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.
There are two basic types of transmembrane proteins:
Alpha-helical. These proteins are present in all types of biological membranes including outer membranes. This is the major category of 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 detergentmicelles 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].
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. 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.
PDBTMAll 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.
