Membrane protein
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{{Mergefrom|Protein Function in Cell Membranes|date=December 2007}}
:''Also see [[transmembrane protein]]''.
A '''membrane protein''' is a [[protein]] molecule that is attached to, or associated with the [[membrane (biology)|membrane]] of a [[cell (biology)|cell]] or an [[organelle]]. More than half of all proteins interact with membranes. Membrane proteins can be classified into two groups, based on the strength of their association with the membrane.
==Main categories==
[[Integral membrane protein]]s are permanently attached to the membrane. They can be defined as those proteins which require a [[detergent]] (such as [[Sodium dodecyl sulfate|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-barrel]]s or [[alpha helix|alpha-helical]]. The alpha-helical domains are present in all types of [[biological membranes]] including [[outer membrane]]s. The [[beta-barrel]]s were found only in [[outer membrane]]s of [[Gram-negative bacteria]], lipid-rich [[cell walls]] of a few [[Gram-positive bacteria]], and [[outer membrane]]s of [[mitochondria]] and [[chloroplasts]].
*Integral monotopic proteins are permanently attached to the membrane from only one side.
[[Peripheral membrane protein]]s 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.
{{further|[[Integral membrane proteins]], [[Transmembrane proteins]], [[Peripheral membrane proteins]]}}
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''.<ref name="pmid10503244">{{cite journal |author=Johnson JE, Cornell RB |title=Amphitropic proteins: regulation by reversible membrane interactions (review) |journal=Mol. Membr. Biol. |volume=16 |issue=3 |pages=217–35 |year=1999 |pmid=10503244 |doi=}}</ref> 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.
==Membrane-bound peptides==
There are also numerous membrane-associated peptides, some of which are [[nonribosomal peptide]]s. They can form transmembrane channels (for example, [[gramicidin]]s and [[peptaibols]]<ref name="titleCrystallography Department, Birkbeck College - Peptaibol Database">{{cite web |url=http://www.cryst.bbk.ac.uk/peptaibol/home.shtml |title=Crystallography Department, Birkbeck College - Peptaibol Database |accessdate=2007-12-18 |format= |work=}}</ref><ref>{{cite web |url=http://opm.phar.umich.edu/families.php?superfamily=76 |title=Orientations of Proteins in Membranes (OPM) database |accessdate=2007-12-18 |format= |work=}}</ref>), travel across the membrane as [[ionophores]] ([[valinomycin]] and others), or associate with lipid bilayer surface, as [[daptomycin]] and other [[lipopeptide]]s. These peptides are usually secreted. So, they probably should be classified as ''amphitropic'', although some of them are poorly soluble in water and associate with membrane irreversibly.
==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.
==See also==
* [[Integral membrane protein]]s
* [[Transmembrane protein]]s
* [[Peripheral membrane protein]]s
* [[Ion pump (biology)]]
* [[Carrier protein]]
* [[Ion channel]]
* [[Receptor (biochemistry)]] (including [[G protein-coupled receptor]])
* [[Wikipedia:MeSH_D12.776#MeSH_D12.776.543_---_membrane_proteins]]
==References==
{{reflist}}
==External links==
* [http://blanco.biomol.uci.edu/mp_assembly.html General Principles of Membrane Protein Folding and Stability ] from Stephen White laboratory
* [http://opm.phar.umich.edu/ Orientations of Proteins in Membranes (OPM) database] 3D structures of integral and amphitropic membrane proteins
* {{MeshName|Membrane+proteins}}
{{Protein topics}}
{{Membrane proteins}}
[[Category:WikiProject Molecular and Cellular Biology articles]]
[[Category:Membrane biology]]
[[Category:Membrane proteins|*]]
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