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.

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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

Main article: Transmembrane proteins

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.

Main article: Peripheral membrane protein

Three-dimensional structures of the following integral monotopic proteins have been determined:

• prostaglandin H2 syntheses 1 and 2 (cyclooxygenases) [1],
• lanosterol synthase and squalene-hopene cyclase [2],
• microsomal prostaglandin E synthase [3],
• carnitine O-palmitoyltransferase 2 [4].

There are also structures of integral monotopic domains of transmembrane proteins:

• monoamine oxidases A and B [5],
• fatty acid amide hydrolase [6],
• mammalian cytochrome P450 oxidases [7],
• corticosteroid 11-beta-dehydrogenases [8].

Such domains require detergents for extraction or crystallization, even after removal of their transmembrane helices. Therefore, they are often classified as integral monotopic proteins [9]

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.

References

• Booth, P.J., Templer, R.H., Meijberg, W., Allen, S.J., Curran, A.R., and Lorch, M. 2001. In vitro studies of membrane protein folding. Crit. Rev. Biochem. Mol. Biol. 36: 501-603.
• Bracey M.H., Cravatt B.F., Stevens R.C., Cravatt B.F. 2004. Structural commonalities among integral membrane enzymes. FEBS Lett. 567: 159-165.
• Bowie J.U. 2001. Stabilizing membrane proteins. Curr. Op. Struct. Biol. 11: 397-402.
• Bowie J.U. 2005. Solving the membrane protein folding problem. Nature 438: 581-589.
• DeGrado W.F., Gratkowski H. and Lear J.D. 2003. How do helix-helix interactions help determine the folds of membrane proteins? Perspectives from the study of homo-oligomeric helical bundles. Protein Sci. 12: 647-665.
• Popot J-L. and Engelman D.M. 2000. Helical membrane protein folding, stability, and evolution. Annu. Rev. Biochem. 69: 881-922.
• Protein-lipid interactions (Ed. L.K. Tamm) Wiley, 2005.

See also

• Membrane proteins
• Transmembrane proteins
• Peripheral membrane proteins

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