Integral membrane protein

An integral membrane protein (IMP) is a protein molecule (or assembly of proteins) that is permanently attached to the biological membrane. Proteins that cross the membrane are surrounded by "annular" lipids, which are defined as lipids that are in direct contact with a membrane protein. 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 an organism's genome.

All transmembrane proteins are IMPs, but not all IMPs are transmembrane proteins.

Structure
Three-dimensional 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) Integral polytopic proteins (Transmembrane proteins)
 * 2) Integral monotopic proteins

Integral Polytopic Protein
The most common type of IMP is the transmembrane protein (TM), which spans the entire biological membrane. Single-pass membrane proteins cross the membrane only once, while multi-pass membrane proteins weave in and out, crossing several times. Single pass TM proteins can be categorized as Type I, which are positioned such that their carboxy-terminus is towards the cytosol, or Type II, which have their amino-terminus towards the cytosol.

Integral Monotopic Proteins
Integral monotopic proteins, are permanently attached to the membrane from one side. Such domains require detergents for extraction or crystallization, even after removal of their transmembrane helices. Therefore, they are often classified as integral monotopic proteins

Determination of Protein Structure
The use of hydropathy plots helps determine integral protein structures based on the hydrophobic and hydrophilic characteristics of alpha helical integral proteins.

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 Transporter Classification database.

Examples
Examples of integral membrane proteins:


 * Insulin receptor
 * Some types of cell adhesion proteins or cell adhesion molecules (CAMs) such as Integrins, Cadherins, NCAMs, or Selectins.
 * Some types of receptor proteins
 * Glycophorin
 * Rhodopsin
 * Band 3
 * CD36
 * GPR30