A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle. Membrane proteins can be classified into two groups, based on their attachment to the membrane.

Integral membrane proteins are firmly attached to the membrane. They can be defined as those proteins which require a detergent 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:

• Most proteins span the entire membrane (i.e. transmembrane protein or plolytopic protein). The region of the protein that is actually crossing the bilayer is in most cases an alpha helix with its hydrophobic side chains pointing outwards. Although some proteins only pass through the membrane once, others may be multipass, having multiple regions that cross the membrane.
• Proteins may anchor in only one layer (monotopic proteins). This may take the form of an amphipathic alpha helix embedded parallel within the layer.
• Also post-translationally modified proteins, with added fatty acid or prenyl chains, or GPI (glycosylphosphatidylinositol), which may be anchored and associated with a single lipid layer.

Peripheral membrane proteins are attached to integral proteins by non-covalent interaction, usually by electrostatic bonds. Peripheral proteins easily dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations. In either case, the lipid bilayer is not disrupted.

Molecular and Cellular Biology | Membrane biology | Membrane proteins

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