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14-September-2008 18:02:49 - protein A transmembrane protein is a protein that spans the entire biological membrane. Transmembrane proteins aggregate and precipitate in water. They require detergents or nonpolar solvents for extraction, although some of them beta-barrels can be also extracted using denaturing agents. Schematic representation of transmembrane proteins: 1. a single transmembrane α-helix bitopic membrane protein 2. a polytopic α-helical protein 3. a transmembrane β barrel The membrane is represented in light brown. Schematic representation of transmembrane proteins: 1. a single transmembrane α-helix bitopic membrane protein 2. a polytopic α-helical protein 3. a transmembrane β barrel The membrane is represented in light brown. Contents 1 Types 2 Thermodynamic stability and folding 2.1 Stability of α-helical transmembrane proteins 2.2 Folding of α-helical transmembrane proteins 2.3 Stability and folding of β-barrel transmembrane proteins 3 3D structures 3.1 Light absorption-driven transporters 3.2 Oxidoreduction-driven transporters 3.3 Electrochemical potential-driven transporters 3.4 P-P-bond hydrolysis-driven transporters 3.5 Porters uniporters, symporters, antiporters 3.6 Alpha-helical channels including ion channels 3.7 Enzymes 3.8 Proteins with alpha-helical transmembrane anchors 3.9 β-barrels composed of a single polypeptide chain 3.10 β-barrels composed of several polypeptide chains 4 References 5 Additional examples 6 External links 7 See also Types There are two basic types of transmembrane proteins: Alpha-helical. These proteins are present in the inner membranes of bacterial cells or the plasma membrane of eukaryotes, and sometimes in the outer membranes 1 This is the major category of transmembrane proteins. Beta-barrels. These proteins are so far found only in outer membranes of Gram-negative bacteria, cell wall of Gram-positive bacteria, and outer membranes of mitochondria and chloroplasts. All beta-barrel transmembrane proteins have simplest up-and-down topology, which may reflect their common evolutionary origin and similar folding mechanism. Thermodynamic stability and folding Stability of α-helical transmembrane proteins Transmembrane α-helical proteins are unusually stable judging from thermal denaturation studies, because they do not unfold completely within the membranes the complete unfolding would require breaking down too many α-helical H-bonds in the nonpolar media. On the other hand, these proteins easily misfold, due to non-native aggregation in membranes, transition to the molten globule states, formation of non-native disulfide bonds, or unfolding of peripheral regions and nonregular loops that are locally less stable. It is also important to properly define the unfolded state. The unfolded state of membrane proteins in detergent micelles is different from that in the thermal denaturation experiments. This state represents a combination of folded hydrophobic α-helices and partially unfolded segments covered by the detergent. For example, the unfolded bacteriorhodopsin in SDS micelles has four transmembrane α-helices folded, while the rest of the protein is situated at the micelle-water interface and can adopt different types of non-native amphiphilic structures. Free energy differences between such detergent-denatured and native states are similar to stabilities of water-soluble proteins 10 kcal/mol. Folding of α-helical transmembrane proteins Refolding of α-helical transmembrane proteins in vitro is technically difficult. There are relatively few examples of the successful refolding experiments, as for bacteriorhodopsin. In vivo all such proteins are normally folded co-translationally within the large transmembrane translocon. The translocon channel provides a highly heterogeneous environment for the nascent transmembane α-helices. A relatively polar amphiphilic α-helix can adopt a transmembrane orientation in the translocon although it would be at the membrane surface or unfolded in vitro, because its polar residues can face the central water-filled channel of the translocon. Such mechanism is necessary for incorporation of polar α-helices into structures of transmembrane proteins. The amphiphilic helices remain attached to the translocon until the protein is completely synthesized and folded. If the protein remains unfolded and attached to the translocon for too long, it is degraded by specific quality control cellular systems. Stability and folding of β-barrel transmembrane proteins Stability of β-barrel transmembrane proteins is similar to stability of water-soluble proteins, based on chemical denaturation studies. Their folding in vivo is facilitated by water-soluble chaperones, such as protein Skp 1. 3D structures Light absorption-driven transporters Bacteriorhodopsin-like proteins including rhodopsin see also opsin2 Bacterial photosynthetic reaction centres and photosystems I and II 3 Light harvesting complexes from bacteria and chloroplasts 4 Oxidoreduction-driven transporters Transmembrane cytochrome b-like proteins 5: coenzyme Q - cytochrome c reductase cytochrome bc1 ; cytochrome b6f complex; formate dehydrogenase, respiratory nitrate reductase; succinate - coenzyme Q reductase fumarate reductase; and succinate dehydrogenase. See electron transport chain. Cytochrome c oxidases 6 from bacteria and mitochondria Electrochemical potential-driven transporters Proton or sodium translocating F-type and V-type ATPases 7 P-P-bond hydrolysis-driven transporters P-type calcium ATPase five different conformations 8 Calcium ATPase regulators phospholamban and sarcolipin9 ABC transporters: BtuCD, multidrug transporter, and molybdate uptake transporter General secretory pathway Sec translocon preprotein translocase SecY 10 Porters uniporters, symporters, antiporters Mitochondrial carrier proteins 11 Major Facilitator Superfamily Glycerol-3-hosphate transporter, Lactose permease, and Multidrug transporter EmrD 