Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D40: Lipid Bilayers and Biological Membranes: Peptide Interactions |
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Sponsoring Units: DBP Chair: Emmanouil Doxastakis, University of Houston Room: A122/123 |
Monday, March 21, 2011 2:30PM - 2:42PM |
D40.00001: Molecular Simulations of Sequence-Specific Association of Transmembrane Proteins in Lipid Bilayers Manolis Doxastakis, Anupam Prakash, Lorant Janosi Association of membrane proteins is central in material and information flow across the cellular membranes. Amino-acid sequence and the membrane environment are two critical factors controlling association, however, quantitative knowledge on such contributions is limited. In this work, we study the dimerization of helices in lipid bilayers using extensive parallel Monte Carlo simulations with recently developed algorithms. The dimerization of Glycophorin A is examined employing a coarse-grain model that retains a level of amino-acid specificity, in three different phospholipid bilayers. Association is driven by a balance of protein-protein and lipid-induced interactions with the latter playing a major role at short separations. Following a different approach, the effect of amino-acid sequence is studied using the four transmembrane domains of the epidermal growth factor receptor family in identical lipid environments. Detailed characterization of dimer formation and estimates of the free energy of association reveal that these helices present significant affinity to self-associate with certain dimers forming non-specific interfaces. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D40.00002: An amino acid composition criterion for membrane active antimicrobials Nathan Schmidt, Ghee Hwee Lai, Abhijit Mishra, Dennis Bong, Paul McCray, Jr., Michael Selsted, Andre Ouellette, Gerard Wong Membrane active antimicrobials (AMPs) are short amphipathic peptides with broad spectrum anti microbial activity. While it is believed that their hydrophobic and cationic moieties are responsible for membrane-based mechanisms of action, membrane disruption by AMPs is manifested in a diversity of outcomes, such as pore formation, blebbing, and budding. This complication, along with others, have made a detailed, molecular understanding of AMPs difficult. We use synchrotron small angle xray scattering to investigate the interaction of model bacterial and eukaryotic cell membranes with archetypes from beta-sheet AMPs (e.g. defensins) and alpha-helical AMPs (e.g. magainins). The relationship between membrane composition and peptide induced changes in membrane curvature and topology is examined. By comparing the membrane rearrangement and phase behavior induced by these different peptides we will discuss the importance of amino acid composition on AMP design. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D40.00003: Interaction between Cell Penetrating pVEC and cell membranes Abhijit Mishra, Ghee Hwee Lai, Nathan Schmidt, Gerard Wong Vascular Endothelial Cadherin (VEC) is a transmembrane-spanning glycoprotein that belongs to the family of cell adhesion molecules and plays an active role in control of vascular permeability and angiogenesis. PVEC, an 18 amino acid domain, has been shown to be able to traverse cell membranes with attached macromolecules. pVEC is an amphiphilic molecule with a high content of basic amino acids resulting in a net positive charge. Electrostatic and hydrophobic interactions can perturb membrane self-assembly and stability and are likely to be responsible for peptide uptake. We use synchrotron x-ray scattering and confocal microscopy to examine the phase behavior of the pVEC lipid system, and its relation to membrane permeation mechanisms. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D40.00004: Interactions between cyclic cell penetrating peptides and lipid membranes Kun Zhao, Tao Liu, Mike Choe, Daniel Kamei, Dehua Pei, Gerard Wong Cyclic peptides exhibit strong enhancement in receptor-binding affinity, specificity, and stability relative to their linear counterparts, partially due to their reduced conformational freedom. In this work, we examine cyclic versions of cell penetrating peptides. Using small-angle x-ray scattering (SAXS) measurements, we show that cyclic polyarginine peptides generate saddle-splay curvature more efficiently than their linear counterparts, We show how this increase in induced saddle splay curvature impinges on the efficiency of cell penetration in a series of giant vesicle and intracellular trafficking experiments. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D40.00005: Interaction of a P. aeruginosa Quorum Sensing Signal with Lipid Membranes Rebecca Morrison, Amelia Hall, Ellen Hutchison, Thuc Nguyen, Benjamin Cooley, Vernita Gordon Bacteria use a signaling and regulatory system called ``quorum sensing'' to alter their gene expressions in response to the concentration of neighboring bacteria and to environmental conditions that make collective activity favorable for bacteria. P. aeruginosa is an opportunistic human pathogen that uses quorum sensing to govern processes such as virulence and biofilm formation. This organism's two main quorum sensing circuits use two different signaling molecules that are amphiphilic and differ primarily in the length of their hydrocarbon side chain and thus in their hydrophobic physical chemistry. How these physical chemistries govern the propagation and spatial localization of signals and thus of quorum sensing is not known. We present preliminary results showing that signals preferentially sequester to amphiphilic lipid membranes, which can act as reservoirs for signal. This is promising for future characterization of how the quorum sensing signals of many bacteria and yeast partition to spatially-differentiated amphiphilic environments, in a host or biofilm. