Integral and peripheral membrane proteins

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Integral Membrane Proteins

These are proteins directly linked to the plasma membrane of cells. They are also considered as proteins that fully pass through the membrane (carrier proteins and channel proteins). Transmembrane proteins can be single-pass, (pass through the membrane once), or multipass (pass through the membrane more than once). These transmembrane proteins have both hydrophilic and hydrophobic regions; they are amphiphilic[1]. The hydrophobic regions are the regions inside the lipid membrane, so they are kept away from water, whereas the hydrophilic regions are on either side of the membrane[2]. The hydrophobic regions interact with the hydrophobic tails on lipids inside the membrane. The transmembrane domains are usually made up of alpha-helices, although can be made up of beta-sheets rolled up into a beta-barrell.

There is also a group of proteins that do not cross the lipid bilayer but are attached to only one side of it. That can be because:

They do not readily dissociate with the cell membrane, and can therefore only be separated from the plasma membrane through the use of detergents (and other agents) to disrupt the plasma membrane.

Peripheral Membrane Proteins

These proteins are connected to the plasma membrane through intermediate molecules (even integral proteins) by noncovalent interactions. They are released from the plasma membrane by agents that disrupts protein-protein interactions, e.g. by simply increasing the salt concentration of the buffer solution which the cells are suspended in.

Membrane proteins association with lipid bilayer

Figure 1. Various ways in which proteins associate with the lipid bilayer
Figure 1. Various ways in which proteins associate with the lipid bilayer[4].
  1. single α-helix
  2. multiple α-helices
  3. β-barrel
  4. association by a hydrophobic face of the α-helix
  5. attached to a lipid
  6. attached via oligosaccharide linker
  7. 8. membrane-associated proteins

References

  1. Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2008) Molecular Biology of the Cell, 5th Edition, Pages 629-630, New York: Garland Science
  2. Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2008) Molecular Biology of the Cell, 5th Edition, Pages 630, New York: Garland Science
  3. Alberts B et al. Molecular Biology of the Cell. 6th ed., New York: Garland Science, Taylor and; Francis Group, LLC. 2015
  4. Alberts B et al. Molecular Biology of the Cell. 6th ed., New York: Garland Science, Taylor and Francis Group, LLC. 2015
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