1
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Reddy KD, Ciftci D, Scopelliti AJ, Boudker O. The archaeal glutamate transporter homologue GltPh shows heterogeneous substrate binding. J Gen Physiol 2022; 154:e202213131. [PMID: 35452090 PMCID: PMC9044058 DOI: 10.1085/jgp.202213131] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2022] [Accepted: 03/10/2022] [Indexed: 12/31/2022] Open
Abstract
Integral membrane glutamate transporters couple the concentrative substrate transport to ion gradients. There is a wealth of structural and mechanistic information about this protein family. Recent studies of an archaeal homologue, GltPh, revealed transport rate heterogeneity, which is inconsistent with simple kinetic models; however, its structural and mechanistic determinants remain undefined. Here, we demonstrate that in a mutant GltPh, which exclusively populates the outward-facing state, at least two substates coexist in slow equilibrium, binding the substrate with different apparent affinities. Wild type GltPh shows similar binding properties, and modulation of the substate equilibrium correlates with transport rates. The low-affinity substate of the mutant is transient following substrate binding. Consistently, cryo-EM on samples frozen within seconds after substrate addition reveals the presence of structural classes with perturbed helical packing of the extracellular half of the transport domain in regions adjacent to the binding site. By contrast, an equilibrated structure does not show such classes. The structure at 2.2-Å resolution details a pattern of waters in the intracellular half of the domain and resolves classes with subtle differences in the substrate-binding site. We hypothesize that the rigid cytoplasmic half of the domain mediates substrate and ion recognition and coupling, whereas the extracellular labile half sets the affinity and dynamic properties.
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Affiliation(s)
- Krishna D. Reddy
- Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY
| | - Didar Ciftci
- Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY
- Tri-Institutional Training Program in Chemical Biology, New York, NY
| | | | - Olga Boudker
- Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY
- Howard Hughes Medical Institute, Weill Cornell Medicine, New York, NY
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2
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Bruguera ES, Mahoney JP, Weis WI. Reconstitution of purified membrane protein dimers in lipid nanodiscs with defined stoichiometry and orientation using a split GFP tether. J Biol Chem 2022; 298:101628. [PMID: 35074428 PMCID: PMC8980801 DOI: 10.1016/j.jbc.2022.101628] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2021] [Revised: 01/11/2022] [Accepted: 01/12/2022] [Indexed: 12/12/2022] Open
Abstract
Many membrane proteins function as dimers or larger oligomers, including transporters, channels, certain signaling receptors, and adhesion molecules. In some cases, the interactions between individual proteins may be weak and/or dependent on specific lipids, such that detergent solubilization used for biochemical and structural studies disrupts functional oligomerization. Solubilized membrane protein oligomers can be captured in lipid nanodiscs, but this is an inefficient process that can produce stoichiometrically and topologically heterogeneous preparations. Here, we describe a technique to obtain purified homogeneous membrane protein dimers in nanodiscs using a split GFP (sGFP) tether. Complementary sGFP tags associate to tether the coexpressed dimers and control both stoichiometry and orientation within the nanodiscs, as assessed by quantitative Western blotting and negative-stain EM. The sGFP tether confers several advantages over other methods: it is highly stable in solution and in SDS-PAGE, which facilitates screening of dimer expression and purification by fluorescence, and also provides a dimer-specific purification handle for use with GFP nanobody–conjugated resin. We used this method to purify a Frizzled-4 homodimer and a Frizzled-4/low-density lipoprotein receptor–related protein 6 heterodimer in nanodiscs. These examples demonstrate the utility and flexibility of this method, which enables subsequent mechanistic molecular and structural studies of membrane protein pairs.
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Affiliation(s)
- Elise S Bruguera
- Departments of Molecular & Cellular Physiology and Structural Biology, Stanford University School of Medicine; Stanford, CA 94305
| | - Jacob P Mahoney
- Departments of Molecular & Cellular Physiology and Structural Biology, Stanford University School of Medicine; Stanford, CA 94305
| | - William I Weis
- Departments of Molecular & Cellular Physiology and Structural Biology, Stanford University School of Medicine; Stanford, CA 94305.
