition of the morphological alterations caused by hypochlorous acid (HOCl) to RBC.

Liposomes encapsulating artificial cytosol as drug delivery system

The fabrication of cell models containing artificial cytosol is challenging. Herein we constructed an artificial cytosol contained cell model by electroformation method. Agarose was selected as the main component of the artificial cytosol, and sucrose was added into the agarose to regulate the sol viscosity and the phase transition temperature. The viscosity of the sol with the mass ratio (agarose-sucrose) 1:9 was closest to the natural cytosol. DSPC/20 mol% cholesterol was used to form large unilamellar vesicles (LUVs) as cell model compartment. The rhodamine release experiment confirmed that the unique release profile of agarose-sucrose@LUVs is suitable as a drug carrier. Doxorubicin is loaded in the agarose-sucrose@LUVs, and their half maximum inhibition concentration on HeLa cells is 0.016 μmol L^-1, which means 28.7 times increase in inhibition efficiency over free doxorubicin.

Molecular recognition and interaction between human plasminogen Kringle 5 and voltage-dependent anion channel-1 by biological specificity technologies and molecular dynamic simulation

Voltage-dependent anion channel-l (VDAC-1) can bind with plasminogen Kringle 5 as the cell surface receptor and induce cell apoptosis, but the detailed information of binding is not clear yet. Thus, the mutual recognition and binding were investigated here utilizing frontal affinity chromatography, surface plasma resonance, mutation analysis combining molecular dynamics simulation. The results showed that Kringle 5 binds with VDAC-1 in equimolar driven mainly by electrostatic force, with 15 amino acid residues participating in Kringle 5 and 21 in VDAC-1. The observed conformational changes indicated the automatic structure regulation providing these two proteins suitable conformations and spatial surroundings for the tighter and stabler binding. Moreover, Glu29 in Kringle 5 was speculated as the key residue maintaining the largest energy contribution. Therefore, this work provided precise information for the recognition and binding of Kringle 5 with VDAC-1 that is valuable for the corresponding treatment of tumours or other angiogenic diseases.