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.
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 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
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
Light absorption-driven transporters
Electrochemical potential-driven transporters
- Proton or sodium translocating F-type and V-type ATPases
P-P-bond hydrolysis-driven transporters
- Mitochondrial carrier proteins
- Major Facilitator Superfamily (Glycerol-3-hosphate 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
Proteins with alpha-helical transmembrane anchors
β-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:
- 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
β-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 α-hemolysin (heptamer n=S=14) These proteins are secreted.
See also Gramicidin A , a peptide that forms a dimeric transmembrane β-helix. It is also secreted by Gram-positive bacteria.
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- Protein-lipid interactions (Ed. L.K. Tamm) Wiley, 2005.
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Transporter Classification database