Membrane protein

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A membrane protein is a protein that is associated with the cell membrane or a biomembrane of cell compartments or organelles of a cell .

A distinction is made between peripheral membrane proteins , as proteins bound to the membrane surface, and integral membrane proteins , which are integrated with a hydrophobic component in the double lipid layer of the membrane and mostly span them as transmembrane proteins . Around 25 to 30 percent of all naturally occurring proteins are stored in cell membranes. The term proteolipids is an outdated term for hydrophobic integral membrane proteins. In contrast to lipoproteins, proteolipids are not water-soluble.

properties

The integral membrane proteins include the transmembrane proteins, while the peripheral membrane proteins include the membrane-based proteins. The difference lies in whether they completely or partially penetrate the biomembrane or whether they are only attached to or anchored in the hydrophilic surface of the lipid bilayer . Cellular proteins with parts on the cell surface are called surface proteins .

Many surface proteins have been discovered as CD antigens . Viral proteins on the surface of a virion are also known as surface proteins and often bind antibodies there , but only in the case of enveloped viruses are they membrane proteins with the characteristic hydrophobic amino acid sequences . Proteins and carbohydrates with at least a portion on the cell surface can trigger a humoral immune response as antigens and are referred to as surface antigens in relation to the immune system .

The structure of the proteins depends on the condition. A protein anchored in the membrane often has phosphorylation , glycosylation and myristoylation sites , which post-translationally influence the tertiary structure .

Membrane proteins are of great medical importance: around half of all currently approved drugs act on this type of protein as a therapeutic target.

Functions

Membrane proteins are key functional proteins because they are an important part of the structural delimitation of the organelles ; they also play a crucial role in almost all cellular functions. To be mentioned here are among others:

  1. Enzyme activity
  2. Signal transmission
  3. Transport (proteins, ions, etc.)
  4. Anchoring to the cytoskeleton and extracellular matrix
  5. Cell-cell recognition
  6. Cell connection

Types

Membrane proteins usually take on important physiological tasks, e.g. B. as cell adhesion molecules , translocases or receptors of signal pathways . Examples are porins (e.g. aquaporins , important for the water and energy balance) and ABC transporters as well as light receptors such as bacteriorhodopsin .

One differentiates between the transmembrane proteins functionally. Transporters and channels are essential to u. A. to ensure the ion balance of a cell, enzymes catalyze important metabolic processes, receptors are used to transmit signals across a lipid layer, connexins establish direct connections between cells, etc.

Transporter classes
  1. ABC transporter
  2. Ion channels
  3. Membrane-bound ATPases
  4. SLC transporter
  5. Water channels (see Aquaporine )

Hydrophobicity

In the transmembrane region (transmembrane domain of about eight to twelve amino acids), transmembrane proteins are built up from the less polar or more hydrophobic amino acids that enter into a protein-lipid interaction with the lipids of the membrane .

Web links

  • Cell, open up! On: Wissenschaft.de of December 7, 2005. American scientists watched the sluices in the cell membrane while working.

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Individual evidence

  1. ↑ Tan, S .: Membrane proteins and membrane proteomics . In: Proteomics . 8, No. 19, 2008, pp. 3924-3932. PMID 18763712 .
  2. ↑ Carpenter, EP et al .: Overcoming the challenges of membrane protein crystallography . In: Curr Opin Struct Biol . 18, No. 5, 2008, pp. 581-586. PMID 18674618 .
  3. ↑ a b c Baker, M .: Making membrane proteins for structures: a trillion tiny tweaks . In: Nat Methods . 7, No. 6, 2010, pp. 429-434. PMID 20508636 .
  4. ↑ JM Lackie: The Dictionary of Cell and Molecular Biology , 4th Edition, 2007, Academic Press, ISBN 978-0-12-373986-5 . P. 347.
  5. ↑ FA Pezold: Lipids and lipoproteins in the blood plasma. Springer, 1961. ISBN 3642873669 . P. 22.
  6. ↑ Sadowski, PG: Sub-cellular localization of membrane proteins . In: Proteomics . 8, No. 19, 2008, pp. 3991-4011. PMID 18780351 .