Membrane proteins

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. Membrane proteins can be classified into two groups, based on the strength of their association with the membrane.

Main categories
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, cell wall of Gram-positive bacteria, and outer membranes of mitochondria and chloroplasts.


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

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.

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 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 peptides. They can form transmembrane channels (for example, gramicidins and peptaibols ), travel across the membrane as ionophores (valinomycin and others), or associate with lipid bilayer surface, as daptomycin and other lipopeptides. 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.