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3
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Ciftci D, Martens C, Ghani VG, Blanchard SC, Politis A, Huysmans GHM, Boudker O. Linking function to global and local dynamics in an elevator-type transporter. Proc Natl Acad Sci U S A 2021; 118:e2025520118. [PMID: 34873050 PMCID: PMC8670510 DOI: 10.1073/pnas.2025520118] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 09/21/2021] [Indexed: 11/24/2022] Open
Abstract
Transporters cycle through large structural changes to translocate molecules across biological membranes. The temporal relationships between these changes and function, and the molecular properties setting their rates, determine transport efficiency-yet remain mostly unknown. Using single-molecule fluorescence microscopy, we compare the timing of conformational transitions and substrate uptake in the elevator-type transporter GltPh We show that the elevator-like movements of the substrate-loaded transport domain across membranes and substrate release are kinetically heterogeneous, with rates varying by orders of magnitude between individual molecules. Mutations increasing the frequency of elevator transitions and reducing substrate affinity diminish transport rate heterogeneities and boost transport efficiency. Hydrogen deuterium exchange coupled to mass spectrometry reveals destabilization of secondary structure around the substrate-binding site, suggesting that increased local dynamics leads to faster rates of global conformational changes and confers gain-of-function properties that set transport rates.
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Affiliation(s)
- Didar Ciftci
- Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY 10065
- Tri-Institutional Training Program in Chemical Biology, New York, NY 10065
| | - Chloe Martens
- Department of Chemistry, King's College London, London SE1 1DB, United Kingdom
| | - Vishnu G Ghani
- Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY 10065
| | - Scott C Blanchard
- Department of Structural Biology, St. Jude Children's Research Hospital, Memphis, TN 38105
| | - Argyris Politis
- Department of Chemistry, King's College London, London SE1 1DB, United Kingdom
| | - Gerard H M Huysmans
- Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY 10065;
| | - Olga Boudker
- Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY 10065;
- Tri-Institutional Training Program in Chemical Biology, New York, NY 10065
- Howard Hughes Medical Institute, Weill Cornell Medicine, New York, NY 10065
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4
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Schiemann O, Heubach CA, Abdullin D, Ackermann K, Azarkh M, Bagryanskaya EG, Drescher M, Endeward B, Freed JH, Galazzo L, Goldfarb D, Hett T, Esteban Hofer L, Fábregas Ibáñez L, Hustedt EJ, Kucher S, Kuprov I, Lovett JE, Meyer A, Ruthstein S, Saxena S, Stoll S, Timmel CR, Di Valentin M, Mchaourab HS, Prisner TF, Bode BE, Bordignon E, Bennati M, Jeschke G. Benchmark Test and Guidelines for DEER/PELDOR Experiments on Nitroxide-Labeled Biomolecules. J Am Chem Soc 2021; 143:17875-17890. [PMID: 34664948 DOI: 10.1021/jacs.1c07371] [Citation(s) in RCA: 105] [Impact Index Per Article: 35.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Distance distribution information obtained by pulsed dipolar EPR spectroscopy provides an important contribution to many studies in structural biology. Increasingly, such information is used in integrative structural modeling, where it delivers unique restraints on the width of conformational ensembles. In order to ensure reliability of the structural models and of biological conclusions, we herein define quality standards for sample preparation and characterization, for measurements of distributed dipole-dipole couplings between paramagnetic labels, for conversion of the primary time-domain data into distance distributions, for interpreting these distributions, and for reporting results. These guidelines are substantiated by a multi-laboratory benchmark study and by analysis of data sets with known distance distribution ground truth. The study and the guidelines focus on proteins labeled with nitroxides and on double electron-electron resonance (DEER aka PELDOR) measurements and provide suggestions on how to proceed analogously in other cases.
