101
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Abu-Arish A, Pandzic E, Goepp J, Matthes E, Hanrahan JW, Wiseman PW. Cholesterol modulates CFTR confinement in the plasma membrane of primary epithelial cells. Biophys J 2016; 109:85-94. [PMID: 26153705 DOI: 10.1016/j.bpj.2015.04.042] [Citation(s) in RCA: 45] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/09/2015] [Accepted: 04/23/2015] [Indexed: 01/01/2023] Open
Abstract
The cystic fibrosis transmembrane conductance regulator (CFTR) is a plasma-membrane anion channel that, when mutated, causes the disease cystic fibrosis. Although CFTR has been detected in a detergent-resistant membrane fraction prepared from airway epithelial cells, suggesting that it may partition into cholesterol-rich membrane microdomains (lipid rafts), its compartmentalization has not been demonstrated in intact cells and the influence of microdomains on CFTR lateral mobility is unknown. We used live-cell imaging, spatial image correlation spectroscopy, and k-space image correlation spectroscopy to examine the aggregation state of CFTR and its dynamics both within and outside microdomains in the plasma membrane of primary human bronchial epithelial cells. These studies were also performed during treatments that augment or deplete membrane cholesterol. We found two populations of CFTR molecules that were distinguishable based on their dynamics at the cell surface. One population showed confinement and had slow dynamics that were highly cholesterol dependent. The other, more abundant population was less confined and diffused more rapidly. Treatments that deplete the membrane of cholesterol caused the confined fraction and average number of CFTR molecules per cluster to decrease. Elevating cholesterol had the opposite effect, increasing channel aggregation and the fraction of channels displaying confinement, consistent with CFTR recruitment into cholesterol-rich microdomains with dimensions below the optical resolution limit. Viral infection caused the nanoscale microdomains to fuse into large platforms and reduced CFTR mobility. To our knowledge, these results provide the first biophysical evidence for multiple CFTR populations and have implications for regulation of their surface expression and channel function.
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Affiliation(s)
| | - Elvis Pandzic
- Physics, McGill University, Montreal, Quebec, Canada
| | - Julie Goepp
- Physiology, McGill University, Montreal, Quebec, Canada
| | | | | | - Paul W Wiseman
- Chemistry & Physics, McGill University, Montreal, Quebec, Canada.
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102
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SH2 Domains Serve as Lipid-Binding Modules for pTyr-Signaling Proteins. Mol Cell 2016; 62:7-20. [PMID: 27052731 DOI: 10.1016/j.molcel.2016.01.027] [Citation(s) in RCA: 58] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2015] [Revised: 12/21/2015] [Accepted: 01/26/2016] [Indexed: 01/26/2023]
Abstract
The Src-homology 2 (SH2) domain is a protein interaction domain that directs myriad phosphotyrosine (pY)-signaling pathways. Genome-wide screening of human SH2 domains reveals that ∼90% of SH2 domains bind plasma membrane lipids and many have high phosphoinositide specificity. They bind lipids using surface cationic patches separate from pY-binding pockets, thus binding lipids and the pY motif independently. The patches form grooves for specific lipid headgroup recognition or flat surfaces for non-specific membrane binding and both types of interaction are important for cellular function and regulation of SH2 domain-containing proteins. Cellular studies with ZAP70 showed that multiple lipids bind its C-terminal SH2 domain in a spatiotemporally specific manner and thereby exert exquisite spatiotemporal control over its protein binding and signaling activities in T cells. Collectively, this study reveals how lipids control SH2 domain-mediated cellular protein-protein interaction networks and suggest a new strategy for therapeutic modulation of pY-signaling pathways.
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103
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Modeling Smith-Lemli-Opitz syndrome with induced pluripotent stem cells reveals a causal role for Wnt/β-catenin defects in neuronal cholesterol synthesis phenotypes. Nat Med 2016; 22:388-96. [PMID: 26998835 PMCID: PMC4823163 DOI: 10.1038/nm.4067] [Citation(s) in RCA: 42] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/22/2014] [Accepted: 02/16/2016] [Indexed: 02/08/2023]
Abstract
Smith-Lemli-Opitz syndrome (SLOS) is a malformation disorder caused by mutations in DHCR7, impairing the reduction of 7-dehydrocholesterol to cholesterol. SLOS results in cognitive impairment, behavioral abnormalities, and nervous system defects, though neither cellular targets nor affected signaling pathways are defined. Whether 7-dehydrocholesterol accumulation or cholesterol loss is primarily responsible for disease pathogenesis is also unclear. Using induced pluripotent stem cells (iPSCs) from SLOS subjects, we identified cellular defects leading to precocious neuronal specification within SLOS derived neural progenitors. We also demonstrated that 7-dehydrocholesterol accumulation, not cholesterol deficiency, is critical for SLOS-associated defects. We further identified downregulation of Wnt/β-catenin signaling as a key initiator of aberrant SLOS iPSCs differentiation through the direct inhibitory effects of 7-dehydrocholesterol on the formation of an active Wnt receptor complex. Activation of canonical Wnt signaling prevented the neural phenotypes observed in SLOS iPSCs, suggesting that Wnt signaling may be a promising therapeutic target for SLOS.
