11 Extracellular Structural and Secretory Proteins
11.1
11.2
11.2.1
11.2.2
11.3
11.4
11.5
Fibronectins
Collagens
Vertebrate Collagens
Invertebrate Collagens
Matrix Proteins of Calcifyin~ TIssues
Elastic Proteins
Albumen Proteins of Birds and Reptiles
The extracellular matrices ()f animal tissues consist of networks of collagens and elastins, the
meshes of which are filled with structural glycoprotein~ and glycosaminoglycans, The cells themselves are in direct contact with the basement
membranes, which are specific regions of the
extracellular matrix with a thickness of 20200 nm. The very varied compositions of the different extracellular matrices determine their
mechanical characters and, in particular, their
interactions with the cells [73]. The binding of
cells to various matrix components is mediated by
specific receptors on the cell surface and these
recognize certain peptide sequences in the matrix
proteins [141). Several particularly important and
well-known protein types of the extracellular
matrix win be dealt with in Sections 11.1 to 11.4,
and the glycosaminoglycans will be discussed in
Chapter 13. There are very few comparative biochemical .data on the other matrix components.
Next to the collagens, the most predominant cartilage protein is the cartilage matrix protein
(CMP), a homotrimer with subunits of 470 amino
acids, the gene for which has been sequenced in
the chicken [69]. Tendons, skin and cornea contain the collagen-binding fibromodulin, and the
gut wall includes the homologous proteoglycans
PG-Sl and PG-S2 (decorin); the 357- to 375amino-acid polypeptide chains of these latter proteins are mainly built up of 23-residue repeats
[92]. In addition to fibronectin, the other cellbinding proteins of the extracellular matrix are
cytotactin and tenascin, which are mainly pro11.6
11.6.1
11.6.2
11.7
11.8
11.9
11.10
Milk Proteins
Caseins
Whey Proteins
Proteins of the Arthropod Cuticula
Chorion Proteins
Silk Proteins
Insect Secretory Proteins
References
duced during the phases of histo- and organogenesis. In contrast to the dimeric fibronectin molecule, these two proteins form oligomers with
a six~armed structure (hexabrachions). Their
sequences of 1777 and 2203 amino acids, respectively, are highly repetitive; some of the repeats
resemble the type-III repeats of fibronectin, and
the others are similar to the repeats of the epidermal growth factor (EGF) [19, 90]. Endothelial
cells, fibroblasts and smooth-muscle cells produce
the large glycoprotein thrombospondin, which is
able to bind to fibronectin, collagen and heparin
in the extracellular matrix [25].
Typical components of the basement membranes are type-IV collagens, glycosaminoglycans,
laminin, merosin and nidogen [129]. The insolubility of the basement membrane makes for large
problems in the characterization of these proteins; significant progress has only been possible
through analysis of cDNA and gene sequences.
Laminin can be extracted with ethylenediaminetetraacetic acid (EDTA) and consists of three
chains, A, Bl and B2. The N-terminal regions of
the subunits take the form of three short, free
arms; the C-terminal regions form a rod-like coil
on the end of which sits a large globule made up
of the C-terminus of the long A chain. Collagen IV binds to both the short terminal globules
of the short arms and the large C-terminal globule of the A chain. The N-terminus of the
A chain carries a cell-binding site. The binding of
neuronal cells to this site stimulates the outgrowth of neurites. Further cell-binding sites, as
11.1
11.2
11.2.1
11.2.2
11.3
11.4
11.5
Fibronectins
Collagens
Vertebrate Collagens
Invertebrate Collagens
Matrix Proteins of Calcifyin~ TIssues
Elastic Proteins
Albumen Proteins of Birds and Reptiles
The extracellular matrices ()f animal tissues consist of networks of collagens and elastins, the
meshes of which are filled with structural glycoprotein~ and glycosaminoglycans, The cells themselves are in direct contact with the basement
membranes, which are specific regions of the
extracellular matrix with a thickness of 20200 nm. The very varied compositions of the different extracellular matrices determine their
mechanical characters and, in particular, their
interactions with the cells [73]. The binding of
cells to various matrix components is mediated by
specific receptors on the cell surface and these
recognize certain peptide sequences in the matrix
proteins [141). Several particularly important and
well-known protein types of the extracellular
matrix win be dealt with in Sections 11.1 to 11.4,
and the glycosaminoglycans will be discussed in
Chapter 13. There are very few comparative biochemical .data on the other matrix components.
Next to the collagens, the most predominant cartilage protein is the cartilage matrix protein
(CMP), a homotrimer with subunits of 470 amino
acids, the gene for which has been sequenced in
the chicken [69]. Tendons, skin and cornea contain the collagen-binding fibromodulin, and the
gut wall includes the homologous proteoglycans
PG-Sl and PG-S2 (decorin); the 357- to 375amino-acid polypeptide chains of these latter proteins are mainly built up of 23-residue repeats
[92]. In addition to fibronectin, the other cellbinding proteins of the extracellular matrix are
cytotactin and tenascin, which are mainly pro11.6
11.6.1
11.6.2
11.7
11.8
11.9
11.10
Milk Proteins
Caseins
Whey Proteins
Proteins of the Arthropod Cuticula
Chorion Proteins
Silk Proteins
Insect Secretory Proteins
References
duced during the phases of histo- and organogenesis. In contrast to the dimeric fibronectin molecule, these two proteins form oligomers with
a six~armed structure (hexabrachions). Their
sequences of 1777 and 2203 amino acids, respectively, are highly repetitive; some of the repeats
resemble the type-III repeats of fibronectin, and
the others are similar to the repeats of the epidermal growth factor (EGF) [19, 90]. Endothelial
cells, fibroblasts and smooth-muscle cells produce
the large glycoprotein thrombospondin, which is
able to bind to fibronectin, collagen and heparin
in the extracellular matrix [25].
Typical components of the basement membranes are type-IV collagens, glycosaminoglycans,
laminin, merosin and nidogen [129]. The insolubility of the basement membrane makes for large
problems in the characterization of these proteins; significant progress has only been possible
through analysis of cDNA and gene sequences.
Laminin can be extracted with ethylenediaminetetraacetic acid (EDTA) and consists of three
chains, A, Bl and B2. The N-terminal regions of
the subunits take the form of three short, free
arms; the C-terminal regions form a rod-like coil
on the end of which sits a large globule made up
of the C-terminus of the long A chain. Collagen IV binds to both the short terminal globules
of the short arms and the large C-terminal globule of the A chain. The N-terminus of the
A chain carries a cell-binding site. The binding of
neuronal cells to this site stimulates the outgrowth of neurites. Further cell-binding sites, as