51
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Ebel C. Sedimentation velocity to characterize surfactants and solubilized membrane proteins. Methods 2011; 54:56-66. [DOI: 10.1016/j.ymeth.2010.11.003] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2010] [Revised: 11/18/2010] [Accepted: 11/19/2010] [Indexed: 02/07/2023] Open
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Schuck P, Zhao H. Editorial for the special issue of methods "Modern Analytical Ultracentrifugation". Methods 2011; 54:1-3. [PMID: 21536133 DOI: 10.1016/j.ymeth.2011.04.003] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 04/22/2011] [Indexed: 10/18/2022] Open
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53
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Ghirlando R. The analysis of macromolecular interactions by sedimentation equilibrium. Methods 2010; 54:145-56. [PMID: 21167941 DOI: 10.1016/j.ymeth.2010.12.005] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2010] [Revised: 11/27/2010] [Accepted: 12/03/2010] [Indexed: 11/29/2022] Open
Abstract
The study of macromolecular interactions by sedimentation equilibrium is a highly technical method that requires great care in both the experimental design and data analysis. The complexity of the interacting system that can be analyzed is only limited by the ability to deconvolute the exponential contributions of each of the species to the overall concentration gradient. This is achieved in part through the use of multi-signal data collection and the implementation of soft mass conservation. We illustrate the use of these constraints in SEDPHAT through the study of an A+B+B⇌AB+B⇌ABB system and highlight some of the technical challenges that arise. We show that both the multi-signal analysis and mass conservation result in a precise and robust data analysis and discuss improvements that can be obtained through the inclusion of data from other methods such as sedimentation velocity and isothermal titration calorimetry.
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Affiliation(s)
- Rodolfo Ghirlando
- Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892-0540, USA.
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54
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Dorwart MR, Wray R, Brautigam CA, Jiang Y, Blount P. S. aureus MscL is a pentamer in vivo but of variable stoichiometries in vitro: implications for detergent-solubilized membrane proteins. PLoS Biol 2010; 8:e1000555. [PMID: 21151884 PMCID: PMC2998437 DOI: 10.1371/journal.pbio.1000555] [Citation(s) in RCA: 58] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2010] [Accepted: 10/27/2010] [Indexed: 12/03/2022] Open
Abstract
Detergent-induced rearrangements of membrane-protein subunits explain why two MscL channel stoichiometries have been resolved by X-ray crystallography - but S. aureus MscL is truly a pentamer in vivo. While the bacterial mechanosensitive channel of large conductance (MscL) is the best studied biological mechanosensor and serves as a paradigm for how a protein can sense and respond to membrane tension, the simple matter of its oligomeric state has led to debate, with models ranging from tetramers to hexamers. Indeed, two different oligomeric states of the bacterial mechanosensitive channel MscL have been resolved by X-ray crystallography: The M. tuberculosis channel (MtMscL) is a pentamer, while the S. aureus protein (SaMscL) forms a tetramer. Because several studies suggest that, like MtMscL, the E. coli MscL (EcoMscL) is a pentamer, we re-investigated the oligomeric state of SaMscL. To determine the structural organization of MscL in the cell membrane we developed a disulfide-trapping approach. Surprisingly, we found that virtually all SaMscL channels in vivo are pentameric, indicating this as the physiologically relevant and functional oligomeric state. Complementing our in vivo results, we purified SaMscL and assessed its oligomeric state using three independent approaches (sedimentation equilibrium centrifugation, crosslinking, and light scattering) and established that SaMscL is a pentamer when solubilized in Triton X-100 and C8E5 detergents. However, performing similar experiments on SaMscL solubilized in LDAO, the detergent used in the crystallographic study, confirmed the tetrameric oligomerization resolved by X-ray crystallography. We further demonstrate that this stoichiometric shift is reversible by conventional detergent exchange experiments. Our results firmly establish the pentameric organization of SaMscL in vivo. Furthermore they demonstrate that detergents can alter the subunit stoichiometry of membrane protein complexes in vitro; thus, in vivo assays are necessary to firmly establish a membrane protein's true functionally relevant oligomeric state. The ability to detect mechanical forces is at the basis of not only the senses of touch hearing and balance but also cardiovascular and osmotic regulation. One of the primary ways that organisms detect forces is through mechanosensitive channels, and mechanosensation is so vital that essentially all organisms have at least one such sensor. Indeed, the best-studied mechanosensitive channel is from bacteria, and because relatively little is known of mechanosensors from higher organisms, these channels are a model for how a protein can sense and respond to mechanical forces. Although the bacterial mechanosensitive channel MscL has been well studied, the simple issue of how many subunits it has is hotly debated. There are even two published crystal structures showing either tetrameric or pentameric complexes. Here we show that the channel is actually pentameric in vivo and that the detergent used to solubilize the protein can rearrange the complexes from pentamers to tetramers. The finding that detergents can have such a profound effect on structure may have broad implications for the study of other membrane proteins.