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Affiliation(s)
- Olav Schiemann
- Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstraße 12, 53115 Bonn, Germany
| | - Caspar A Heubach
- Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstraße 12, 53115 Bonn, Germany
| | - Dinar Abdullin
- Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstraße 12, 53115 Bonn, Germany
| | - Katrin Ackermann
- EaStCHEM School of Chemistry, Biomedical Sciences Research Complex, and Centre of Magnetic Resonance, University of St Andrews North Haugh, St Andrews KY16 9ST, U.K
| | - Mykhailo Azarkh
- Department of Chemistry and Konstanz Research School Chemical Biology, University of Konstanz, Universitätsstraße 10, 78457 Konstanz, Germany
| | - Elena G Bagryanskaya
- N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Lavrentieva aven 9, 630090 Novosibirsk, Russia
| | - Malte Drescher
- Department of Chemistry and Konstanz Research School Chemical Biology, University of Konstanz, Universitätsstraße 10, 78457 Konstanz, Germany
| | - Burkhard Endeward
- Institute of Physical and Theoretical Chemistry and Center of Biomolecular Magnetic Resonance, Goethe University, 60438 Frankfurt am Main, Germany
| | - Jack H Freed
- Department of Chemistry and Chemical Biology, and ACERT, National Biomedical Center for Advanced Electron Spin Resonance Technology, Cornell University, Ithaca, New York 14853-1301, United States
| | - Laura Galazzo
- Faculty of Chemistry and Biochemistry, Ruhr University Bochum, 44801 Bochum, Germany
| | - Daniella Goldfarb
- Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot 76100, Israel
| | - Tobias Hett
- Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstraße 12, 53115 Bonn, Germany
| | - Laura Esteban Hofer
- Department of Chemistry and Applied Biosciences, ETH Hönggerberg, Vladimir-Prelog-Weg 2, 8093 Zürich, Switzerland
| | - Luis Fábregas Ibáñez
- Department of Chemistry and Applied Biosciences, ETH Hönggerberg, Vladimir-Prelog-Weg 2, 8093 Zürich, Switzerland
| | - Eric J Hustedt
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232, United States
| | - Svetlana Kucher
- Faculty of Chemistry and Biochemistry, Ruhr University Bochum, 44801 Bochum, Germany
| | - Ilya Kuprov
- School of Chemistry, University of Southampton, Highfield Campus, Southampton SO17 1BJ, U.K
| | - Janet Eleanor Lovett
- SUPA School of Physics and Astronomy and BSRC, University of St Andrews, North Haugh, St Andrews KY16 9SS, U.K
| | - Andreas Meyer
- Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany
| | - Sharon Ruthstein
- Department of Chemistry, Bar Ilan University, Ramat Gan 5290002, Israel
| | - Sunil Saxena
- Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
| | - Stefan Stoll
- Department of Chemistry, University of Washington, Seattle, Washington 98195, United States
| | - Christiane R Timmel
- Department of Chemistry, Centre for Advanced Electron Spin Resonance, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K
| | - Marilena Di Valentin
- Department of Chemical Sciences, University of Padova, via Marzolo 1, 35131 Padova, Italy
| | - Hassane S Mchaourab
- Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, Tennessee 37232, United States
| | - Thomas F Prisner
- Institute of Physical and Theoretical Chemistry and Center of Biomolecular Magnetic Resonance, Goethe University, 60438 Frankfurt am Main, Germany
| | - Bela Ernest Bode
- EaStCHEM School of Chemistry, Biomedical Sciences Research Complex, and Centre of Magnetic Resonance, University of St Andrews North Haugh, St Andrews KY16 9ST, U.K
| | - Enrica Bordignon
- Faculty of Chemistry and Biochemistry, Ruhr University Bochum, 44801 Bochum, Germany
| | - Marina Bennati
- Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany
| | - Gunnar Jeschke
- Department of Chemistry and Applied Biosciences, ETH Hönggerberg, Vladimir-Prelog-Weg 2, 8093 Zürich, Switzerland
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5
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Ernst M, Robertson JL. The Role of the Membrane in Transporter Folding and Activity. J Mol Biol 2021; 433:167103. [PMID: 34139219 PMCID: PMC8756397 DOI: 10.1016/j.jmb.2021.167103] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/02/2021] [Revised: 06/07/2021] [Accepted: 06/08/2021] [Indexed: 12/23/2022]
Abstract
The synthesis, folding, and function of membrane transport proteins are critical factors for defining cellular physiology. Since the stability of these proteins evolved amidst the lipid bilayer, it is no surprise that we are finding that many of these membrane proteins demonstrate coupling of their structure or activity in some way to the membrane. More and more transporter structures are being determined with some information about the surrounding membrane, and computational modeling is providing further molecular details about these solvation structures. Thus, the field is moving towards identifying which molecular mechanisms - lipid interactions, membrane perturbations, differential solvation, and bulk membrane effects - are involved in linking membrane energetics to transporter stability and function. In this review, we present an overview of these mechanisms and the growing evidence that the lipid bilayer is a major determinant of the fold, form, and function of membrane transport proteins in membranes.