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104
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Abstract
Membrane-protein interaction plays key roles in a wide variety of biological processes. To facilitate rapid and sensitive measurement of membrane binding of soluble proteins, we developed a fluorescence-based quantitative assay that is universally applicable to all proteins. This fluorescence-quenching assay employs fluorescence protein (FP)-tagged proteins whose fluorescence intensity is greatly decreased when they bind vesicles containing synthetic lipid dark quenchers, such as N-dimethylaminoazobenzenesulfonylphosphatidylethanolamine (dabsyl-PE). This simple assay can be performed with either a spectrofluorometer or a plate reader and optimized for different proteins with various combinations of FPs and quenching lipids. The assay allows rapid, sensitive, and accurate determination of lipid specificity and affinity for various lipid binding domains and proteins, and also high-throughput screening of small molecules that modulate membrane binding of proteins.
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Affiliation(s)
- Wonhwa Cho
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL, 60607, USA.
| | - Hyunjin Kim
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL, 60607, USA
| | - Yusi Hu
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL, 60607, USA
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105
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Sezgin E, Can FB, Schneider F, Clausen MP, Galiani S, Stanly TA, Waithe D, Colaco A, Honigmann A, Wüstner D, Platt F, Eggeling C. A comparative study on fluorescent cholesterol analogs as versatile cellular reporters. J Lipid Res 2015; 57:299-309. [PMID: 26701325 DOI: 10.1194/jlr.m065326] [Citation(s) in RCA: 62] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/16/2015] [Indexed: 12/12/2022] Open
Abstract
Cholesterol (Chol) is a crucial component of cellular membranes, but knowledge of its intracellular dynamics is scarce. Thus, it is of utmost interest to develop tools for visualization of Chol organization and dynamics in cells and tissues. For this purpose, many studies make use of fluorescently labeled Chol analogs. Unfortunately, the introduction of the label may influence the characteristics of the analog, such as its localization, interaction, and trafficking in cells; hence, it is important to get knowledge of such bias. In this report, we compared different fluorescent lipid analogs for their performance in cellular assays: 1) plasma membrane incorporation, specifically the preference for more ordered membrane environments in phase-separated giant unilamellar vesicles and giant plasma membrane vesicles; 2) cellular trafficking, specifically subcellular localization in Niemann-Pick type C disease cells; and 3) applicability in fluorescence correlation spectroscopy (FCS)-based and super-resolution stimulated emission depletion-FCS-based measurements of membrane diffusion dynamics. The analogs exhibited strong differences, with some indicating positive performance in the membrane-based experiments and others in the intracellular trafficking assay. However, none showed positive performance in all assays. Our results constitute a concise guide for the careful use of fluorescent Chol analogs in visualizing cellular Chol dynamics.
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Affiliation(s)
- Erdinc Sezgin
- MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, University of Oxford, OX39DS Oxford, United Kingdom
| | - Fatma Betul Can
- MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, University of Oxford, OX39DS Oxford, United Kingdom
| | - Falk Schneider
- MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, University of Oxford, OX39DS Oxford, United Kingdom
| | - Mathias P Clausen
- MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, University of Oxford, OX39DS Oxford, United Kingdom MEMPHYS-Center for Biomembrane Physics, Department of Physics, Chemistry, and Pharmacy, University of Southern Denmark, 5230 Odense M, Denmark
| | - Silvia Galiani
- MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, University of Oxford, OX39DS Oxford, United Kingdom
| | - Tess A Stanly
- MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, University of Oxford, OX39DS Oxford, United Kingdom
| | - Dominic Waithe
- MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, University of Oxford, OX39DS Oxford, United Kingdom
| | - Alexandria Colaco
- Department of Pharmacology, University of Oxford, OX13QT Oxford, United Kingdom
| | - Alf Honigmann
- Max Planck Institute of Cell Biology and Genetics, 01307 Dresden, Germany
| | - Daniel Wüstner
- Department of Biochemistry and Molecular Biology, University of Southern Denmark, 5230 Odense M, Denmark
| | - Frances Platt
- Department of Pharmacology, University of Oxford, OX13QT Oxford, United Kingdom
| | - Christian Eggeling
- MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, University of Oxford, OX39DS Oxford, United Kingdom
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106
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Abstract
The fundamental mechanisms of protein and lipid organization at the plasma membrane have continued to engage researchers for decades. Among proposed models, one idea has been particularly successful which assumes that sterol-dependent nanoscopic phases of different lipid chain order compartmentalize proteins, thereby modulating protein functionality. This model of membrane rafts has sustainably sparked the fields of membrane biophysics and biology, and shifted membrane lipids into the spotlight of research; by now, rafts have become an integral part of our terminology to describe a variety of cell biological processes. But is the evidence clear enough to continue supporting a theoretical concept which has resisted direct proof by observation for nearly twenty years? In this essay, we revisit findings that gave rise to and substantiated the raft hypothesis, discuss its impact on recent studies, and present alternative mechanisms to account for plasma membrane heterogeneity.
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Affiliation(s)
- Eva Sevcsik
- Institute of Applied Physics, Vienna University of Technology, Vienna, Austria
| | - Gerhard J Schütz
- Institute of Applied Physics, Vienna University of Technology, Vienna, Austria
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107
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Akt activation increases cellular cholesterol by promoting the proteasomal degradation of Niemann-Pick C1. Biochem J 2015; 471:243-53. [PMID: 26283546 DOI: 10.1042/bj20150602] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2015] [Accepted: 08/17/2015] [Indexed: 11/17/2022]
Abstract
Null mutations of the Niemann-Pick type C1 (NPC1) gene cause NPC disease, a lysosomal storage disorder characterized by cholesterol accumulation in late endosomes (LE) and lysosomes (Ly). Nascent or mutated NPC1 is degraded through the ubiquitin-proteasome pathway, but how NPC1 degradation is regulated remains currently unknown. In the present study, we demonstrated a link between NPC1 degradation and the Akt (protein kinase B)/mTOR [mammalian (or mechanistic) target of rapamycin] signalling pathway in cervical cancer cell lines. We provided evidence that activated Akt/mTOR pathway increased NPC1 degradation by ∼50% in C33A cells when compared with SiHa or HeLa cells. NPC1 degradation in C33A cells was reversed when Akt/mTOR activation was blocked by specific inhibitors or when mTORC1 (mTOR complex 1) was disrupted by regulatory associated protein of mTOR (Raptor) knockdown. Importantly, inhibition of the Akt/mTOR pathway led to decreased NPC1 ubiquitination in C33A cells, pointing to a role of Akt/mTOR in the proteasomal degradation of NPC1. Moreover, we found that NPC1 depletion in several cancer cell lines inhibited cell proliferation and migration. Our results uncover Akt as a key regulator of NPC1 degradation and link NPC1 to cancer cell proliferation and migration.