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Affiliation(s)
- Michael R. Dorwart
- Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, United States of America
- Howard Hughes Medical Institute, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, United States of America
| | - Robin Wray
- Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, United States of America
| | - Chad A. Brautigam
- Department of Biochemistry, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, United States of America
| | - Youxing Jiang
- Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, United States of America
- Howard Hughes Medical Institute, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, United States of America
| | - Paul Blount
- Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, United States of America
- * E-mail:
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55
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Padrick SB, Deka RK, Chuang JL, Wynn RM, Chuang DT, Norgard MV, Rosen MK, Brautigam CA. Determination of protein complex stoichiometry through multisignal sedimentation velocity experiments. Anal Biochem 2010; 407:89-103. [PMID: 20667444 PMCID: PMC3089910 DOI: 10.1016/j.ab.2010.07.017] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2010] [Revised: 07/19/2010] [Accepted: 07/20/2010] [Indexed: 01/21/2023]
Abstract
Determination of the stoichiometry of macromolecular assemblies is fundamental to an understanding of how they function. Many different biophysical methodologies may be used to determine stoichiometry. In the past, both sedimentation equilibrium and sedimentation velocity analytical ultracentrifugation have been employed to determine component stoichiometries. Recently, a method of globally analyzing multisignal sedimentation velocity data was introduced by Schuck and coworkers. This global analysis removes some of the experimental inconveniences and inaccuracies that could occur in the previously used strategies. This method uses spectral differences between the macromolecular components to decompose the well-known c(s) distribution into component distributions c(k)(s); that is, each component k has its own c(k)(s) distribution. Integration of these distributions allows the calculation of the populations of each component in cosedimenting complexes, yielding their stoichiometry. In our laboratories, we have used this method extensively to determine the component stoichiometries of several protein-protein complexes involved in cytoskeletal remodeling, sugar metabolism, and host-pathogen interactions. The overall method is described in detail in this work, as are experimental examples and caveats.
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Affiliation(s)
- Shae B. Padrick
- Department of Biochemistry, The University of Texas, Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390-8816
| | - Ranjit K. Deka
- Department of Microbiology, The University of Texas, Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390-8816
| | - Jacinta L. Chuang
- Department of Biochemistry, The University of Texas, Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390-8816
| | - R. Max Wynn
- Department of Biochemistry, The University of Texas, Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390-8816
- Department of Internal Medicine, The University of Texas, Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390-8816
| | - David T. Chuang
- Department of Biochemistry, The University of Texas, Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390-8816
- Department of Internal Medicine, The University of Texas, Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390-8816
| | - Michael V. Norgard
- Department of Microbiology, The University of Texas, Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390-8816
| | - Michael K. Rosen
- Department of Biochemistry, The University of Texas, Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390-8816
- Howard Hughes Medical Institute, The University of Texas, Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390-8816
| | - Chad A. Brautigam
- Department of Biochemistry, The University of Texas, Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, TX 75390-8816
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56
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Qin X, Tan Y, Wang L, Wang Z, Wang B, Wen X, Yang G, Xi Z, Shen Y. Structural insight into human variegate porphyria disease. FASEB J 2010; 25:653-64. [PMID: 21048046 DOI: 10.1096/fj.10-170811] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Human protoporphyrinogen IX oxidase (hPPO), a mitochondrial inner membrane protein, converts protoporphyrinogen IX to protoporphyrin IX in the heme biosynthetic pathway. Mutations in the hPPO gene cause the inherited human disease variegate porphyria (VP). In this study, we report the crystal structure of hPPO in complex with the coenzyme flavin adenine dinucleotide (FAD) and the inhibitor acifluorfen at a resolution of 1.9 Å. The structural and biochemical analyses revealed the molecular details of FAD and acifluorfen binding to hPPO as well as the interactions of the substrate with hPPO. Structural analysis and gel chromatography indicated that hPPO is a monomer rather than a homodimer in vitro. The founder-effect mutation R59W in VP patients is most likely caused by a severe electrostatic hindrance in the hydrophilic binding pocket involving the bulky, hydrophobic indolyl ring of the tryptophan. Forty-seven VP-causing mutations were purified by chromatography and kinetically characterized in vitro. The effect of each mutation was demonstrated in the high-resolution crystal structure.
