Editing Membrane protein (section)

===Integral membrane proteins===
[[File:Polytopic membrane protein.png|thumb|right|300px|Schematic representation of [[Transmembrane protein|transmembrane proteins]]: 1. a single transmembrane [[α-helix]] ([[bitopic membrane protein]]) 2. a polytopic transmembrane α-helical protein 3. a polytopic transmembrane [[β-sheet]] protein <br/>The membrane is represented in light-brown.]]

{{Main|Integral membrane protein|Transmembrane protein}}
[[Integral membrane protein]]s are permanently attached to the membrane. Such proteins can be separated from the biological membranes only using [[detergent]]s, [[nonpolar solvent]]s, or sometimes [[Denaturation (biochemistry)|denaturing]] agents.{{cn|date=June 2023}} They can be classified according to their relationship with the [[bilayer]]:

* [[Integral polytopic protein]]s are transmembrane proteins that span across the membrane more than once. These proteins may have different [[transmembrane topology]].<ref>{{cite journal |vauthors=[[Gunnar von Heijne|von Heijne G]] |date=December 2006 |title=Membrane-protein topology |journal=[[Nature Reviews Molecular Cell Biology|Nature Reviews. Molecular Cell Biology]] |volume=7 |issue=12 |pages=909–18 |doi=10.1038/nrm2063 |pmid=17139331 |s2cid=22218266}}</ref><ref name="Karp2009">{{cite book |author=Gerald Karp |url=https://books.google.com/books?id=arRGYE0GxRQC&pg=PA128 |title=Cell and Molecular Biology: Concepts and Experiments |publisher=[[Wiley (publisher)|John Wiley and Sons]] |year=2009 |isbn=978-0-470-48337-4 |pages=128– |access-date=13 November 2010 |url-access=limited |via=[[Google Books]]}}</ref> These proteins have one of two structural architectures:
**[[Helix bundle]] proteins, which are present in all types of [[biological membranes]];
** [[Beta barrel]] proteins, which are found only in [[Bacterial outer membrane|outer membrane]]s of [[Gram-negative bacteria]], and outer membranes of [[mitochondria]] and [[chloroplasts]].<ref name="Selkrig2014">{{cite journal |vauthors=Selkrig J, Leyton DL, Webb CT, Lithgow T |date=August 2014 |title=Assembly of β-barrel proteins into bacterial outer membranes |journal=Biochimica et Biophysica Acta (BBA) - Molecular Cell Research |volume=1843 |issue=8 |pages=1542–50 |doi=10.1016/j.bbamcr.2013.10.009 |pmid=24135059 |via=Elsevier [[Science Direct]] |doi-access=free}}</ref>
* [[Bitopic protein]]s are transmembrane proteins that span across the membrane only once. Transmembrane helices from these proteins have significantly different amino acid distributions to transmembrane helices from polytopic proteins.<ref>{{cite journal | vauthors = Baker JA, Wong WC, Eisenhaber B, Warwicker J, Eisenhaber F | title = Charged residues next to transmembrane regions revisited: "Positive-inside rule" is complemented by the "negative inside depletion/outside enrichment rule" | journal = BMC Biology | volume = 15 | issue = 1 | pages = 66 | date = July 2017 | pmid = 28738801 | pmc = 5525207 | doi = 10.1186/s12915-017-0404-4 | doi-access = free }}{{open access}}</ref>
* [[Integral monotopic protein]]s are integral membrane proteins that are attached to only one side of the membrane and do not span the whole way across.