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108
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Du X, Brown AJ, Yang H. Novel mechanisms of intracellular cholesterol transport: oxysterol-binding proteins and membrane contact sites. Curr Opin Cell Biol 2015; 35:37-42. [DOI: 10.1016/j.ceb.2015.04.002] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2015] [Revised: 04/12/2015] [Accepted: 04/13/2015] [Indexed: 01/08/2023]
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109
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Listowski MA, Leluk J, Kraszewski S, Sikorski AF. Cholesterol Interaction with the MAGUK Protein Family Member, MPP1, via CRAC and CRAC-Like Motifs: An In Silico Docking Analysis. PLoS One 2015; 10:e0133141. [PMID: 26186446 PMCID: PMC4505867 DOI: 10.1371/journal.pone.0133141] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2014] [Accepted: 06/24/2015] [Indexed: 12/22/2022] Open
Abstract
Cholesterol is essential for the proper organization of the biological membrane. Therefore, predicting which proteins can bind cholesterol is important in understanding how proteins participate in lateral membrane organization. In this study, a simple bioinformatics approach was used to establish whether MPP1, a member of the MAGUK protein family, is capable of binding cholesterol. Modelled and experimentally-validated fragment structures were mined from online resources and searched for CRAC and CRAC-like motifs. Several of these motifs were found in the primary structure of MPP1, and these were structurally visualized to see whether they localized to the protein surface. Since all of the CRAC and CRAC-like motifs were found at the surface of MPP1 domains, in silico docking experiments were performed to assess the possibility of interaction between CRAC motifs and cholesterol. The results obtained show that MPP1 can bind cholesterol via CRAC and CRAC-like motifs with moderate to high affinity (KI in the nano- to micro-molar range). It was also found that palmitoylation-mimicking mutations (C/F or C/M) did not affect the affinity of MPP1 towards cholesterol. Data presented here may help to understand at least one of the molecular mechanisms via which MPP1 affects lateral organization of the membrane.
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Affiliation(s)
- Marcin A. Listowski
- Department of Cytobiochemistry, Faculty of Biotechnology, University of Wrocław, Wrocław, Poland
| | - Jacek Leluk
- Department of Molecular Biology, University of Zielona Góra, Zielona Góra, Poland
| | - Sebastian Kraszewski
- Department of Biomedical Engineering, Wrocław University of Technology, Wrocław, Poland
| | - Aleksander F. Sikorski
- Department of Cytobiochemistry, Faculty of Biotechnology, University of Wrocław, Wrocław, Poland
- Department of Molecular Biology, University of Zielona Góra, Zielona Góra, Poland
- * E-mail:
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110
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Tello-Lafoz M, Ghai R, Collins B, Mérida I. A role for novel lipid interactions in the dynamic recruitment of SNX27 to the T-cell immune synapse. BIOARCHITECTURE 2015; 4:215-20. [PMID: 25996807 DOI: 10.1080/19490992.2015.1031950] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
SNX27 is a member of the sorting nexin family that plays an important role in the recycling of receptors from endosomes to the cell surface. In addition to a PX (Phox homology) domain that regulates its endosomal localization, SNX27 has a unique PDZ (Psd-95/Dlg/ZO1) domain and an atypical FERM (4.1, ezrin, radixin, moesin) domain that both function to bind short peptide sequence motifs in the cytoplasmic domains of the cargo receptors. Using the T cell immune synapse (IS) as a model for polarized protein recycling, we recently identified an additional mechanism that enhances SNX27 localization to the endosomal recycling compartment (ERC). Our study defined a phosphoinositide (PI) lipid-binding site within the SNX27 FERM domain, with a clear preference for bi- and triphosphorylated PIs, which may promote SNX27 localization to phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) and/or PtdIns(3,4,5)P3-enriched membrane domains. Using fluorescently tagged lipid-binding probes, we studied the kinetics of distinct PIs in living T cells during IS formation. Our results suggest that PtdIns(3,4,5)P3 accumulates at the contact site simultaneously with early SNX27 recruitment to the plasma membrane (PM), and this is partly controlled by by lipid binding through the FERM domain. These studies define 2 independent binding sites for PtdIns-derived lipids in SNX27, that contribute to the dynamic recruitment of SNX27 to distinct membranes during T cell activation.