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Affiliation(s)
- Xiaohong Qin
- College of Life Science, Nankai University, Tianjin, China 300071
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57
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Prunetti L, Infossi P, Brugna M, Ebel C, Giudici-Orticoni MT, Guiral M. New functional sulfide oxidase-oxygen reductase supercomplex in the membrane of the hyperthermophilic bacterium Aquifex aeolicus. J Biol Chem 2010; 285:41815-26. [PMID: 20971847 DOI: 10.1074/jbc.m110.167841] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Aquifex aeolicus, a hyperthermophilic and microaerophilic bacterium, obtains energy for growth from inorganic compounds alone. It was previously proposed that one of the respiratory pathways in this organism consists of the electron transfer from hydrogen sulfide (H(2)S) to molecular oxygen. H(2)S is oxidized by the sulfide quinone reductase, a membrane-bound flavoenzyme, which reduces the quinone pool. We have purified and characterized a novel membrane-bound multienzyme supercomplex that brings together all the molecular components involved in this bioenergetic chain. Our results indicate that this purified structure consists of one dimeric bc(1) complex (complex III), one cytochrome c oxidase (complex IV), and one or two sulfide quinone reductases as well as traces of the monoheme cytochrome c(555) and quinone molecules. In addition, this work strongly suggests that the cytochrome c oxidase in the supercomplex is a ba(3)-type enzyme. The supercomplex has a molecular mass of about 350 kDa and is enzymatically functional, reducing O(2) in the presence of the electron donor, H(2)S. This is the first demonstration of the existence of such a respirasome carrying a sulfide oxidase-oxygen reductase activity. Moreover, the kinetic properties of the sulfide quinone reductase change slightly when integrated in the supercomplex, compared with the free enzyme. We previously purified a complete respirasome involved in hydrogen oxidation and sulfur reduction from Aquifex aeolicus. Thus, two different bioenergetic pathways (sulfur reduction and sulfur oxidation) are organized in this bacterium as supramolecular structures in the membrane. A model for the energetic sulfur metabolism of Aquifex aeolicus is proposed.
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Affiliation(s)
- Laurence Prunetti
- Laboratoire de Bioénergétique et Ingénierie des Protéines, UPR 9036, IMM, IFR88-CNRS, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France
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58
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Casutt MS, Huber T, Brunisholz R, Tao M, Fritz G, Steuber J. Localization and function of the membrane-bound riboflavin in the Na+-translocating NADH:quinone oxidoreductase (Na+-NQR) from Vibrio cholerae. J Biol Chem 2010; 285:27088-27099. [PMID: 20558724 PMCID: PMC2930708 DOI: 10.1074/jbc.m109.071126] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2009] [Revised: 06/16/2010] [Indexed: 12/29/2022] Open
Abstract
The sodium ion-translocating NADH:quinone oxidoreductase (Na(+)-NQR) from the human pathogen Vibrio cholerae is a respiratory membrane protein complex that couples the oxidation of NADH to the transport of Na(+) across the bacterial membrane. The Na(+)-NQR comprises the six subunits NqrABCDEF, but the stoichiometry and arrangement of these subunits are unknown. Redox-active cofactors are FAD and a 2Fe-2S cluster on NqrF, covalently attached FMNs on NqrB and NqrC, and riboflavin and ubiquinone-8 with unknown localization in the complex. By analyzing the cofactor content and NADH oxidation activity of subcomplexes of the Na(+)-NQR lacking individual subunits, the riboflavin cofactor was unequivocally assigned to the membrane-bound NqrB subunit. Quantitative analysis of the N-terminal amino acids of the holo-complex revealed that NqrB is present in a single copy in the holo-complex. It is concluded that the hydrophobic NqrB harbors one riboflavin in addition to its covalently attached FMN. The catalytic role of two flavins in subunit NqrB during the reduction of ubiquinone to ubiquinol by the Na(+)-NQR is discussed.
