Integral membrane protein

An Integral Membrane Protein (IMP) is a protein molecule (or assembly of proteins) that is permanently attached to the biological membrane. Such proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes denaturing agents.

IMPs comprise a very significant fraction of the proteins encoded in the genome.

Structure
3D structures of only ~160 different integral membrane proteins are currently determined at atomic resolution by X-ray crystallography or Nuclear magnetic resonance spectroscopy due to the difficulties with extraction and crystallization. In addition, structures of many water-soluble domains of IMPs are available in the Protein Data Bank. Their membrane-anchoring α-helices have been removed to facilitate the extraction and crystallization.

IMPs can be divided into two groups:
 * 1) Transmembrane proteins
 * 2) Integral monotopic proteins

Integral transmembrane protein
Transmembrane proteins span the entire biological membrane. This is the most common type of IMP.

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

Three-dimensional structures of the following integral monotopic proteins have been determined: There are also structures of integral monotopic domains of transmembrane proteins: Such domains require detergents for extraction or crystallization, even after removal of their transmembrane helices. Therefore, they are often classified as integral monotopic proteins
 * prostaglandin H2 synthases 1 and 2 (cyclooxygenases) ,
 * lanosterol synthase and squalene-hopene cyclase ,
 * microsomal prostaglandin E synthase ,
 * carnitine O-palmitoyltransferase 2.
 * monoamine oxidases A and B ,
 * fatty acid amide hydrolase ,
 * mammalian cytochrome P450 oxidases ,
 * corticosteroid 11-beta-dehydrogenases.

Function
IMPs include transporters, channels, receptors, enzymes, structural membrane-anchoring domains, proteins involved in accumulation and transduction of energy, and proteins responsible for cell adhesion. Classification of transporters can be found in TCDB database.

Examples
Examples of integral membrane proteins:
 * Integrin
 * Cadherin
 * Insulin receptor
 * NCAM
 * Selectin
 * Some types of cell adhesion proteins
 * Some types of receptor proteins
 * Glycophorin
 * Rhodopsin
 * Band 3
 * CD36