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Affiliation(s)
- Melanie Ernst
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine in St. Louis, 660 South Euclid Avenue, St. Louis, MO 63110, USA
| | - Janice L Robertson
- Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine in St. Louis, 660 South Euclid Avenue, St. Louis, MO 63110, USA.
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6
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Electron paramagnetic resonance spectroscopy on G-protein-coupled receptors: Adopting strategies from related model systems. Curr Opin Struct Biol 2021; 69:177-186. [PMID: 34304006 DOI: 10.1016/j.sbi.2021.06.003] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2021] [Revised: 05/27/2021] [Accepted: 06/06/2021] [Indexed: 11/23/2022]
Abstract
Membrane proteins, including ion channels, transporters and G-protein-coupled receptors (GPCRs), play a significant role in various physiological processes. Many of these proteins are difficult to express in large quantities, imposing crucial experimental restrictions. Nevertheless, there is now a wide variety of studies available utilizing electron paramagnetic resonance (EPR) spectroscopic techniques that expand experimental accessibility by using relatively small quantities of protein. Here, we give an overview starting from basic strategies in EPR on membrane proteins with a focus on GPCRs, while emphasizing several applications from recent years. We highlight how the arsenal of EPR-based techniques may provide significant further contributions to understanding the complex molecular machinery and energetic phenomena responsible for seamless workflow in essential biological processes.
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7
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Alvarez-Sieiro P, Sikkema HR, Poolman B. Heterodimer Formation of the Homodimeric ABC Transporter OpuA. Int J Mol Sci 2021; 22:ijms22115912. [PMID: 34072847 PMCID: PMC8199443 DOI: 10.3390/ijms22115912] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2021] [Revised: 05/28/2021] [Accepted: 05/28/2021] [Indexed: 11/16/2022] Open
Abstract
Many proteins have a multimeric structure and are composed of two or more identical subunits. While this can be advantageous for the host organism, it can be a challenge when targeting specific residues in biochemical analyses. In vitro splitting and re-dimerization to circumvent this problem is a tedious process that requires stable proteins. We present an in vivo approach to transform homodimeric proteins into apparent heterodimers, which then can be purified using two-step affinity-tag purification. This opens the door to both practical applications such as smFRET to probe the conformational dynamics of homooligomeric proteins and fundamental research into the mechanism of protein multimerization, which is largely unexplored for membrane proteins. We show that expression conditions are key for the formation of heterodimers and that the order of the differential purification and reconstitution of the protein into nanodiscs is important for a functional ABC-transporter complex.
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8
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Wort JL, Ackermann K, Norman DG, Bode BE. A general model to optimise Cu II labelling efficiency of double-histidine motifs for pulse dipolar EPR applications. Phys Chem Chem Phys 2021; 23:3810-3819. [PMID: 33533341 DOI: 10.1039/d0cp06196d] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Electron paramagnetic resonance (EPR) distance measurements are making increasingly important contributions to studies of biomolecules underpinning health and disease by providing highly accurate and precise geometric constraints. Combining double-histidine (dH) motifs with CuII spin labels shows promise for further increasing the precision of distance measurements, and for investigating subtle conformational changes. However, non-covalent coordination-based spin labelling is vulnerable to low binding affinity. Dissociation constants of dH motifs for CuII-nitrilotriacetic acid were previously investigated via relaxation induced dipolar modulation enhancement (RIDME), and demonstrated the feasibility of exploiting the dH motif for EPR applications at sub-μM protein concentrations. Herein, the feasibility of using modulation depth quantitation in CuII-CuII RIDME to simultaneously estimate a pair of non-identical independent KD values in such a tetra-histidine model protein is addressed. Furthermore, we develop a general speciation model to optimise CuII labelling efficiency, depending upon pairs of identical or disparate KD values and total CuII label concentration. We find the dissociation constant estimates are in excellent agreement with previously determined values, and empirical modulation depths support the proposed model.
