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EmrE, a puzzling transporter: Assembly, biogenesis and evolution of a dual-topology membrane protein
Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biological membranes are the key to cell existence, as they are able to both isolate and connect their interior with the environment. Membranes are composed of lipids and proteins that create a semi-permeable barrier; because the lipid bilayer stops free diffusion of most molecules and ions, membrane proteins play an important role in connecting the interior of the cell with its environment. They function as receptors, sensing signals to trigger a response; cell adhesion molecules, holding neighboring cells together, or transporters and channels importing nutrients and extruding waste, among other chemical compounds, in a controlled manner.

In order for membrane proteins to function correctly, proper insertion, folding and oligomerization in the bilayer is essential. While most membrane proteins adopt a unique orientation in the membrane, some proteins adopt multiple topologies. A well-known case is the dual-topology membrane proteins that adopt two opposite orientations in the membrane. The best-studied dual-topology protein is EmrE, a dimeric multidrug transporter found in Escherichia coli, and other bacteria.

The existence of dual-topology proteins raises many questions regarding oligomerization, biogenesis and evolution of membrane proteins. In this thesis, EmrE has been used as a model protein to study some of these issues. Our goals were (i) to settle the controversy regarding whether the arrangement of the monomers within the EmrE dimer is parallel or antiparallel, (ii) to test the validity of the published X-ray structure by in vivo experiments and, (iii) to elucidate the mechanism of membrane insertion (iv) and the evolution of dual-topology membrane proteins.

Place, publisher, year, edition, pages
Stockholm: Department of Biochemistry and Biophysics, Stockholm University , 2013. , 68 p.
National Category
Biochemistry and Molecular Biology
Research subject
Biochemistry
Identifiers
URN: urn:nbn:se:su:diva-94830ISBN: 978-91-7447-797-9 (print)OAI: oai:DiVA.org:su-94830DiVA: diva2:658719
Public defence
2013-12-06, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defence the following papers were unpublished and had a status as follows: Paper 2: Epub ahead of print; Paper 4: Manuscript

Available from: 2013-11-14 Created: 2013-10-15 Last updated: 2013-11-05Bibliographically approved
List of papers
1. Antiparallel Dimers of the Small Multidrug Resistance Protein EmrE Are More Stable Than Parallel Dimers
Open this publication in new window or tab >>Antiparallel Dimers of the Small Multidrug Resistance Protein EmrE Are More Stable Than Parallel Dimers
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2012 (English)In: Journal of Biological Chemistry, ISSN 0021-9258, E-ISSN 1083-351X, Vol. 287, no 31, 26052-26059 p.Article in journal (Refereed) Published
Abstract [en]

The bacterial multidrug transporter EmrE is a dual-topology membrane protein and as such is able to insert into the membrane in two opposite orientations. The functional form of EmrE is a homodimer; however, the relative orientation of the subunits in the dimer is under debate. Using EmrE variants with fixed, opposite orientations in the membrane, we now show that, although the proteins are able to form parallel dimers, an antiparallel organization of the subunits in the dimer is preferred. Blue-native PAGE analyses of intact oligomers and disulfide cross-linking demonstrate that in membranes, the proteins form parallel dimers only if no oppositely orientated partner is present. Co-expression of oppositely orientated proteins almost exclusively yields antiparallel dimers. Finally, parallel dimers can be disrupted and converted into antiparallel dimers by heating of detergent-solubilized protein. Importantly, in vivo function is correlated clearly to the presence of antiparallel dimers. Our results suggest that an antiparallel arrangement of the subunits in the dimer is more stable than a parallel organization and likely corresponds to the functional form of the protein.

National Category
Biochemistry and Molecular Biology
Identifiers
urn:nbn:se:su:diva-80590 (URN)10.1074/jbc.M112.357590 (DOI)000306916300032 ()
Note

AuthorCount:6;

Available from: 2012-09-28 Created: 2012-09-25 Last updated: 2017-12-07Bibliographically approved
2. In Vivo Trp Scanning of the Small Multidrug Resistance Protein EmrE Confirms 3D Structure Models
Open this publication in new window or tab >>In Vivo Trp Scanning of the Small Multidrug Resistance Protein EmrE Confirms 3D Structure Models
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2013 (English)In: Journal of Molecular Biology, ISSN 0022-2836, E-ISSN 1089-8638, Vol. 425, no 22, 4642-4651 p.Article in journal (Refereed) Published
Abstract [en]

The quaternary structure of the homodimeric small multidrug resistance protein EmrE has been studied intensely over the past decade. Structural models derived from both two- and three-dimensional crystals show EmrE as an anti-parallel homodimer. However, the resolution of the structures is rather low and their relevance for the in vivo situation has been questioned. Here, we have challenged the available structural models by a comprehensive in vivo Trp scanning of all four transmembrane helices in EmrE. The results are in close agreement with the degree of lipid exposure of individual residues predicted from coarse-grained molecular dynamics simulations of the anti-parallel dimeric structure obtained by X-ray crystallography, strongly suggesting that the X-ray structure provides a good representation of the active in vivo form of EmrE.

Keyword
EmrE, multidrug resistance, Trp scan
National Category
Biochemistry and Molecular Biology
Identifiers
urn:nbn:se:su:diva-94831 (URN)10.1016/j.jmb.2013.07.039 (DOI)000328100500048 ()
Available from: 2013-10-22 Created: 2013-10-15 Last updated: 2017-12-06Bibliographically approved
3. Control of Membrane Protein Topology by a Single C-Terminal Residue
Open this publication in new window or tab >>Control of Membrane Protein Topology by a Single C-Terminal Residue
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2010 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 328, no 5986, 1698-1700 p.Article in journal (Refereed) Published
Abstract [en]

The mechanism by which multispanning helix-bundle membrane proteins are inserted into their target membrane remains unclear. In both prokaryotic and eukaryotic cells, membrane proteins are inserted cotranslationally into the lipid bilayer. Positively charged residues flanking the transmembrane helices are important topological determinants, but it is not known whether they act strictly locally, affecting only the nearest transmembrane helices, or can act globally, affecting the topology of the entire protein. Here we found that the topology of an Escherichia coli inner membrane protein with four or five transmembrane helices could be controlled by a single positively charged residue placed in different locations throughout the protein, including the very C terminus. This observation points to an unanticipated plasticity in membrane protein insertion mechanisms.

National Category
Biological Sciences
Identifiers
urn:nbn:se:su:diva-50108 (URN)10.1126/science.1188950 (DOI)000279107400044 ()
Note

authorCount :5

Available from: 2010-12-29 Created: 2010-12-21 Last updated: 2017-12-11Bibliographically approved
4. Why have Small Multidrug Resistance proteins not evolved into fused, internally homologous structures?
Open this publication in new window or tab >>Why have Small Multidrug Resistance proteins not evolved into fused, internally homologous structures?
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(English)Manuscript (preprint) (Other academic)
National Category
Biochemistry and Molecular Biology
Identifiers
urn:nbn:se:su:diva-94832 (URN)
Available from: 2013-10-22 Created: 2013-10-15 Last updated: 2013-10-25

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