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D40.00006: Criticality in Plasma Membranes Benjamin Machta, Stefanos Papanikolaou, James Sethna, Sarah Veatch We are motivated by recent observations of micron-sized critical fluctuations in the 2d Ising Universality class in plasma membrane vesicles that are isolated from cortical cytoskeleton. We construct a minimal model of the plasma membrane's interaction with intact cytoskeleton which explains why large scale phase separation has not been observed in Vivo. In addition, we use analytical techniques from conformal field theory and numerical simulations to investigate the form of effective forces mediated by the membrane's proximity to criticality. We show that the range of this force is maximized near a critical point and we quantify its usefulness in mediating communication using techniques from information theory. Finally we use theoretical techniques from statistical physics in conjunction with Monte-Carlo simulations to understand how criticality can be used to increase the efficiency of membrane bound receptor mediated signaling. We expect that this sort of analysis will be broadly useful in understanding and quantifying the role of lipid ``rafts'' in a wide variety of membrane bound processes. Generally, we demonstrate that critical fluctuations provide a physical mechanism to organize and spatially segregate membrane components by providing channels for interaction over relatively large distances. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D40.00007: Fluctuation-induced forces between inclusions in a fluid membrane under tension Hsiang-Ku Lin, Roya Zandi, Leonid P. Pryadko We discuss the fluctuation-induced force, a finite-temperature analog of the Casimir force, between two inclusions embedded in a fluid membrane under tension. We suggest a method to calculate this Casimir interaction in the most general case, where membrane fluctuations are governed by the combined action of surface tension, bending modulus, and Gaussian rigidity. We find that the surface tension strongly modifies the power law in the separation dependence of the Casimir interaction. Furthermore, the method allows us to calculate the Casimir force both at short and large separations. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D40.00008: Nanoparticles Induced Microscaled Pore Formation on Supported Lipid Bilayer Benxin Jing, Y. Elaine Zhu Most of recent researches on the cytotoxicity of nanomaterials focused on hydrophilic nanomaterials because of their good dispersion in water, but much less on hydrophobic ones. In this work, we have investigated the effect of semi-hydrophobic nanoparticles (NPs) on the dynamics and morphology of model cell membrane. We have found carboxyl functionalized polystyrene nanoparticles can induce the formation of microscaled pores on neutral supported Egg PC lipid bilayer at the ionic strength range similar to that in the human body with a strong dependence on nanoparticle size and concentration. The hydrophobic interaction between the NP surface and lipid bilayer is accounted for the induced line tension in lipid bilayer; when the tension exceeds a critical value, pores are formed and grow rapidly with dependence on nanoparticle size and ionic strength. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D40.00009: Modeling the Elastic Properties of Lipid Bilayer Membranes Edward Barry, Thomas Gibaud, Mark Zakhary, Zvonimir Dogic Model membranes such as lipid bilayers have been indispensable tools for our understanding of the elastic properties of biological membranes. In this talk, I will introduce a colloidal model for membranes and demonstrate that the physical properties of these colloidal membranes are identical to lipid bilayers. The model system is unique in that the constituent molecules are homogenous and non-amphiphilic, yet their self-assembly into membranes and other hierarchical assemblages, such as a lamellar type phases and chiral ribbons, proceeds spontaneously in solution. Owing to the large size of the constituent molecules, individual molecules can be directly visualized and simultaneous observations at the continuum and molecular lengthscales are used to characterize the behavior of model membranes with unprecedented detail. Moreover, once assembled in solution, molecular interactions can be controlled in situ. In particular, the strength of chiral interactions can be varied, leading to fascinating transitions in behavior that resembles the formation of starfish vesicles. These observations point towards the important role of line tension, and have potential implications for phase separated lipid mixtures or lipid rafts. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D40.00010: Dielectric sensors for measuring membrane potential Kimal Rajapakshe, Asanga Wijesinghe, Jie Fang, William Widger, John Miller Membrane potential in a biological cell depends on the ionic concentration difference between the extracellular and the intracellular medium. Ions close to the membrane show high polarizations under an electric field. Recent theoretical studies have related these polarizations to the alpha ($\alpha )$ dispersions in the impedance spectroscopy of a cell suspension. Therefore these dispersions can be used to measure the membrane potential of a single cell. Here we report the dielectric properties of phosphatidylcholine liposomes and its changes with the membrane potential. Liposomes have been prepared to have a higher concentration of potassium ions (K$^{+})$ inside the membrane compared to external medium. Under valinomycin (K$^{+}$ ionophores) these liposomes generate a negative membrane potential, as verified by fluorescent voltage sensitive dye measurements. Both dielectric and conductivity spectra display low frequency dispersions that are dependent on membrane potential. Possible future applications include noninvasive sensors for in vitro testing of new drugs and other applications. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D40.00011: Laser Transmission Spectroscopy and applications to liposome studies Frank Li, James Marr, Ching-Ting Hwang, Robert Schafer, Zachary Schultz, Steven Ruggiero, Carol Tanner We describe the implementation of precision laser transmission spectroscopy (LTS) for sizing nanoparticles in suspension. Our apparatus incorporates a tunable laser and balanced optical system which measures light transmission over a wide (210 -- 2300 nm) wavelength range with high precision and sensitivity. Spectral inversion was employed to determine both the particle size distribution and absolute density of particles with diameters over a total range of 5 to 3000 nm. LTS has a dynamic range of $\sim$ $10^{3}$ particles/mL to $\sim$ $10^{10}$ particles/mL ($5x10^{-8}$ vol.\% to 0.5 vol.\%). Currently, LTS is being applied as a tool to investigate the behavior of liposomes, dipalmitoylphosphatidycholine (DPPC) and dipalmitoylphosphatidylserine (DPPS), under the presence of fusing and de-aggregating agents. Our measurements indicate a maximum diameter of 400 nm for liposomes suspended in solution after fusion. [Preview Abstract] |
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