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Affiliation(s)
- María Tello-Lafoz
- a Lipid Signaling Laboratory ; Centro Nacional de Biotecnología (CNB)/CSIC ; Madrid , Spain
| | - Rajesh Ghai
- b Institute for Molecular Bioscience ; The University of Queensland ; St. Lucia , Australia.,c Current address: The School of Biotechnology and Biomolecular Sciences , The University of New South Wales , Sydney , NSW , Australia
| | - Brett Collins
- b Institute for Molecular Bioscience ; The University of Queensland ; St. Lucia , Australia
| | - Isabel Mérida
- a Lipid Signaling Laboratory ; Centro Nacional de Biotecnología (CNB)/CSIC ; Madrid , Spain
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111
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Sevcsik E, Brameshuber M, Fölser M, Weghuber J, Honigmann A, Schütz GJ. GPI-anchored proteins do not reside in ordered domains in the live cell plasma membrane. Nat Commun 2015; 6:6969. [PMID: 25897971 PMCID: PMC4430820 DOI: 10.1038/ncomms7969] [Citation(s) in RCA: 91] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2014] [Accepted: 03/18/2015] [Indexed: 02/07/2023] Open
Abstract
The organization of proteins and lipids in the plasma membrane has been the subject of a long-lasting debate. Membrane rafts of higher lipid chain order were proposed to mediate protein interactions, but have thus far not been directly observed. Here we use protein micropatterning combined with single-molecule tracking to put current models to the test: we rearranged lipid-anchored raft proteins (glycosylphosphatidylinositol(GPI)-anchored-mGFP) directly in the live cell plasma membrane and measured the effect on the local membrane environment. Intriguingly, this treatment does neither nucleate the formation of an ordered membrane phase nor result in any enrichment of nanoscopic-ordered domains within the micropatterned regions. In contrast, we find that immobilized mGFP-GPIs behave as inert obstacles to the diffusion of other membrane constituents without influencing their membrane environment over distances beyond their physical size. Our results indicate that phase partitioning is not a fundamental element of protein organization in the plasma membrane.
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Affiliation(s)
- Eva Sevcsik
- Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10, Vienna 1040, Austria
| | - Mario Brameshuber
- Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10, Vienna 1040, Austria
| | - Martin Fölser
- Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10, Vienna 1040, Austria
| | - Julian Weghuber
- School of Engineering and Environmental Sciences, University of Applied Sciences Upper Austria, Stelzhamerstrasse 23, Wels 4600, Austria
| | - Alf Honigmann
- Department of NanoBiophotonics, Max-Planck-Institute for Biophysical Chemistry, Am Fassberg 11, Göttingen 37077, Germany
| | - Gerhard J Schütz
- Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10, Vienna 1040, Austria
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112
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Grouleff J, Irudayam SJ, Skeby KK, Schiøtt B. The influence of cholesterol on membrane protein structure, function, and dynamics studied by molecular dynamics simulations. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2015; 1848:1783-95. [PMID: 25839353 DOI: 10.1016/j.bbamem.2015.03.029] [Citation(s) in RCA: 123] [Impact Index Per Article: 13.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/19/2014] [Revised: 03/24/2015] [Accepted: 03/25/2015] [Indexed: 12/12/2022]
Abstract
The plasma membrane, which encapsulates human cells, is composed of a complex mixture of lipids and embedded proteins. Emerging knowledge points towards the lipids as having a regulating role in protein function. Furthermore, insight from protein crystallography has revealed several different types of lipids intimately bound to membrane proteins and peptides, hereby possibly pointing to a site of action for the observed regulation. Cholesterol is among the lipid membrane constituents most often observed to be co-crystallized with membrane proteins, and the cholesterol levels in cell membranes have been found to play an essential role in health and disease. Remarkably little is known about the mechanism of lipid regulation of membrane protein function in health as well as in disease. Herein, we review molecular dynamics simulation studies aimed at investigating the effect of cholesterol on membrane protein and peptide properties. This article is part of a Special Issue entitled: Lipid-protein interactions.
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Affiliation(s)
- Julie Grouleff
- Department of Chemistry and Interdisciplinary Nanoscience Center, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark
| | - Sheeba Jem Irudayam
- Department of Chemistry and Interdisciplinary Nanoscience Center, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark
| | - Katrine K Skeby
- Department of Chemistry and Interdisciplinary Nanoscience Center, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark
| | - Birgit Schiøtt
- Department of Chemistry and Interdisciplinary Nanoscience Center, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark.
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113
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Maekawa M, Fairn GD. Complementary probes reveal that phosphatidylserine is required for the proper transbilayer distribution of cholesterol. J Cell Sci 2015; 128:1422-33. [PMID: 25663704 DOI: 10.1242/jcs.164715] [Citation(s) in RCA: 163] [Impact Index Per Article: 18.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Cholesterol is an essential component of metazoan cellular membranes and it helps to maintain the structural integrity and fluidity of the plasma membrane. Here, we developed a cholesterol biosensor, termed D4H, based on the fourth domain of Clostridium perfringens theta-toxin, which recognizes cholesterol in the cytosolic leaflet of the plasma membrane and organelles. The D4H probe disassociates from the plasma membrane upon cholesterol extraction and after perturbations in cellular cholesterol trafficking. When used in combination with a recombinant version of the biosensor, we show that plasmalemmal phosphatidylserine is essential for retaining cholesterol in the cytosolic leaflet of the plasma membrane. In vitro experiments reveal that 1-stearoy-2-oleoyl phosphatidylserine can induce phase separation in cholesterol-containing lipid bilayers and shield cholesterol from cholesterol oxidase. Finally, the altered transbilayer distribution of cholesterol causes flotillin-1 to relocalize to endocytic organelles. This probe should be useful in the future to study pools of cholesterol in the cytosolic leaflet of the plasma membrane and organelles.