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Affiliation(s)
- Marco S Casutt
- Department of Neuropathology, Breisacherstrasse 64, University of Freiburg, 79106 Freiburg, Germany
| | - Tamara Huber
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - René Brunisholz
- Functional Genomics Centre Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland
| | - Minli Tao
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland
| | - Günter Fritz
- Department of Neuropathology, Breisacherstrasse 64, University of Freiburg, 79106 Freiburg, Germany
| | - Julia Steuber
- Department of Biochemistry, University of Zurich, 8057 Zurich, Switzerland.
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Crystallography of membrane proteins: from crystallization to structure. Methods Mol Biol 2010. [PMID: 20665262 DOI: 10.1007/978-1-60761-762-4_5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
Abstract
Although crystallographic studies of membrane proteins have progressed in the last 5 years, the field still remains challenging with several severe bottlenecks. The chapter focuses on the crystallization and describes two approaches, the classical vapor diffusion method and the more recent use of lipidic phases. General aspects on the crystallization principles as well as more practical details are given. In a more synthetic way, the chapter also addresses how structures are solved by X-ray crystallography, and highlights aspects that are specific to membrane proteins.
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60
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Zhao H, Brown PH, Balbo A, Fernández-Alonso MDC, Polishchuck N, Chaudhry C, Mayer ML, Ghirlando R, Schuck P. Accounting for solvent signal offsets in the analysis of interferometric sedimentation velocity data. Macromol Biosci 2010; 10:736-45. [PMID: 20480511 PMCID: PMC7469924 DOI: 10.1002/mabi.200900456] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
Sedimentation velocity (SV) analytical ultracentrifugation has re-emerged as an important tool in the characterization of biological macromolecules and nanoparticles. The computational analysis of the evolution of the macromolecular concentration profile allows the characterization of many hydrodynamic and thermodynamic properties of the macromolecules and their interactions. The Rayleigh interference optical system is often the detection method of choice, for its usually superior data quality and the wide applicability of refractive index sensitive detection. However, the interference optical system is also sensitive to the redistribution of co-solvent molecules, which are not of primary experimental interest. In principle, their contribution can be eliminated by an exact geometric and compositional match of the sample solution and the reference solution, achieving the complete optical subtraction of unwanted buffer signals. Unfortunately, in practice, this can often not be perfectly achieved for various reasons, leading to signal offsets arising from unmatched sedimentation of solvent components. If unrecognized, this can lead to significant misfit, accompanied by significant errors in the macromolecular sedimentation parameters. In the present work, we describe an approach of computationally accounting for signals from sedimenting buffer components through explicitly modeling their redistribution with Lamm equation solutions, implemented in the software SEDFIT. We demonstrate how this can restore the SV analysis to yield a high quality fit of the data and to provide correct macromolecular sedimentation parameters.
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Affiliation(s)
- Huaying Zhao
- Dynamics of Macromolecular Assembly, Laboratory of Bioengineering and Physical Science, NIBIB, National Institutes of Health, Bethesda, Maryland, USA.