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Affiliation(s)
- Joshua L Wort
- EaStCHEM School of Chemistry, Biomedical Sciences Research Complex, and Centre of Magnetic Resonance, University of St Andrews North Haugh, St Andrews KY16 9ST, UK.
| | - Katrin Ackermann
- EaStCHEM School of Chemistry, Biomedical Sciences Research Complex, and Centre of Magnetic Resonance, University of St Andrews North Haugh, St Andrews KY16 9ST, UK.
| | - David G Norman
- School of Life Sciences, University of Dundee, Medical Sciences Institute, Dundee, DD1 5EH, UK
| | - Bela E Bode
- EaStCHEM School of Chemistry, Biomedical Sciences Research Complex, and Centre of Magnetic Resonance, University of St Andrews North Haugh, St Andrews KY16 9ST, UK.
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9
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Probing Structural Dynamics of Membrane Proteins Using Electron Paramagnetic Resonance Spectroscopic Techniques. BIOPHYSICA 2021. [DOI: 10.3390/biophysica1020009] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
Membrane proteins are essential for the survival of living organisms. They are involved in important biological functions including transportation of ions and molecules across the cell membrane and triggering the signaling pathways. They are targets of more than half of the modern medical drugs. Despite their biological significance, information about the structural dynamics of membrane proteins is lagging when compared to that of globular proteins. The major challenges with these systems are low expression yields and lack of appropriate solubilizing medium required for biophysical techniques. Electron paramagnetic resonance (EPR) spectroscopy coupled with site directed spin labeling (SDSL) is a rapidly growing powerful biophysical technique that can be used to obtain pertinent structural and dynamic information on membrane proteins. In this brief review, we will focus on the overview of the widely used EPR approaches and their emerging applications to answer structural and conformational dynamics related questions on important membrane protein systems.
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10
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Functional (un)cooperativity in elevator transport proteins. Biochem Soc Trans 2020; 48:1047-1055. [PMID: 32573703 DOI: 10.1042/bst20190970] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2020] [Revised: 05/26/2020] [Accepted: 05/28/2020] [Indexed: 11/17/2022]
Abstract
The activity of enzymes is subject to regulation at multiple levels. Cooperativity, the interconnected behavior of active sites within a protein complex, directly affects protein activity. Cooperativity is a mode of regulation that requires neither extrinsic factors nor protein modifications. Instead, it allows enzymes themselves to modulate reaction rates. Cooperativity is an important regulatory mechanism in soluble proteins, but also examples of cooperative membrane proteins have been described. In this review, we summarize the current knowledge on interprotomer cooperativity in elevator-type proteins, a class of membrane transporters characterized by large rigid-body movements perpendicular to the membrane, and highlight well-studied examples and experimental approaches.
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11
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Sahu ID, Lorigan GA. Electron Paramagnetic Resonance as a Tool for Studying Membrane Proteins. Biomolecules 2020; 10:E763. [PMID: 32414134 PMCID: PMC7278021 DOI: 10.3390/biom10050763] [Citation(s) in RCA: 25] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2020] [Revised: 04/24/2020] [Accepted: 04/24/2020] [Indexed: 12/13/2022] Open
Abstract
Membrane proteins possess a variety of functions essential to the survival of organisms. However, due to their inherent hydrophobic nature, it is extremely difficult to probe the structure and dynamic properties of membrane proteins using traditional biophysical techniques, particularly in their native environments. Electron paramagnetic resonance (EPR) spectroscopy in combination with site-directed spin labeling (SDSL) is a very powerful and rapidly growing biophysical technique to study pertinent structural and dynamic properties of membrane proteins with no size restrictions. In this review, we will briefly discuss the most commonly used EPR techniques and their recent applications for answering structure and conformational dynamics related questions of important membrane protein systems.
