Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N31: Membrane and Membrane Protein Interactions |
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Sponsoring Units: DPOLY Chair: Yossef Elabd, Drexel University Room: 339 |
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N31.00001: Investigating the Structural Properties of Integral Membrane Proteins with Pulsed EPR Spectroscopy Invited Speaker: Gary Lorigan Very limited structural and dynamic information on proteins embedded inside a membrane currently exist, because they are difficulty to crystalize. New biophysical/structural biology methods are needed to probe these systems in a lipid bilayer. The Lorigan lab is applying unique hybrid NMR and spin-label EPR spectroscopic techniques to study membrane proteins. Magnetic resonance spectroscopic data of $^{\mathrm{15}}$N-, $^{\mathrm{2}}$H-labeled and/or spin-labeled membrane proteins incorporated into vesicles and bicelles will be presented. State-of-the-art pulsed EPR techniques such as Electron Spin Echo Envelope Modulation (ESEEM) spectroscopy, and Double Electron-Electron Resonance (DEER) spectroscopy will be used. The ESEEM technique can determine short to medium range distances (out to about 8 {\AA}) between a site-specific nitroxide spin label and a nearby NMR-active isotopic labeled residue for a variety of different peptides and proteins which ultimately can be used to determine the difference between an $\alpha $-helical and $\beta $-sheet secondary structure. DEER can be used to measure distances between 2 spin labels out to about 70 {\AA}. We have shown a huge improvement is sensitivity with DEER measurements at Q-band when compared to X-band. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N31.00002: Tocopherol activity correlates with its location in a membrane: A new perspective on the anti-oxidant Vitamin E Drew Marquardt, Justin Williams, Norbert Kucerka, Jeffrey Atkinson, John Katsaras, Stephen Wassall, Thad Harroun There are no proven health benefits to supplementing with Vitamin E, so why do we require it for healthy living? The whole notion that vitamin E is an in-vivo antioxidant is now being seriously questioned. Using neutron diffraction and supporting techniques, we have correlated vitamin E's location in model membranes with its antioxidant activity. experiments were conducted using phosphatidylcholine (PC) bilayers whose fatty acid chains varied in their degree of unsaturation. We observe vitamin E up-right in all lipids examined, with its overall height in the bilayer lipid dependant. Interestingly we observe vitamin E's hydroxyl in the headgroup region of the bilayer for both the fully saturated and poly unsaturated lipids. Vitamin E was most effective at intercepting water borne oxidants than radical initiated within the bilayer core. However for lipids where vitamin E resides slightly lower (glycerol backbone) we observe comparable antioxidant activity against both water borne and hydrocarbon borne oxidants. Thus showing lipid species can modulate the location of vitamin E's activity. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N31.00003: Redistribution of Cholesterol in Model Lipid Membranes in Response to the Membrane-Active Peptide Alamethicin William Heller, Shuo Qian The cellular membrane is a heterogeneous, dynamic mixture of molecules and macromolecules that self-assemble into a tightly-regulated functional unit that provides a semipermeable barrier between the cell and its environment. Among the many compositional differences between mammalian and bacterial cell membranes that impact its physical properties, one key difference is cholesterol content, which is more prevalent in mammals. Cholesterol is an amphiphile that associates with membranes and serves to maintain its fluidity and permeability. Membrane-active peptides, such as the alpha-helical peptide alamethicin, interact with membranes in a concentration- and composition-dependent manner to form transmembrane pores that are responsible for the lytic action of the peptide. Through the use of small-angle neutron scattering and deuterium labeling, it was possible to observe a redistribution of the lipid and cholesterol in unilamellar vesicles in response to the presence of alamethicin at a peptide-to-lipid ratio of 1/200. The results demonstrate that the membrane remodeling powers of alamethicin reach beyond the membrane thinning effect to altering the localization of specific components in the bilayer, complementing the accepted two-state mechanism of pore formation. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N31.00004: Relationship between peptide membrane curvature generation and bactericidal activities Nathan Schmidt, Michelle Lee, David Kuo, Andre Ouellette, Gerard Wong Many amphipathic peptides and amphipathic domains in proteins can restructure biological membranes. Two examples are host defense antimicrobial peptides (AMPs) which disrupt and destabilize the cell membranes of microbes, and apolipoproteins which help stabilize nanoscale lipid aggregates. We use complementary x-ray and bacterial cell assays to elucidate the molecular length scale membrane deformations generated by amphipathic peptides with different structural motifs and relate these deformations to their activities on bacteria. Small angle x-ray scattering is used to study the interactions of model membranes with prototypical AMPs and consensus peptides from the amphipathic domains in apolipoproteins. By characterizing the nanoscale curvature deformations induced by these two distinct classes of membrane restructuring peptides we will discuss the role of amino acid composition on curvature generation. Bactericidal assays are used to access the in vivo activities of different amphipathic peptide motifs in order to understand the relationships between cell viability and membrane curvature generation. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N31.00005: Observing Stepwise Unzipping of Neuronal Snare Protein with Steered Molecular Dynamics Mustafa Tekpinar, Wenjun Zheng Soluble N-ethylmaleimide-sensitive factor Attachment Protein Receptors (SNARE) play a crucial role in membrane fusion. Neuronal SNAREs are made up of four helices: a snaptobrevin, a syntaxin 1, and two SNAP-25 helices. We applied constant velocity pulling forces to C terminal of snaptobrevin in SNARE complex to understand unzipping mechanism of neuronal SNAREs. We successfully observed unzipping of snaptobrevin from the other three helices in two steps: C terminal unzipping and N terminal unzipping. Our results have good agreement with recent optical tweezer experiments that observe this stepwise unzipping. Additionally, our simulations reveal that these two steps differ from each other. We believe that these different mechanisms can help us to understand SNARE mediated membrane fusion process better. