in SDS micelles has four transmembrane ?-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 ?-helical transmembrane proteins

Refolding of ?-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 transmembrane ?-helices. A relatively polar amphiphilic ?-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 ?-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 beta-barrel transmembrane proteins

Stability of beta barrel (?-barrel) transmembrane proteins is similar to stability of water-soluble proteins, based on chemical denaturation studies. Some of them are very stable even in chaotropic agents and high temperature. Their folding in vivo is facilitated by water-soluble chaperones, such as protein Skp. It is thought that ?-barrel membrane proteins come from one ancestor even having different number of sheets which could be added or doubled during evolution. Some studies show a huge sequence conservation among different organisms and also conserved amino acids which hold the structure and help with folding.

3D structures

Light absorption-driven transporters

* Bacteriorhodopsin-like proteins including rhodopsin (see also opsin)
* Bacterial photosynthetic reaction centres and photosystems I and II
* Light-harvesting complexes from bacteria and chloroplasts

Oxidoreduction-driven transporters

* Transmembrane cytochrome b-like proteins: 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 from bacteria and mitochondria

Electrochemical potential-driven transporters

* Proton or sodium translocating F-type and V-type ATPases

P-P-bond hydrolysis-driven transporters

* P-type calcium ATPase (five different conformations)
* Calcium ATPase regulators phospholamban and sarcolipin
* ABC transporters
* General secretory pathway (Sec) translocon (preprotein translocase SecY)

Porters (uniporters, symporters, antiporters)

* Mitochondrial carrier proteins
* Major Facilitator Superfamily (Glycerol-3-phosphate transporter, Lactose permease, and Multidrug transporter EmrD)
* Resistance-nodulation-cell division (multidrug efflux transporter AcrB, see multidrug resistance)
* Dicarboxylate/amino acid:cation symporter (proton glutamate symporter)
* Monovalent cation/proton antiporter (Sodium/proton antiporter 1 NhaA)
* Neurotransmitter sodium symporter
* Ammonia transporters
* Drug/Metabolite Transporter (small multidrug resistance transporter EmrE - the structures are retracted as erroneous)

Alpha-helical channels including ion channels

* Voltage-gated ion channel like, including potassium channels KcsA and KvAP, and inward-rectifier potassium ion channel Kirbac
* Large-conductance mechanosensitive channel, MscL
* Small-conductance mechanosensitive ion channel (MscS)
* CorA metal ion transporters
* Ligand-gated ion channel of neurotransmitter receptors (acetylcholine receptor)
* Aquaporins
* Chloride channels
* Outer membrane auxiliary proteins (polysaccharide transporter) - ?-helical transmembrane proteins from the outer bacterial membrane

Enzymes

* Methane monooxygenase
* Rhomboid protease
* Disulfide bond formation protein (DsbA-DsbB complex)

Proteins with single transmembrane alpha-helices

* Subunits of T cell receptor complex
* Cytochrome c nitrite reductase complex
* Glycophorin A dimer
* Inovirus (filamentous phage) major coat protein
* Pilin
* Pulmonary surfactant-associated protein
* Monoamine oxidases A and B
* Fatty acid amide hydrolase
* Cytochrome P450 oxidases
* Corticosteroid 11?-dehydrogenases .
* Signal Peptide Peptidase

Beta-barrels composed of a single polypeptide chain

* Beta barrels from eight beta-strands and with shear number of ten (n=8, S=10). 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
** Opacity family porins (NspA)
* Autotransporter domain (n=12,S=14)
* FadL outer membrane protein transport family, including Fatty acid transporter FadL (n=14,S=14)
* General bacterial porin family, known as trimeric porins (n=16,S=20)
* Maltoporin, or sugar porins (n=18,S=22)
* Nucleoside-specific porin (n=12,S=16)
* Outer membrane phospholipase A1(n=12,S=16)
* TonB-dependent receptors and their plug domain. They are ligand-gated outer membrane channels (n=22,S=24), including cobalamin transporter BtuB, Fe(III)-pyochelin receptor FptA, receptor FepA, ferric hydroxamate uptake receptor FhuA, transporter FecA, and pyoverdine receptor FpvA
* Outer membrane protein OpcA family (n=10,S=12) that includes outer membrane protease OmpT and adhesin/invasin OpcA protein
* Outer membrane protein G porin family (n=14,S=16)

Note: n and S are, respectively, the number of beta-strands and the shear number of the beta-barrel

Beta-barrels composed of several polypeptide chains

* Trimeric autotransporter (n=12,S=12)
* Outer membrane efflux proteins, also known as trimeric outer membrane factors (n=12,S=18) including TolC and multidrug resistance proteins
* MspA porin (octamer, n=S=16) and ?-hemolysin (heptamer n=S=14) . These proteins are secreted.