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Affiliation(s)
- Masashi Maekawa
- Keenan Research Centre for Biomedical Science, St. Michael's Hospital, 209 Victoria Street, Toronto, ON, M5S 1T8, Canada
| | - Gregory D Fairn
- Keenan Research Centre for Biomedical Science, St. Michael's Hospital, 209 Victoria Street, Toronto, ON, M5S 1T8, Canada Department of Biochemistry, University of Toronto, Toronto, ON M5S 1A8, Canada Department of Surgery, University of Toronto, Toronto, ON M5T 1P5, Canada Institute of Medical Science, Faculty of Medicine, University of Toronto, Toronto, ON M5S 1A8, Canada Institute for Biomedical Engineering and Science Technology (iBEST) at Ryerson University and St. Michael's Hospital, Toronto, ON M5B 2K3, Canada
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114
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Lee HJ, Zhang W, Zhang D, Yang Y, Liu B, Barker EL, Buhman KK, Slipchenko LV, Dai M, Cheng JX. Assessing cholesterol storage in live cells and C. elegans by stimulated Raman scattering imaging of phenyl-Diyne cholesterol. Sci Rep 2015; 5:7930. [PMID: 25608867 PMCID: PMC4302291 DOI: 10.1038/srep07930] [Citation(s) in RCA: 94] [Impact Index Per Article: 10.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2014] [Accepted: 12/22/2014] [Indexed: 02/07/2023] Open
Abstract
We report a cholesterol imaging method using rationally synthesized phenyl-diyne cholesterol (PhDY-Chol) and stimulated Raman scattering (SRS) microscope. The phenyl-diyne group is biologically inert and provides a Raman scattering cross section that is 88 times larger than the endogenous C = O stretching mode. SRS microscopy offers an imaging speed that is faster than spontaneous Raman microscopy by three orders of magnitude, and a detection sensitivity of 31 μM PhDY-Chol (~1,800 molecules in the excitation volume). Inside living CHO cells, PhDY-Chol mimics the behavior of cholesterol, including membrane incorporation and esterification. In a cellular model of Niemann-Pick type C disease, PhDY-Chol reflects the lysosomal accumulation of cholesterol, and shows relocation to lipid droplets after HPβCD treatment. In live C. elegans, PhDY-Chol mimics cholesterol uptake by intestinal cells and reflects cholesterol storage. Together, our work demonstrates an enabling platform for study of cholesterol storage and trafficking in living cells and vital organisms.
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Affiliation(s)
- Hyeon Jeong Lee
- Interdisciplinary Life Science Program, Purdue University, West Lafayette, IN, USA
- Department of Comparative Pathobiology, Purdue University, West Lafayette, IN, USA
| | - Wandi Zhang
- Department of Chemistry, Purdue University, West Lafayette, IN, USA
| | - Delong Zhang
- Department of Chemistry, Purdue University, West Lafayette, IN, USA
| | - Yang Yang
- Department of Chemistry, Purdue University, West Lafayette, IN, USA
| | - Bin Liu
- National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China
| | - Eric L. Barker
- Interdisciplinary Life Science Program, Purdue University, West Lafayette, IN, USA
- Medicinal Chemistry and Molecular Pharmacology, Purdue University, West Lafayette, IN, USA
| | - Kimberly K. Buhman
- Interdisciplinary Life Science Program, Purdue University, West Lafayette, IN, USA
- Department of Nutrition Science, Purdue University, West Lafayette, IN, USA
| | | | - Mingji Dai
- Interdisciplinary Life Science Program, Purdue University, West Lafayette, IN, USA
- Department of Chemistry, Purdue University, West Lafayette, IN, USA
| | - Ji-Xin Cheng
- Interdisciplinary Life Science Program, Purdue University, West Lafayette, IN, USA
- Department of Chemistry, Purdue University, West Lafayette, IN, USA
- Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN, USA
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115
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Kim DH, Park MJ, Gwon GH, Silkov A, Xu ZY, Yang EC, Song S, Song K, Kim Y, Yoon HS, Honig B, Cho W, Cho Y, Hwang I. An ankyrin repeat domain of AKR2 drives chloroplast targeting through coincident binding of two chloroplast lipids. Dev Cell 2014; 30:598-609. [PMID: 25203210 DOI: 10.1016/j.devcel.2014.07.026] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2013] [Revised: 05/22/2014] [Accepted: 07/31/2014] [Indexed: 10/24/2022]
Abstract
In organellogenesis of the chloroplast from endosymbiotic cyanobacteria, the establishment of protein-targeting mechanisms to the chloroplast should have been pivotal. However, it is still mysterious how these mechanisms were established and how they work in plant cells. Here we show that AKR2A, the cytosolic targeting factor for chloroplast outer membrane (COM) proteins, evolved from the ankyrin repeat domain (ARD) of the host cell by stepwise extensions of its N-terminal domain and that two lipids, monogalactosyldiacylglycerol (MGDG) and phosphatidylglycerol (PG), of the endosymbiont were selected to function as the AKR2A receptor. Structural analysis, molecular modeling, and mutational analysis of the ARD identified two adjacent sites for coincidental and synergistic binding of MGDG and PG. Based on these findings, we propose that the targeting mechanism of COM proteins was established using components from both the endosymbiont and host cell through a modification of the protein-protein-interacting ARD into a lipid binding domain.