12 Resistance-nodulation-cell division multidrug efflux transporter AcrB, see multidrug resistance13 Dicarboxylate/amino acid:cation symporter proton glutamate symporter 14 Monovalent cation/proton antiporter Sodium/proton antiporter 1 NhaA 15 Neurotransmitter sodium symporter 16 Ammonia transporters 17 Drug/Metabolite Transporter small multidrug resistance transporter EmrE - the structures are retracted as erroneous 18 Alpha-helical channels including ion channels Voltage-gated ion channel like, including potassium channels KcsA and KvAP, and inward-rectifier potassium ion channel Kirbac 19 Large-conductance mechanosensitive channel, MscL 20 Small-conductance mechanosensitive ion channel MscS 21 CorA metal ion transporters 22 Ligand-gated ion channel of neurotransmitter receptors acetylcholine receptor 23 Aquaporins 24 Chloride channels 25 Outer membrane auxiliary proteins polysaccharide transporter 26 - α-helical transmembrane proteins from the outer bacterial membrane Enzymes Methane monooxygenase 27 Rhomboid protease 28 Disulfide bond formation protein DsbA-DsbB complex 29 Proteins with alpha-helical transmembrane anchors T cell receptor transmembrane dimerization domain 30 Cytochrome c nitrite reductase complex 31 Steryl-sulfate sulfohydrolase 32 Stannin 33 Glycophorin A dimer 34 Inovirus filamentous phage major coat protein 35 Pilin 36 Pulmonary surfactant-associated protein 37 Monoamine oxidases A and B 38, Cytochrome P450 oxidases 39, Corticosteroid 11β-dehydrogenases 40. Signal Peptide Peptidase 41 Membrane protease specific for a stomatin homolog 42 β-barrels composed of a single polypeptide chain Beta barrels from eight beta-strands and with shear number of ten n=8, S=10 43. They include: OmpA-like transmembrane domain OmpA, Virulence-related outer membrane protein family OmpX, Outer membrane protein W family OmpW, Antimicrobial peptide resistance and lipid A acylation protein family PagP Lipid A deacylase PagL, and Opacity family porins NspA Autotransporter domain n=12,S=14' 44 FadL outer membrane protein transport family, including Fatty acid transporter FadL n=14,S=14 45 General bacterial porin family, known as trimeric porins n=16,S=20 46 Maltoporin, or sugar porins n=18,S=22 47 Nucleoside-specific porin n=12,S=16 48 Outer membrane phospholipase A1n=12,S=16 49 TonB-dependent receptors and their plug domain. They are ligand-gated outer membrane channels n=22,S=24, including cobalamin transporter BtuB, FeIII-pyochelin receptor FptA, receptor FepA, ferric hydroxamate uptake receptor FhuA, transporter FecA, and pyoverdine receptor FpvA 50 Outer membrane protein OpcA family n=10,S=12 that includes outer membrane protease OmpT and adhesin/invasin OpcA protein 51 Outer membrane protein G porin family n=14,S=16 52 Note: n and S are, respectively, the number of beta-strands and the shear number 53 of the beta-barrel β-barrels composed of several polypeptide chains Trimeric autotransporter n=12,S=12 54 Outer membrane efflux proteins, also known as trimeric outer membrane factors n=12,S=18 including TolC and multidrug resistance proteins 55 MspA porin octamer, n=S=16 and α-hemolysin heptamer n=S=14 56. These proteins are secreted. See also Gramicidin A 57, a peptide that forms a dimeric transmembrane β-helix. It is also secreted by Gram-positive bacteria. References Booth, P.J., Templer, R.H., Meijberg, W., Allen, S.J., Curran, A.R., and Lorch, M. 2001. In vitro studies of membrane protein folding. Crit. Rev. Biochem. Mol. Biol. 36: 501-603. Bowie J.U. 2001. Stabilizing membrane proteins. Curr. Op. Struct. Biol. 11: 397-402. Bowie J.U. 2005. Solving the membrane protein folding problem. Nature 438: 581-589. DeGrado W.F., Gratkowski H. and Lear J.D. 2003. How do helix-helix interactions help determine the folds of membrane proteins? Perspectives from the study of homo-oligomeric helical bundles. Protein Sci. 12: 647-665. Lee, A.G. 2003 Lipid-protein interactions in biological membranes: a structural perspective. Biochim. Biophys. Acta 1612: 1-40. Lee, A.G. 2004. How lipids affect the activities of integral membrane proteins. Biochim. Biophys. Acta 1666: 62-87. le Maire, M., Champeil, P., and Moller, J.V. 2000. Interaction of membrane proteins and lipids with solubilizing detergents. Biochim. Biophys. Acta 1508: 86-111. Popot J-L. and Engelman D.M. 2000. Helical membrane protein folding, stability, and evolution. Annu. Rev. Biochem. 69: 881-922. Protein-lipid interactions Ed. L.K. Tamm Wiley, 2005. Tamm, L.K., Hong, H., and Liang, B.Y. 2004. Folding and assembly of beta-barrel membrane proteins. Biochim. Biophys. Acta 1666: 250-263. Additional examples Some cell adhesion proteins Some receptor proteins Insulin receptor GLUTI Integrin Cadherin External links TCDB - Transporter classification database from Milton H. Saier, Jr. laboratory TransportDB Genomics-oriented database of transporters from TIGR Membrane PDB Database of 3D structures of integral membrane proteins and hydrophobic peptides with an emphasis on crystallization conditions Membrane proteins of known 3D structure from Stephen White laboratory PDBTM All 3D models of transmembrane peptides and proteins currently in the PDB including theoretical models. Approximate positions of membrane boundary planes were calculated for each PDB entry. Orientations of proteins in membranes database - Calculated spatial positions of transmembrane, integral monotopic, and peripheral proteins in membranes See also cell membrane transmembrane receptors membrane topology transmembrane helix membrane protein integral membrane protein peripheral membrane proteinTransporter Classification database Retrieved from http://en..org/wiki/Transmembrane_protein Categories: Integral membrane proteins | Cell-surface receptors | Transmembrane receptors Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Deutsch Español Français Italiano עברית Polski Português This page was last modified on 9 September 2008, at 15:02
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