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61
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Mayer CL, Snyder WK, Swietlicka MA, Vanschoiack AD, Austin CR, McFarland BJ. Size-exclusion chromatography can identify faster-associating protein complexes and evaluate design strategies. BMC Res Notes 2009; 2:135. [PMID: 19604395 PMCID: PMC2717102 DOI: 10.1186/1756-0500-2-135] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2009] [Accepted: 07/15/2009] [Indexed: 11/10/2022] Open
Abstract
Background We previously developed a set of rationally designed mutant MICA protein ligands for the NKG2D immunoreceptor in which MICA was mutated at residues that do not contact NKG2D. Some of these MICA mutants, predicted by RosettaDesign to be destabilized, bound NKG2D with affinities enhanced by more than an order of magnitude when evaluated by surface plasmon resonance (SPR). Findings Small-zone size-exclusion chromatography (SEC) detected persistent high-affinity MICA mutant-NKG2D complexes in solution as early-eluting peaks. The SEC binding assay used standard protein purification instrumentation to evaluate complex stability, qualitatively paralleled the SPR results, and successfully discriminated among complexes that differed only in on-rates. We used the SEC binding assay, along with SPR, to assess the results of a follow-up design strategy targeting the non-interfacial redesigned region. Both SEC and SPR agreed that these mutations did not enhance affinity as much as previous mutants. When the SEC binding assay was run in 1 M urea, only the highest affinity complex was detected. Conclusion This SEC binding assay provides a correlation with SPR results for protein complex affinities, detecting changes in complex on-rates, and tunable to lower sensitivity with 1 M urea. The SEC binding assay is complementary to other protein design evaluation methods, can be adapted to the undergraduate research laboratory, and may provide additional structural information about changes in hydrodynamic radii from elution times. Our assay allowed us to conclude that further alteration of MICA at non-contacting residues is unlikely to further enhance NKG2D affinity.
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Affiliation(s)
- Chad L Mayer
- Department of Chemistry and Biochemistry, Seattle Pacific University, 3307 Third Avenue West, Seattle, WA 98119-1997, USA.
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62
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Detergent binding explains anomalous SDS-PAGE migration of membrane proteins. Proc Natl Acad Sci U S A 2009. [PMID: 19181854 DOI: 10.1073/pnas.0813167106.] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) that does not correlate with formula molecular weights, termed "gel shifting," appears to be common for membrane proteins but has yet to be conclusively explained. In the present work, we investigate the anomalous gel mobility of helical membrane proteins using a library of wild-type and mutant helix-loop-helix ("hairpin") sequences derived from transmembrane segments 3 and 4 of the human cystic fibrosis transmembrane conductance regulator (CFTR), including disease-phenotypic residue substitutions. We find that these hairpins migrate at rates of -10% to +30% vs. their actual formula weights on SDS-PAGE and load detergent at ratios ranging from 3.4-10 g SDS/g protein. We additionally demonstrate that mutant gel shifts strongly correlate with changes in hairpin SDS loading capacity (R(2) = 0.8), and with hairpin helicity (R(2) = 0.9), indicating that gel shift behavior originates in altered detergent binding. In some cases, this differential solvation by SDS may result from replacing protein-detergent contacts with protein-protein contacts, implying that detergent binding and folding are intimately linked. The CF-phenotypic V232D mutant included in our library may thus disrupt CFTR function via altered protein-lipid interactions. The observed interdependence between hairpin migration, SDS aggregation number, and conformation additionally suggests that detergent binding may provide a rapid and economical screen for identifying membrane proteins with robust tertiary and/or quaternary structures.
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63
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Detergent binding explains anomalous SDS-PAGE migration of membrane proteins. Proc Natl Acad Sci U S A 2009; 106:1760-5. [PMID: 19181854 DOI: 10.1073/pnas.0813167106] [Citation(s) in RCA: 596] [Impact Index Per Article: 39.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
Migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) that does not correlate with formula molecular weights, termed "gel shifting," appears to be common for membrane proteins but has yet to be conclusively explained. In the present work, we investigate the anomalous gel mobility of helical membrane proteins using a library of wild-type and mutant helix-loop-helix ("hairpin") sequences derived from transmembrane segments 3 and 4 of the human cystic fibrosis transmembrane conductance regulator (CFTR), including disease-phenotypic residue substitutions. We find that these hairpins migrate at rates of -10% to +30% vs. their actual formula weights on SDS-PAGE and load detergent at ratios ranging from 3.4-10 g SDS/g protein. We additionally demonstrate that mutant gel shifts strongly correlate with changes in hairpin SDS loading capacity (R(2) = 0.8), and with hairpin helicity (R(2) = 0.9), indicating that gel shift behavior originates in altered detergent binding. In some cases, this differential solvation by SDS may result from replacing protein-detergent contacts with protein-protein contacts, implying that detergent binding and folding are intimately linked. The CF-phenotypic V232D mutant included in our library may thus disrupt CFTR function via altered protein-lipid interactions. The observed interdependence between hairpin migration, SDS aggregation number, and conformation additionally suggests that detergent binding may provide a rapid and economical screen for identifying membrane proteins with robust tertiary and/or quaternary structures.
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