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Affiliation(s)
- Indra D. Sahu
- Natural Science Division, Campbellsville University, Campbellsville, KY 42718, USA
- Department of Chemistry and Biochemistry, Miami University, Oxford, OH 45056, USA
| | - Gary A. Lorigan
- Department of Chemistry and Biochemistry, Miami University, Oxford, OH 45056, USA
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12
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Scalise M, Console L, Galluccio M, Pochini L, Indiveri C. Chemical Targeting of Membrane Transporters: Insights into Structure/Function Relationships. ACS OMEGA 2020; 5:2069-2080. [PMID: 32064367 PMCID: PMC7016923 DOI: 10.1021/acsomega.9b04078] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/30/2019] [Accepted: 01/16/2020] [Indexed: 05/09/2023]
Abstract
Chemical modification of proteins is a vintage strategy that is still fashionable due to the information that can be obtained from this approach. An interesting application of chemical modification is linked with membrane transporters. These proteins have peculiar features such as the presence of hydrophobic and hydrophilic domains, which show different degree of accessibility to chemicals. The presence of reactive residues in the membrane transporters is at the basis of the chemical targeting strategy devoted to investigating structure/function relationships; in particular, information on the substrate binding site, regulatory domains, dimerization domains, and the interface between hydrophilic loops and transmembrane domains has been obtained over the years by chemical targeting. Given the difficulty in handling membrane transporters, their study experienced a great delay, particularly concerning structural information. Chemical targeting has been applied with reasonable success to some membrane transporters belonging to the families SLC1, SLC6, SLC7, and SLC22. Furthermore, some data on the potential application of chemical targeting in pharmacology are also discussed.
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13
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Abstract
The cytoplasm of bacterial cells is a highly crowded cellular compartment that possesses considerable osmotic potential. As a result, and owing to the semipermeable nature of the cytoplasmic membrane and the semielastic properties of the cell wall, osmotically driven water influx will generate turgor, a hydrostatic pressure considered critical for growth and viability. Both increases and decreases in the external osmolarity inevitably trigger water fluxes across the cytoplasmic membrane, thus impinging on the degree of cellular hydration, molecular crowding, magnitude of turgor, and cellular integrity. Here, we assess mechanisms that permit the perception of osmotic stress by bacterial cells and provide an overview of the systems that allow them to genetically and physiologically cope with this ubiquitous environmental cue. We highlight recent developments implicating the secondary messenger c-di-AMP in cellular adjustment to osmotic stress and the role of osmotic forces in the life of bacteria-assembled in biofilms.
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Affiliation(s)
- Erhard Bremer
- Laboratory for Microbiology, Department of Biology; and Center for Synthetic Microbiology, Philipps-Universität Marburg, 35043 Marburg, Germany;
| | - Reinhard Krämer
- Institute of Biochemistry, University of Cologne, 50674 Cologne, Germany;
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14
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Balo AR, Lee J, Ernst OP. Stationary Phase EPR Spectroscopy for Monitoring Membrane Protein Refolding by Conformational Response. Anal Chem 2018; 91:1071-1079. [DOI: 10.1021/acs.analchem.8b04542] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
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15
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Abstract
Robertson recounts the historical experiments that gave rise to our current understanding of cell membranes. In 1918, the year the Journal of General Physiology was founded, there was little understanding of the structure of the cell membrane. It was evident that cells had invisible barriers separating the cytoplasm from the external solution. However, it would take decades before lipid bilayers were identified as the essential constituent of membranes. It would take even longer before it was accepted that there existed hydrophobic proteins that were embedded within the membrane and that these proteins were responsible for selective permeability in cells. With a combination of intuitive experiments and quantitative thinking, the last century of cell membrane research has led us to a molecular understanding of the structure of the membrane, as well as many of the proteins embedded within. Now, research is turning toward a physical understanding of the reactions of membrane proteins and lipids in this unique and incredibly complex solvent environment.
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Affiliation(s)
- Janice L Robertson
- Department of Molecular Physiology and Biophysics, The University of Iowa, Iowa City, IA
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