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Affiliation(s)
- Dae Heon Kim
- Division of Molecular and Life Sciences, Pohang University of Science and Technology, Pohang 790-784, Korea
| | - Mi-Jeong Park
- Division of Integrative Biosciences and Biotechnology, Pohang University of Science and Technology, Pohang 790-784, Korea
| | - Gwang Hyeon Gwon
- Division of Molecular and Life Sciences, Pohang University of Science and Technology, Pohang 790-784, Korea
| | - Antonina Silkov
- Department of Biochemistry and Molecular Biophysics, Howard Hughes Medical Institute, Columbia University, New York, NY 11032, USA
| | - Zheng-Yi Xu
- Division of Molecular and Life Sciences, Pohang University of Science and Technology, Pohang 790-784, Korea
| | - Eun Chan Yang
- Department of Biological Sciences, Sungkyunkwan University, Suwon 440-746, Korea
| | - Seohyeon Song
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, USA
| | - Kyungyoung Song
- Division of Molecular and Life Sciences, Pohang University of Science and Technology, Pohang 790-784, Korea
| | - Younghyun Kim
- Division of Molecular and Life Sciences, Pohang University of Science and Technology, Pohang 790-784, Korea
| | - Hwan Su Yoon
- Department of Biological Sciences, Sungkyunkwan University, Suwon 440-746, Korea
| | - Barry Honig
- Department of Biochemistry and Molecular Biophysics, Howard Hughes Medical Institute, Columbia University, New York, NY 11032, USA
| | - Wonhwa Cho
- Division of Integrative Biosciences and Biotechnology, Pohang University of Science and Technology, Pohang 790-784, Korea; Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, USA.
| | - Yunje Cho
- Division of Molecular and Life Sciences, Pohang University of Science and Technology, Pohang 790-784, Korea.
| | - Inhwan Hwang
- Division of Molecular and Life Sciences, Pohang University of Science and Technology, Pohang 790-784, Korea; Division of Integrative Biosciences and Biotechnology, Pohang University of Science and Technology, Pohang 790-784, Korea.
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116
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Abstract
“Lipid raft” is the name given to the tiny, dynamic, and ordered domains of cholesterol and sphingolipids that are hypothesized to exist in the plasma membranes of eukaryotic cells. According to the lipid raft hypothesis, these cholesterol- and sphingolipid-enriched domains modulate the protein–protein interactions that are essential for cellular function. Indeed, many studies have shown that cellular levels of cholesterol and sphingolipids influence plasma membrane organization, cell signaling, and other important biological processes. Despite 15 years of research and the application of highly advanced imaging techniques, data that unambiguously demonstrate the existence of lipid rafts in mammalian cells are still lacking. This Perspective summarizes the results that challenge the lipid raft hypothesis and discusses alternative hypothetical models of plasma membrane organization and lipid-mediated cellular function.
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Affiliation(s)
- Mary L Kraft
- Department of Chemical and Biomolecular Engineering, University of Illinois, Urbana, IL 61801
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117
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Live cell imaging of membrane/cytoskeleton interactions and membrane topology. Sci Rep 2014; 4:6056. [PMID: 25205456 PMCID: PMC4159633 DOI: 10.1038/srep06056] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2014] [Accepted: 06/20/2014] [Indexed: 01/25/2023] Open
Abstract
We elucidate the interaction between actin and specific membrane components, using real time live cell imaging, by delivering probes that enable access to components, that cannot be accessed genetically. We initially investigated the close interplay between Phosphatidylinositol 4,5-bisphosphate (PIP2) and the F-actin network. We show that, during the early stage of cell adhesion, PIP2 forms domains within the filopodia membrane. We studied these domains alongside cell spreading and observed that these very closely follow the actin tread-milling. We show that this mechanism is associated with an active transport of PIP2 rich organelles from the cell perinuclear area to the edge, along actin fibers. Finally, mapping other phospholipids and membrane components we observed that the PIP2 domains formation is correlated with sphingosine and cholesterol rafts.
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118
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Saka SK, Honigmann A, Eggeling C, Hell SW, Lang T, Rizzoli SO. Multi-protein assemblies underlie the mesoscale organization of the plasma membrane. Nat Commun 2014; 5:4509. [PMID: 25060237 PMCID: PMC4124874 DOI: 10.1038/ncomms5509] [Citation(s) in RCA: 133] [Impact Index Per Article: 13.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/05/2014] [Accepted: 06/25/2014] [Indexed: 02/07/2023] Open
Abstract
Most proteins have uneven distributions in the plasma membrane. Broadly speaking, this may be caused by mechanisms specific to each protein, or may be a consequence of a general pattern that affects the distribution of all membrane proteins. The latter hypothesis has been difficult to test in the past. Here, we introduce several approaches based on click chemistry, through which we study the distribution of membrane proteins in living cells, as well as in membrane sheets. We found that the plasma membrane proteins form multi-protein assemblies that are long lived (minutes), and in which protein diffusion is restricted. The formation of the assemblies is dependent on cholesterol. They are separated and anchored by the actin cytoskeleton. Specific proteins are preferentially located in different regions of the assemblies, from their cores to their edges. We conclude that the assemblies constitute a basic mesoscale feature of the membrane, which affects the patterning of most membrane proteins, and possibly also their activity. Although many proteins adopt uneven distributions in the plasma membrane, it is not clear how these nanoscale heterogeneities relate to the general protein patterning of the membrane. Saka et al. use click chemistry to reveal the mesoscale organization of membrane proteins into multi-protein assemblies.
