Structure, Self-Assembly, and Phase Behavior of Neuroactive N-Acyl GABAs: Doxorubicin Encapsulation in NPGABA/DPPC Liposomes and Release Studies.

N-Acylated amino acids and neurotransmitters in mammals exert significant biological effects on the nervous system, immune responses, and vasculature. N-Acyl derivatives of γ-aminobutyric acid (N-acyl GABA), which belong to both classes mentioned above, are prominent among them. In this work, a homologous series of N-acyl GABAs bearing saturated N-acyl chains (C8-C18) have been synthesized and characterized with respect to self-assembly, thermotropic phase behavior, and supramolecular organization. Differential scanning calorimetric studies revealed that the transition enthalpies and entropies of N-acyl GABAs are linearly dependent on the acyl chain length. The crystal structure of N-tridecanoyl GABA showed that the molecules are packed in bilayers with the acyl chains aligned parallel to the bilayer normal and that the carboxyl groups from opposite layers associate to form dimeric structures involving strong O-H···O hydrogen bonds. In addition, N-H···O and C-H···O hydrogen bonds between amide moieties of adjacent molecules within each layer stabilize the molecular packing. Powder X-ray diffraction studies showed odd-even alternation in the d spacings, suggesting that the odd chain and even chain compounds pack differently. Equimolar mixtures of N-palmitoyl GABA and dipalmitoylphosphatidylcholine (DPPC) were found to form stable unilamellar vesicles with diameters of ∼300-340 nm, which could encapsulate doxorubicin, an anticancer drug, with higher efficiency and better release characteristics than DPPC liposomes at physiologically relevant pH. These liposomes exhibit faster release of doxorubicin at acidic pH (<7.0), indicating their potential utility as drug carriers in cancer chemotherapy.

Contrasting effects of molecular crowding on the membrane-perturbing and chaperone-like activities of major bovine seminal plasma protein, PDC-109.

Crowded environments inside cells and biological fluids greatly affect protein stability and activity. PDC-109, a polydisperse oligomeric protein of the bovine seminal plasma selectively binds choline phospholipids on the sperm cell surface and causes membrane destabilization and lipid efflux, leading to acrosome reaction. PDC-109 also exhibits chaperone-like activity (CLA) and protects client proteins against various kinds of stress, such as high temperature and low pH. In the present work, we have investigated the effect of molecular crowding on these two different activities of PDC-109 employing Dextran 70 (D70) - a widely used polymeric dextran - as the crowding agent. The results obtained show that presence of D70 markedly increases membrane destabilization by PDC-109. Isothermal titration calorimetric studies revealed that under crowded condition the binding affinity of PDC-109 for choline phospholipids increases approximately 3-fold, which could in turn facilitate membrane destabilization. In contrast, under identical conditions, its CLA was reduced significantly. The decreased CLA could be correlated to reduced surface hydrophobicity, which was due to stabilization of the protein oligomers. These results establish that molecular crowding exhibits contrasting effects on two different functional activities of PDC-109 and highlight the importance of microenvironment of proteins in modulating their functional activities.

Supported Natural Membranes on Microspheres for Protein-Protein Interaction Studies.

Multiple biological and pathological processes, such as signaling, cell-cell communication, and infection by various viruses, occur at the plasma membrane. The eukaryotic plasma membrane is made up of thousands of different lipids, membrane proteins, and glycolipids, and its composition is dynamic and constantly changing. Due to the central importance of membranes on the one hand and their complexity on the other, membrane model systems are instrumental for interrogating membrane-related biological processes. Here, we develop a new tool for protein-membrane interaction studies. Our method is based on natural membranes obtained from extracellular vesicles. We form membrane bilayers supported on polystyrene microspheres that can be trapped and manipulated using optical tweezers. This method allows working with membrane proteins of interest within a background of native membrane components where their correct orientation is preserved. We demonstrate our method's applicability by successfully measuring the interaction forces between the Spike protein of SARS-CoV-2 and its human receptor, ACE2. We further show that these interactions are blocked by the addition of an antibody against the receptor binding domain of the Spike protein. Our approach is versatile and broadly applicable for various membrane biology and biophysics questions.

