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The cell. 2. Extracellular matrix.

STRUCTURAL PROTEINS


Collagen and elastin are structural proteins of the extracellular matrix.

Collagen
Most abundant protein of the extracellular matrix.
Made up of repeats of 3 polypeptides alpha chains.
Mainly organized in fibers.
It resists mechanical forces.

Elastin
Part of elastic fibers.
It allows tissue recovery after mechanical stretching.

Extracellular matrix of animal tissues is a molecular network mainly composed of proteins, proteoglycans, glycosaminoglycans, and glycoproteins, that comine together to form a diverse set of extracellular matrices. The more abundant structural protein are collagen and elastin.

Collagen

Collagens are a family of proteins found in animals, so abundant that they can be up to 25 to 35 % of the total protein content of the body. Actually, they are the most abundant proteins of the body. Traditionally, collagen has been used to produce glue and adhesives; that is why the name collagen (from Greek kolla: glue). In vertebrates, there are more than 40 genes for polypeptides known as alpha chains, and three alpha chains join together to form a triple helix, which is the repeated unit used for making macromolecular collagen structures. The combination of different alpha chains makes up to 28 types of collagen. In tissues, the main function of collagen is providing mechanical support and withstand tensile forces, working as the steel bars inside the pillars of buildings. Collagen molecules may be organized in several ways, such as fibers and sheets. Cells bind to collagen molecules through a number of membrane adhesion proteins such as integrins, immunoglobulins, annexins, and some others. Other molecules of the extracellular matrix like proteoglycans and glycoproteins are also able to recognize and bind to collagen molecules.

Main features of collagen:

a) A distinct amino acid composition. Glycine is very abundant in the collagen molecule, together with proline and hydroxyproline. Glycine is found every three amino acids in the sequence x-y-Gly-x-y-Gly-x-y-Gly-, where x and y are often proline and hydroxyproline, respectively. Thanks to the low molecular size of the glycine amino acid, this repeated sequence enables polypeptides to adopt left-handed helix conformations, i.e., alpha chains.

Collagen fibers

Collagen fibers in the extracellular matrix of the instestine. Scanning electron microscopy.


Dermal collagen

Collagen fibers in the dermis.
Masson trichome staining.


Collagen fibers

Collagen fibers in the intestinal mucosa. Transmission electron microscopy.

b) Collagen can be arranged in fibers, sheets, or it may work as a linking/anchoring molecule. It depends on the amino acid composition of the alpha chains and on the type of alpha chains in the triple helix. The combination of alpha chains may be homotypic (the three alpha chais are the same) or heterotypic (there are different alpha chains).

Fibers. Collagen is mostly arranged in fibers made up of aligned and repeated collagen molecules. Collagen type I is the most abundant of fibrillar collagens. It is abundant in bones, cartilage and skin, and is up to 90 % of the total collagen content of the body. Other types of fibrillar collagens are type II, mainly located in cartilage, and type III, found in skin and blood vessels.

Sheets. There are collagen molecules forming molecular networks arranged in sheets. They are found surrounding organs, in the basal part of epithelial tissues, and around muscle fibers. For example, type IV collagen is a main component of the basal lamina located between epithelia and connective tissue.

Cross-linking. These collagens anchor extracellular molecules to other collagens arranged in either fibrils or sheets. For example, type IX collagen binds both glycosaminoglycans and type II collagen fibers, so that they form a cross-linked set of molecules.

Collagen synthesis

Steps during the collagen synthesis.


Moreover, there are some collagen molecules containing sequences of hydrophobic amino acids inserted in the plasma membrane. For example, type XIII and XVII collagens. Collagen type XVII is part of the molecular structure of hemidesmosomes.

No matter the type, collagen is synthesized inside the cell as a precursor: the pro-collagen molecule. First, alpha chains are synthesized in the endoplasmic reticulum, where proline becomes hydroxyproline by hydroxylation. Proline and hydroxyproline may be up to 20 % of the alpha chain. GLycosylation (type O-glycosylation) of alpha chains also occurs in the reticulum. At this point, three alpha chains join together by hydrogen bonds to finally form the pro-collagen molecule. Disulfide bridges are also formed between the alpha chains. Pro-collagen molecules are recognized and linked by transmembrane receptors and packaged in COPII coated vesicles. Typical COPII vesicles are 60 to 90 nm in diameter, but COPII vesicles transporting pro-collagen are 500 nm in diameter because pro-collagen molecules are around 300 nm in length. It is not known how pro-collagen molecules arrive at the Golgi apparatus, cross it, and are released from the TGN to the extracellular space by exocytosis. It is of notice that some cells are able to select different cell membrane domains for releasing different types of collagen. During or after the releasing process, terminal amino acid sequences are enzymatically removed from alpha chains of the pro-collagen molecules so that pro-collagen becomes collagen molecules. These terminal sequences were preventing spontaneous assembling of collagen molecules inside the cell (see the figure).

In fibrillar collagens, collagen molecules are spontaneously assembled into microfibrils that in turn join together to form collagen fibrils. Types V and XI collagens have been suggested to collaborate during the formation of these collagen fibers, and other molecules like proteoglycans influence the size and organization of these fibers. During the final assembling stage, fibers get stabilized by enzymes like lysyl oxidase.

Collagen synthesis

Transmission electron microscopy image of connective tissue of an invertebrate, sea ear gasteropod. Black asterisks indicate assembled collagen fibers in the extracellular space. White asterisks indicate large vesicles filled with pro-collagen molecules. White arrow indicates a likely releasing site for vesicles containing pro-collagen.


Collagen is mainly synthesized by fibroblasts, myofibroblasts, osteoblasts and chondrocytes. Some collagen types are synthesized by other cells such as epithelial cells.

Elastic fibers

Fibras de elastina

Elastic fibers of connective tissue

Unlike collagen fibers, elastic fibers can be stretched in response to tensile forces and later are able to recover their original length. They are mainly found in the dermis, artery walls, elastic cartilage and connective tissue of lungs. The elasticity of our tissues mostly depend on the elastic fibers. Other functions of elastic fibers are mechanical support of tissues and regulation of the activity of growth factors such as TGF-β. Elastic fibers are mostly made up of elastin, up to 90 %, and fibrillin, but also contain a low proportion glycoproteins and proteoglycans.

Elastic fiber

Parts of a elastic fiber. Lysine amino acid rich regions allow elastin fibers to be cross-linked. (Modified from Kielty, 2007).

Elastin contains long amino acid sequences with many hydrophobic amino acids, separated between each other by couples of glycine and other small amino acids. This distinct amino acid sequences provides the elastic properties, because hydrophobic amino acids allow elastin to be a rolled up structure, and lysine permit an alpha helix structure. Non hydrophobic amino acids are the points where elastin molecules are cross-linked. Tensile forces stretch elastin molecules. But they can later recover the original conformation thanks to hydrophobic amino acids which try to avoid the water environment. Elastin appears to be a vertebrate invention because it has not been found in invertebrates.


Bibliography

Heino J. 2007. The collagen family members as cell adhesion proteins. Bioessays. 29:1001-1010.

Canty EG, Kadler KE. 2005. Procollagen trafficking, processing and fibrillogenesis. Journal of cell sciences. 118:1341-1353.

Kadler CM, et al.,. 2007. Collagens at a glance. Journal of cell science. 120:1955-1958.

Kielty CM. 2006. Elastic fibres in health and disease. Expert reviews in molecular medicine. 8:1-23.


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Updated: 2017-09-25. 08:31