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Affiliation(s)
- Sinem K Saka
- 1] Department of Neuro- and Sensory Physiology, University of Göttingen Medical Centre, and Centre for Nanoscale Microscopy and Molecular Physiology of the Brain (CNMPB), 37075 Göttingen, Germany [2] International Max Planck Research School Molecular Biology, 37077 Göttingen, Germany
| | - Alf Honigmann
- Department of Nanobiophotonics, Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany
| | - Christian Eggeling
- MRC Human Immunology Unit, Weatherall Institute of Molecular Medicine, Oxford OX3 9DS, UK
| | - Stefan W Hell
- Department of Nanobiophotonics, Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany
| | - Thorsten Lang
- Department of Membrane Biochemistry, Life and Medical Sciences (LIMES) Institute, University of Bonn, 53115 Bonn, Germany
| | - Silvio O Rizzoli
- Department of Neuro- and Sensory Physiology, University of Göttingen Medical Centre, and Centre for Nanoscale Microscopy and Molecular Physiology of the Brain (CNMPB), 37075 Göttingen, Germany
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119
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Sheng R, Kim H, Lee H, Xin Y, Chen Y, Tian W, Cui Y, Choi JC, Doh J, Han JK, Cho W. Cholesterol selectively activates canonical Wnt signalling over non-canonical Wnt signalling. Nat Commun 2014; 5:4393. [PMID: 25024088 PMCID: PMC4100210 DOI: 10.1038/ncomms5393] [Citation(s) in RCA: 94] [Impact Index Per Article: 9.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2014] [Accepted: 06/13/2014] [Indexed: 12/19/2022] Open
Abstract
Wnt proteins control diverse biological processes through β-catenin-dependent canonical signalling and β-catenin-independent non-canonical signalling. The mechanisms by which these signalling pathways are differentially triggered and controlled are not fully understood. Dishevelled (Dvl) is a scaffold protein that serves as the branch point of these pathways. Here, we show that cholesterol selectively activates canonical Wnt signalling over non-canonical signalling under physiological conditions by specifically facilitating the membrane recruitment of the PDZ domain of Dvl and its interaction with other proteins. Single-molecule imaging analysis shows that cholesterol is enriched around the Wnt-activated Frizzled and low-density lipoprotein receptor-related protein 5/6 receptors and plays an essential role for Dvl-mediated formation and maintenance of the canonical Wnt signalling complex. Collectively, our results suggest a new regulatory role of cholesterol in Wnt signalling and a potential link between cellular cholesterol levels and the balance between canonical and non-canonical Wnt signalling activities.
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Affiliation(s)
- Ren Sheng
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, USA
| | | | | | - Yao Xin
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, USA
| | - Yong Chen
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, USA
| | - Wen Tian
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, USA
| | - Yang Cui
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, USA
| | - Jong-Cheol Choi
- Mechanical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea
| | - Junsang Doh
- Mechanical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea
| | | | - Wonhwa Cho
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, USA
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120
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Elucidation of different inhibition mechanism of small chemicals on PtdInsP-binding domains using in silico docking experiments. Bioorg Med Chem Lett 2014; 24:2256-62. [PMID: 24736117 DOI: 10.1016/j.bmcl.2014.03.085] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2014] [Revised: 03/24/2014] [Accepted: 03/26/2014] [Indexed: 10/25/2022]
Abstract
Phosphatidylinositides, most negatively charged lipids in cellular membranes, regulate diverse effector proteins through the interaction with their lipid binding domains. We have previously reported inhibitory effect of small chemicals on the interaction between PtdIns(3,4,5)P3 and Btk PH domain. Here, we report that the inhibitory effects of same sets of chemicals on Grp1 PH domain and epsin1 ENTH domain to elucidate diversity of inhibitory mechanisms upon different lipid binding domains. Among the chemicals, chemical 8 showed best inhibition in vitro assay for Grp1 PH domain and epsin1 ENTH domain, and then the interaction between small chemicals and lipid binding domains was further investigated by in silico docking experiments. As a result, it was concluded that the diverse inhibitory effects on different lipid binding domains were dependent on not only the number of interactions between small chemical and domain, but also additional interaction with positively charged surfaces as the secondary binding sites. This finding will help to develop lipid binding inhibitors as antagonists for lipid-protein interactions, and these inhibitors would be novel therapeutic drug candidates via regulating effector proteins involved in severe human diseases.
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121
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Lu G, Wu Y, Jiang Y, Wang S, Hou Y, Guan X, Brunzelle J, Sirinupong N, Sheng S, Li C, Yang Z. Structural insights into neutrophilic migration revealed by the crystal structure of the chemokine receptor CXCR2 in complex with the first PDZ domain of NHERF1. PLoS One 2013; 8:e76219. [PMID: 24098448 PMCID: PMC3788737 DOI: 10.1371/journal.pone.0076219] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/29/2013] [Accepted: 08/21/2013] [Indexed: 11/17/2022] Open
Abstract
Neutrophil plays an essential role in host defense against infection, but uncontrolled neutrophilic infiltration can cause inflammation and severe epithelial damage. We recently showed that CXCR2 formed a signaling complex with NHERF1 and PLC-2, and that the formation of this complex was required for intracellular calcium mobilization and neutrophilic transepithelial migration. To uncover the structural basis of the complex formation, we report here the crystal structure of the NHERF1 PDZ1 domain in complex with the C-terminal sequence of CXCR2 at 1.16 Å resolution. The structure reveals that the CXCR2 peptide binds to PDZ1 in an extended conformation with the last four residues making specific side chain interactions. Remarkably, comparison of the structure to previously studied PDZ1 domains has allowed the identification of PDZ1 ligand-specific interactions and the mechanisms that govern PDZ1 target selection diversities. In addition, we show that CXCR2 can bind both NHERF1 PDZ1 and PDZ2 in pulldown experiments, consistent with the observation that the peptide binding pockets of these two PDZ domains are highly structurally conserved. The results of this study therefore provide structural basis for the CXCR2-mediated neutrophilic migration and could have important clinical applications in the prevention and treatment of numerous neutrophil-dependent inflammatory disorders.