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N31.00006: Information processing in the plasma membrane Benjamin Machta The plasma membrane is a 2D liquid where information from the world is received and processed. Motivated by the recent discovery that these membranes seem to be tuned close to a 2D liquid-liquid critical point, we set out to understand the different channels through which membrane bound proteins can communicate with each other. Diffusing proteins can carry out reactions when they come in contact with each other. Near criticality, proteins can also exert long-ranged critical Casimir forces on one another by coupling to the local composition order parameter. By modulating the growth and breakdown of the rigid cytoskeleton, they can direct forces on even more distant regions. In addition, proteins can control the release and production of second messengers that diffuse either through the bulk, or in the plane of the membrane itself. By making simple models for these processes we bound functional measures for them as communication channels. These include information theoretic measures of bandwidth, as well as physical measures of energetic efficiency and speed. Our results will likely prove useful in understanding functional reasons underlying the clustering and collective behavior often seen in experiments. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N31.00007: Yeast mitochondrial fission proteins induce antagonistic Gaussian membrane curvatures to regulate apoptosis Michelle Lee, Ghee Hwee Lai, Nathan Schmidt, Wujing Xian, Gerard C. L. Wong Mitochondria form a dynamic and interconnected network, which disintegrates during apoptosis to generate numerous smaller mitochondrial fragments. This process is at present not well understood. Yeast mitochondrial fission machinery proteins, Dnm1 and Fis1, are believed to regulate programmed cell death in yeast. Yeast Dnm1 has been previously shown to promote mitochondrial fragmentation and degradation characteristic of apoptotic cells, while yeast Fis1 inhibits cell death by limiting the mitochondrial fission induced by Dnm1 [Fannjiang et al, \textit{Genes {\&} Dev.} 2004. 18: 2785-2797]. To better understand the mechanisms of these antagonistic fission proteins, we use synchrotron small angle x-ray scattering (SAXS) to investigate their interaction with model cell membranes. The relationship between each protein, Dnm1 and Fis1, and protein-induced changes in membrane curvature and topology is examined. Through the comparison of the membrane rearrangement and phase behavior induced by each protein, we will discuss their respective roles in the regulation of mitochondrial fission. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N31.00008: On the modeling of endocytosis Tao Zhang, Rastko Sknepnek, Jennifer Schwarz, Mark Bowick Endocytosis is the primary mechanism by which extracellular material enters the cell. During endocytosis, the cell membrane deforms to surround the extracellular material and draw it into the cell, followed by a pinch-off to produce an internal vesicle. Recent experiments on clathrin-mediated endocytosis all agree that the actin cytoskeleton plays a crucial role in the deformation of the cell membrane. The actin cytoskeleton is a crosslinked network of filaments exerting active forces. However, competing ideas remain as to precisely how the actin cytoskeleton organizes itself to help drive the deformation. To begin to resolve this controversy, we mathematically model clathrin-mediated endocytosis using variational methods and Monte Carlo simulations. In particular, we investigate how the deformation of the cell membrane depends on the organization of the actin cytoskeletal network, and its associated active forces, to rule out one or more of the competing ideas. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N31.00009: Structural studies of lipid-protein interactions on cushioned bilayers S.K. Ghosh, M.K. Mukhopadhyay, Y. Ma, I. Lopez, S. Bera, L.B. Lurio, A. Chakrabarti, J.E. Kim, M.K. Sanyal, S.K. Sinha Biological membranes are heterogeneous and dynamical organizations of lipids and proteins, which perform functions fundamental to cell survival. Lipid-protein interactions control these functions by influencing folding and stability of integral or peripheral membrane proteins. Further, the incorporation or adsorption of these proteins into the membrane can in turn influence the lipid bilayer properties. In spite of some progress in understanding this process, a detailed structural analysis is lacking. Towards a better understanding of this interaction, we have performed an advanced interface sensitive scattering experiment using synchrotron x-rays. To accurately mimic the biological membranes with their natural thermal fluctuations and in-plane mobility of lipid molecules, polymer cushioned lipid bilayers have been used. This study shows that the adsorption of peripheral membrane protein\textit{spectrin}depends on the lipid headgroups, exhibiting different types of binding to phosphatidylcholine (PC) and phosphatidylethanolamie (PE). Further, the interaction of \textit{outer membrane protein A (OMP-A)}, an integral membrane protein is sensitive to the thermodynamic phase of the lipids. A detailed physical modeling of the lipid-protein interactions is under way. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N31.00010: Critical cell wall hole size for lysis in Gram-positive bacteria Gabriel Mitchell, Kurt Wiesenfeld, Daniel Nelson, Joshua Weitz Gram-positive bacteria transport molecules necessary for their survival through holes in their cell wall. The holes in cell walls need to be large enough to let critical nutrients pass through. However, the cell wall must also function to prevent the bacteria's membrane from protruding through a large hole into the environment and lysing the cell. As such, we hypothesize that there exists a range of cell wall hole sizes that allow for molecule transport but prevent membrane protrusion. Here we develop and analyze a biophysical theory of the response of a Gram-positive cell's membrane to the formation of a hole in the cell wall. We predict a critical hole size in the range 15-24nm beyond which lysis occurs. To test our theory, we measured hole sizes in \textit{Streptococcus pyogenes} cells undergoing enzymatic lysis via transmission electron microscopy. The measured hole sizes are in strong agreement with our theoretical prediction. Together, the theory and experiments provide a means to quantify the mechanisms of death of Gram-positive cells via enzymatically mediated lysis and provides insight into the range of cell wall hole sizes compatible with bacterial homeostasis. [Preview Abstract] |
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