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The cell. 3. Cell membrane

PROTEINS

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Many cell membrane functions rely on membrane proteins.

There are two groups of proteins: integral and associated.

Integral proteins (peripheral and transmembrane) are permanent components of membranes.

Associated proteins (peripheral) are transiently joined to membranes by electrical forces.

Many functions of cell membranes rely on proteins. There are two main groups of proteins regarding how they are associated to membranes: integral and associated.

Integral proteins

Transmembrane proteins
Types of transmembrane proteins. There are transmembrane proteins, such as glycophorin, that have an amino acid chain that spans once the cell membrane, whereas others, such as many receptors, can have several crosses. In these examples the amino acid chain located among the fatty acids shows an alpha helix structure. Aquaporin is also a transmembrane protein forming a channel with several crosses, but the amino acid chain among fatty acid show a beta structure (modified from Pollard et al., 2007).

Proteins which are permanent components of the membrane are known as integral proteins. They may be transmembrane proteins, proteins spanning just one monolayer or proteins that are chemically linked to a molecule which is part of the membrane.

Transmembrane proteins show three molecular domains: extracellular (if they are in the plasma membrane), intra-membrane and cytosolic. They contain sequences of hydrophobic amino acids that are located among the fatty acid chains of membrane lipids, whereas the extra and intracellular parts contain hydrophilic amino acids. These proteins are mostly synthesized in the endoplasmic reticulum and distributed to other cell membranes by vesicular traffic.

Some transmembrane proteins have amino acid sequences that span the membrane one time, whereas others can cross it up to 7 times. Although most of transmembrane proteins work alone, many others need to be associated to other proteins to carry out their functions working as oligomeric groups. The functions of the transmembrane proteins are diverse. a) Integrins, cadherins, and selectins are adhesion proteins. b) Bombs and ion channels generate and modify ion gradients between both sides of the membrane, for example, to produce ATP and to interchange ions, such as calcium, sodium, and potassium. c) Transporters introduce in the cell molecules such as glucose across the plasma membrane. d) Receptors allow cell communication by binding to their ligands, such as hormones, growth factors, neurotransmitters, and others, and transducing these signals into intracellular downstream molecular events. This can be done because transmembrane proteins have two hydrophilic domains, located at both sides of the membrane, respectively, connected by one or severalintramembrane domains. For example, an extracellular signal triggers a molecular conformational change which is transmitted to the intracellular domain, which in turn triggers a cytosolic molecular process, sometimes ending up with changes in gene expression.

Peripheral proteins

Main types of peripheral membrane proteins: integral and associated. From left to the right: proteins with a part of the amino acid chain spanning just one monolayer, proteins linked to glycolipids, proteins linked to fatty acid chains inserted among the membrane lipids, proteins associated to the membrane lipids heads by electric affinity and proteins associated to hydrophilic domain of transmembrane proteins (modified from Alberts et al., 2002).

There are integral proteins which are not transmembrane proteins. Some of them have part of the amino acid sequence inserted in only one of the monolayers of the membrane, so they have one intramembrane domain and one external domain, either cytosolic or extracellular (in they are in the plasma membrane), depending on the monolayer where they are integrated. Another type of integral proteins are those anchored to other membrane molecule, mostly glycolipids, by chemical bonds. Other proteins may be chemically linked to fatty acids which are inserted between the lipid fatty acids. In these cases, the entire protein is cytosolic or extracellular, that is why they are also known as peripheral proteins. Associated proteins are also classified as peripheral proteins (see below).

Associated proteins

Associated proteins are not permanently joined to membranes, i.e. they are not integral proteins. Electrical interactions, such as van der Waals forces, instead of chemical bonds, keep these proteins sticked to the hydrophilic surfaces of membranes. These forces are weak and proteins may switch between be attached to or detached from membranes. Associated proteins are hydrosoluble.


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Updated: 2017-09-11. 13:02