Also see transmembrane protein.

A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. More than half of all proteins interact with membranes.

Biological membranes consist of a phospholipid bilayer and a variety of proteins that accomplish vital biological functions. Structural proteins are attached to microfilaments in the cytoskeleton which ensures stability of the cell. Cell recognition proteins allow cells to identify each other and interact. Such proteins are involved in immune response, for example. Membrane enzymes produce a variety of substances essential for cell function. Membrane receptor proteins serve as connection between the cell's internal and external environments. Finally, transport proteins play an important role in the maintenance of concentrations of ions. These transport proteins come in two forms: carrier proteins and channel proteins. Carrier proteins are involved in using the energy released from ATP being broken down to facilitate active transport and ion exchange. These processes ensure that useful substances are able to enter the cell and that toxic substances are pumped out of the cell.

Contents 1 Main categories 1.1 Integral membrane proteins 1.2 Peripheral membrane proteins 1.3 Polypeptide toxins 2 Intracellular localization 3 Further reading 4 See also 5 References 6 External links

[edit] Main categories

[edit] Integral membrane proteins

Integral membrane proteins are permanently attached to the membrane. They can be defined as those proteins which require a detergent (such as SDS or Triton X-100) or some other apolar solvent to be displaced. They can be classified according to their relationship with the bilayer:

• Transmembrane proteins span the entire membrane. The transmembrane regions of the proteins are either beta-barrels or alpha-helical. The alpha-helical domains are present in all types of biological membranes including outer membranes. The beta-barrels were found only in outer membranes of Gram-negative bacteria, lipid-rich cell walls of a few Gram-positive bacteria, and outer membranes of mitochondria and chloroplasts.

• Integral monotopic proteins are permanently attached to the membrane from only one side.

[edit] Peripheral membrane proteins

Peripheral membrane proteins are temporarily attached either to the lipid bilayer or to integral proteins by a combination of hydrophobic, electrostatic, and other non-covalent interactions. Peripheral proteins dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations.

Integral and peripheral proteins may be post-translationally modified, with added fatty acid or prenyl chains, or GPI (glycosylphosphatidylinositol), which may be anchored in the lipid bilayer.

[edit] Polypeptide toxins

Classification of membrane proteins to integral and peripheral does not include some polypeptide toxins, such as colicin A or alpha-hemolysin, and certain proteins involved in apoptosis. These proteins are water-soluble but can aggregate and associate irreversibly with the lipid bilayer and form alpha-helical or beta-barrel transmembrane channels. An alternative classification is to divide all membrane proteins to integral and amphitropic.^[1] The amphitropic are proteins that can exist in two alternative states: a water-soluble and a lipid bilayer-bound, whereas integral proteins can be found only in the membrane-bound state. The amphitropic protein category includes water-soluble channel-forming polypeptide toxins, which associate irreversibly with membranes, but excludes peripheral proteins that interact with other membrane proteins rather than with lipid bilayer.

[edit] Intracellular localization

Proteins are specifically targeted to many different types of biological membranes ^[2]

[edit] Further reading

• Protein-lipid interactions (Ed. L.K. Tamm) Wiley, 2005.
• Popot J-L. and Engelman D.M. 2000. Helical membrane protein folding, stability, and evolution. Annu. Rev. Biochem. 69: 881-922.
• Bowie J.U. 2005. Solving the membrane protein folding problem. Nature 438: 581-589.
• Cho, W. and Stahelin, R.V. 2005. Membrane-protein interactions in cell signaling and membrane trafficking. Annu. Rev. Biophys. Biomol. Struct. 34: 119–151.
• Goni F.M. 2002. Non-permanent proteins in membranes: when proteins come as visitors. Mol. Membr. Biol. 19: 237-245.
• Johnson J.E. and Cornell R.B. 1999. Amphitropic proteins: regulation by reversible membrane interactions. Mol. Membr. Biol. 16: 217-235.
• Seaton B.A. and Roberts M.F. Peripheral membrane proteins. pp. 355-403. In Biological Membranes (Eds. K. Mertz and B.Roux), Birkhauser Boston, 1996.
• Dürr U.H.N., Waskell L., and Ramamoorthy A. The cytochromes P450 and b5 and their reductases-Promising targets for structural studies by advanced solid-state NMR spectroscopy, 2007. BBA Biomembranes 1768: 3235-3259.

[edit] See also

• Integral membrane proteins
• Transmembrane proteins
• Peripheral membrane proteins
• Ion pump (biology)
• Carrier protein
• Ion channel
• Receptor (biochemistry) (including G protein-coupled receptor)
• Wikipedia:MeSH_D12.776#MeSH_D12.776.543_---_membrane_proteins

[edit] References

[edit] External links

• General Principles of Membrane Protein Folding and Stability from Stephen White laboratory
• Orientations of Proteins in Membranes (OPM) database 3D structures of integral and amphitropic membrane proteins
• MeSH Membrane+proteins
• The Human Membrane Proteome - A comprehensive article covering the transmembrane protein component of the human proteome

v • d • e Proteins Processes Protein biosynthesis - Posttranslational modification - Protein folding - Protein targeting - Proteome - Protein methods Structures Protein structure - Protein structural domains - Proteasome Types List of types of proteins - List of proteins - Membrane protein - Globular protein (Globulin, Albumin) - Fibrous protein

v • d • e Protein: membrane proteins / membrane glycoproteins / integral membrane proteins Transmembrane protein Transmembrane receptors (CD33) Peripheral membrane protein Lipid anchored protein Pore-forming toxin antimicrobial cationic peptide (Cathelicidin, Cecropin, Defensin, Dermcidin, Histatin, Magainin, Melitten, Polymyxin, Thionin) · Bacteriocin · Gramicidin (Gramicidin S) · Hemolysin · Leukocidin · Perforin · Streptolysin Transfer/transport Membrane transport proteins · Phospholipid transfer proteins · Vesicular transport proteins Cytoskeleton Ankyrin · Dystrophin · Spectrin · Utrophin Ungrouped Arrestin · Calnexin · Connexin · Ephrin · Heterotrimeric GTP-binding proteins · LDL-receptor-related protein associated protein · Membrane fusion protein (VAMP) · Myelin basic protein · Neurofibromin 2 · Presenilin (PSEN1, PSEN2) · Protoporphyrinogen oxidase · pulmonary surfactant (Pulmonary surfactant-associated protein B, Pulmonary surfactant-associated protein C) · Tetraspanin · Peroxin