Forces of Change: Optical Tweezers in Membrane Remodeling Studies.

Optical tweezers allow precise measurement of forces and distances with piconewton and nanometer precision, and have thus been instrumental in elucidating the mechanistic details of various biological processes. Some examples include the characterization of motor protein activity, studies of protein-DNA interactions, and characterizing protein folding trajectories. The use of optical tweezers (OT) to study membranes is, however, much less abundant. Here, we review biophysical studies of membranes that utilize optical tweezers, with emphasis on various assays that have been developed and their benefits and limitations. First, we discuss assays that employ membrane-coated beads, and overview protein-membrane interactions studies based on manipulation of such beads. We further overview a body of studies that make use of a very powerful experimental tool, the combination of OT, micropipette aspiration, and fluorescence microscopy, that allow detailed studies of membrane curvature generation and sensitivity. Finally, we describe studies focused on membrane fusion and fission. We then summarize the overall progress in the field and outline future directions.

Purification, molecular characterization and ligand binding properties of the major donkey seminal plasma protein DSP-1.

Fibronectin type-II (FnII) family proteins are the major proteins in many mammalian species including bull, horse and pig. In the present study, a major FnII protein has been identified and isolated from donkey (Equus hemionus) seminal plasma, which we refer to as Donkey Seminal Plasma protein-1 (DSP-1). The amino acid sequence determined by mass spectrometry and computational modeling studies revealed that DSP-1 is homologous to other mammalian seminal plasma proteins, including bovine PDC-109 (also known as BSP-A1/A2) and equine HSP-1/2. High-resolution LC-MS analysis indicated that the protein is heterogeneously glycosylated and also contains multiple acetylations, occurring in the attached glycans. Structural and thermal stability studies on DSP-1 employing CD spectroscopy and differential scanning calorimetry showed that the protein unfolds at ~43 °C and binding to phosphorylcholine (PrC) - the head group moiety of choline phospholipids - increases its thermal stability. Intrinsic fluorescence titrations revealed that DSP-1 recognizes lyso-phosphatidylcholine with over 100-fold higher affinity than PrC. Further, interaction of DSP-1 with erythrocytes, a model cell membrane, revealed that DSP-1 binding is mediated by a specific interaction with choline phospholipids and results in membrane perturbation, suggesting that binding of this protein to sperm plasma membrane could be physiologically significant.

Structure, thermotropic phase behavior and membrane interaction of N-acyl-ß-alaninols. Homologs of stress-combating N-acylethanolamines.

ß-Alaninol and its derivatives were reported to exhibit interesting biological and pharmacological activities and showed potential application in formulating drug delivery vehicles. In the present study, we report the synthesis and characterization of N-acyl-ß-alaninols (NABAOHs) bearing saturated acyl chains (n = 8-20) with respect to thermotropic phase behavior, supramolecular organization and interaction with diacylphosphatidylcholine, a major membrane lipid. Results obtained from DSC and powder XRD studies revealed that the transition temperatures (Tt), transition enthalpies (ΔHt), transition entropies (ΔSt) and d-spacings of NABAOHs show odd-even alteration. A linear dependence was observed in the values of ΔHt and ΔSt on the acyl chain length, independently for even and odd acyl chains in both dry and hydrated states; further, the even chainlength molecules exhibited higher values than the odd chainlength series. The crystals structures of N-lauroyl-ß-alaninol and N-palmitoyl-ß-alaninol, solved in monoclinic system in the P21/c space group, show that the NABAOHs adopt a tilted bilayer structure. A number of NH⋯O, O-H⋯O, and C-H⋯O hydrogen bonds between the hydroxyl and amide moieties of the head groups of NABAOH molecules belonging to adjacent and opposite layers stabilize the overall supramolecular organization of the self-assembled bilayer system. DSC studies on the interaction of N-myristoyl-ß-alaninol (NMBAOH) with dimyristoylphosphatidylcholine (DMPC) indicate that these two lipids mix well up to 45 mol% NMBAOH, whereas phase separation was observed at higher contents of NMBAOH. Transmission electron microscopic studies reveal that mixtures containing 20-50 mol% NMBAOH form stable ULVs of 90-150 nm diameter, suitable for use in drug delivery applications.