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Affiliation(s)
- Guorong Lu
- Department of Biochemistry and Molecular Biology, Wayne State University School of Medicine, Detroit, Michigan, United States of America
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122
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Kim H, Afsari HS, Cho W. High-throughput fluorescence assay for membrane-protein interaction. J Lipid Res 2013; 54:3531-8. [PMID: 24006510 DOI: 10.1194/jlr.d041376] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Membrane-protein interaction plays key roles in a wide variety of biological processes. Although various methods have been employed to measure membrane binding of soluble proteins, a robust high-throughput assay that is universally applicable to all proteins is lacking at present. Here we report a new fluorescence quenching assay utilizing enhanced green fluorescence protein (EGFP)-fusion proteins and a lipid containing a dark quencher, N-dimethylaminoazobenzenesulfonyl-phosphatidylethanolamine (dabsyl-PE). The EGFP fluorescence emission intensity showed a large decrease (i.e., >50%) when EGFP-fusion proteins bound the vesicles containing 5 mol% dabsyl-PE. This simple assay, which can be performed using either a cuvette-based spectrofluorometer or a fluorescence plate reader, allowed rapid, sensitive, and accurate determination of lipid specificity and affinity for various lipid binding domains, including two pleckstrin homology domains, an epsin N-terminal homology domain, and a phox homology domain. The assay can also be applied to high-throughput screening of small molecules that modulate membrane binding of proteins.
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Affiliation(s)
- Hyunjin Kim
- Department of Chemistry, University of Illinois at Chicago, Chicago, IL 60607
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123
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Neumeier M, Krautbauer S, Schmidhofer S, Hader Y, Eisinger K, Eggenhofer E, Froehner SC, Adams ME, Mages W, Buechler C. Adiponectin receptor 1 C-terminus interacts with PDZ-domain proteins such as syntrophins. Exp Mol Pathol 2013; 95:180-6. [PMID: 23860432 DOI: 10.1016/j.yexmp.2013.07.002] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2013] [Accepted: 07/02/2013] [Indexed: 01/22/2023]
Abstract
Adiponectin receptor 1 (AdipoR1) is one of the two signaling receptors of adiponectin with multiple beneficial effects in metabolic diseases. AdipoR1 C-terminal peptide is concordant with the consensus sequence of class I PSD-95, disc large, ZO-1 (PDZ) proteins, and screening of a liver yeast two hybrid library identified binding to β2-syntrophin (SNTB2). Hybridization of a PDZ-domain array with AdipoR1 C-terminal peptide shows association with PDZ-domains of further proteins including β1- and α-syntrophin (SNTA). Interaction of PDZ proteins and C-terminal peptides requires a free carboxy terminus next to the PDZ-binding region and is blocked by carboxy terminal added tags. N-terminal tagged AdipoR1 is more highly expressed than C-terminal tagged receptor suggesting that the free carboxy terminus may form a complex with PDZ proteins to regulate cellular AdipoR1 levels. The C- and N-terminal tagged AdipoR1 proteins are mainly localized in the cytoplasma. N-terminal but not C-terminal tagged AdipoR1 colocalizes with syntrophins in adiponectin incubated Huh7 cells. Adiponectin induced hepatic phosphorylation of AMPK and p38 MAPK which are targets of AdipoR1 is, however, not blocked in SNTA and SNTB2 deficient mice. Further, AdipoR1 protein is similarly abundant in the liver of knock-out and wild type mice when kept on a standard chow or a high fat diet. In summary these data suggest that AdipoR1 protein levels are regulated by so far uncharacterized class I PDZ proteins which are distinct from SNTA and SNTB2.
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Affiliation(s)
- Markus Neumeier
- Department of Internal Medicine I, University of Regensburg, Regensburg, Germany
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124
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Frisz JF, Klitzing HA, Lou K, Hutcheon ID, Weber PK, Zimmerberg J, Kraft ML. Sphingolipid domains in the plasma membranes of fibroblasts are not enriched with cholesterol. J Biol Chem 2013; 288:16855-16861. [PMID: 23609440 PMCID: PMC3675618 DOI: 10.1074/jbc.m113.473207] [Citation(s) in RCA: 121] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
The plasma membranes of mammalian cells are widely expected to contain domains that are enriched with cholesterol and sphingolipids. In this work, we have used high-resolution secondary ion mass spectrometry to directly map the distributions of isotope-labeled cholesterol and sphingolipids in the plasma membranes of intact fibroblast cells. Although acute cholesterol depletion reduced sphingolipid domain abundance, cholesterol was evenly distributed throughout the plasma membrane and was not enriched within the sphingolipid domains. Thus, we rule out favorable cholesterol-sphingolipid interactions as dictating plasma membrane organization in fibroblast cells. Because the sphingolipid domains are disrupted by drugs that depolymerize the cells actin cytoskeleton, cholesterol must instead affect the sphingolipid organization via an indirect mechanism that involves the cytoskeleton.
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Affiliation(s)
| | | | - Kaiyan Lou
- Chemical and Biomolecular Engineering, University of Illinois, Urbana, Illinois 61801
| | - Ian D Hutcheon
- Glenn T. Seaborg Institute, Lawrence Livermore National Laboratory, Livermore, California 94551
| | - Peter K Weber
- Glenn T. Seaborg Institute, Lawrence Livermore National Laboratory, Livermore, California 94551
| | - Joshua Zimmerberg
- Program in Physical Biology, Eunice Kennedy Shriver NICHD, National Institutes of Health, Bethesda, Maryland 20892
| | - Mary L Kraft
- Departments of Chemistry, Urbana, Illinois 61801; Chemical and Biomolecular Engineering, University of Illinois, Urbana, Illinois 61801.
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