384
11 Extracellular Structural and Secretory Proteins
the composition (al)3 and a high content of glycosylated hydroxylysine (19-23 residues per 1000)
[81, 144].
The collagens of the Porifera are known as
spongins. A distinction is drawn between fibrils
with a diameter of 20 nm and clear cross-stripes
with an interval of 63-65 nm (spongin A) and
thinner fibrils with no distinct stripes (spongin B). The spongins contain one-third glycine
and large amounts of hydroxyproline, hydroxylysine and cysteine [Table 11.1 (2a/b)]. Partial
cDNA and gene sequences show that the sponges
have both fibrillar collagens, with almost uninterrupted triple-helix sequences, and short-chain
collagens, with shorter triple helices and terminal, non-helical domains. The exons encoding the
triple helices almost exclusively have lengths of
54 bp or mUltiples thereof [31].
The mesogloea of the Cnidaria contains striped
collagen fibrils with a diameter of 22-66 nm;
these are made up from two or three different collagen chains and are rich in hydroxylysine-bound
carbohydrate [82, 150]. The extracellular matrix
of the Platyhelminthes also contains disulphidelinked, highly glycosylated 500-kDa collagens,
i.e. about twice the size of interstitial vertebrate
collagens and therefore possibly dimers [150].
The fibrils of the extracellular matrix of the Mollusca contain collagens with one or two chain
types. The head cartilage of the cephalopod Todarodes pacificus is histologically similar to vertebrate cartilage, and contains a collagen which
resembles vertebrate type II in its high content of
glycosylated hydroxylysine, but otherwise has the
amino acid composition of vertebrate collagen
type I [66]. The byssus threads of the edible mussel Mytilus edulis are composed of a type-l-Iike
collagen with two different subunits, but also contain a tyrosine-rich non-collagen protein. Collagens of type I, III and IV are immunologically
detectable in the Echinodermata. The sea urchin
Paracentrotus lividus possesses tissue-specific collagen fibrils with a much smaller periodicity than
vertebrate collagens I-III: 55 nm in the foot,
47 nm in the epidermis, and 44 nm in the peristome. A partial eDNA sequence from this sea
urchin encodes an uninterrupted triple helix of
480 amino acids and a C-terminal pro-peptide of
252 amino acids. The gene organization is apparently similar to that of the fibrillar vertebrate collagens [14, 24]. The collagens of the Tunicata and
the Acrania differ from vertebrate interstitial collagens by having a high content of glycosylated
hydroxylysines; they therefore resemble more
closely the basement-membrane collagens [150].
The cuticular collagens of the Annelida Lumbricus, Pheretima, Nereis and Neanthes are rather
unusual. The molecules are 950 nm long with a
mass of 1700-1900 kDa and are thus several
times larger than the classical vertebrate collagens. The cuticular collagen of Nereis is made up
of two different subunits of about 470 kDa with
6000 amino acids, i.e. five to six times larger than
the subunits of vertebrate collagens, and is one of
the largest known polypeptides. The cuticular collagens are not striped, in contrast to the collagens
of the internal organs of annelids, which in their
molecular size and periodicity are very similar to
the fibril-forming collagens of the vertebrates.
Furthermore, annelid cuticular collagens are unique in that 4-hydroxyproline is found not only in
position Y but also in position X of the sequence
Gly-X-Y. Thus the 4-hydroxyproline content is
particularly high at 14-16.5 % ; 3-hydroxyproline
is also present [Table 11.1 (4)]. The prolylhydroxylase of the earthworm is very similar to the vertebrate enzyme but, in contrast, is also able to
hydroxylate the synthetic substrate (Gly-ProAla)n. The cuticular collagens contain 5-15 % carbohydrate, mainly galactose; however, this is
bound not to hydroxylysine but to serine and
threonine [150].
The collagen from the cuticle of the nematode
Ascaris lumbricoides, with a length of 410 nm and
a mass of 900 kDa, is also significantly larger than
vertebrate fibrillar collagens. However, reduction
with, for example, mercaptoethanol increases the
solubility and releases three different chains of
52 kDa. The native collagen is thus cross-linked
by disulphide bridges and also by bis- and isotertyrosine residues (Fig. 11.4); cross-linking structures based on allysine or hydroxyallysine are not
detectable. The total proline content of cuticular
collagens is very high (22 % in Caenorhabditis,
23 % in Panagrellus and 34 % in Ascaris) but the
proportions of 4-hydroxyproline differ (1 in 2 prolines in the case of Caenorhabditis, 1 in 7 in
Panagrellus, and 1 in 20 in Ascaris). 3Hydroxyproline is not present and, as in the
annelid cuticle, hydroxylysine is also absent [Table 11.1 (3 a)]; the carbohydrate content is low.
Lysylhydroxylase shows no activity with the cuticular collagen of Ascaris, although this contains
45 lysines per chain. The collagen fibrils of the
connective tissues of internal organs are not
striped but have the typical X-ray pattern; they
are also much more similar to the vertebrate collagens in their amino acid composition than are
the cuticular collagens [Table 11.1 (3b)]. The
basement membrane of the gut epithelium con-
11 Extracellular Structural and Secretory Proteins
the composition (al)3 and a high content of glycosylated hydroxylysine (19-23 residues per 1000)
[81, 144].
The collagens of the Porifera are known as
spongins. A distinction is drawn between fibrils
with a diameter of 20 nm and clear cross-stripes
with an interval of 63-65 nm (spongin A) and
thinner fibrils with no distinct stripes (spongin B). The spongins contain one-third glycine
and large amounts of hydroxyproline, hydroxylysine and cysteine [Table 11.1 (2a/b)]. Partial
cDNA and gene sequences show that the sponges
have both fibrillar collagens, with almost uninterrupted triple-helix sequences, and short-chain
collagens, with shorter triple helices and terminal, non-helical domains. The exons encoding the
triple helices almost exclusively have lengths of
54 bp or mUltiples thereof [31].
The mesogloea of the Cnidaria contains striped
collagen fibrils with a diameter of 22-66 nm;
these are made up from two or three different collagen chains and are rich in hydroxylysine-bound
carbohydrate [82, 150]. The extracellular matrix
of the Platyhelminthes also contains disulphidelinked, highly glycosylated 500-kDa collagens,
i.e. about twice the size of interstitial vertebrate
collagens and therefore possibly dimers [150].
The fibrils of the extracellular matrix of the Mollusca contain collagens with one or two chain
types. The head cartilage of the cephalopod Todarodes pacificus is histologically similar to vertebrate cartilage, and contains a collagen which
resembles vertebrate type II in its high content of
glycosylated hydroxylysine, but otherwise has the
amino acid composition of vertebrate collagen
type I [66]. The byssus threads of the edible mussel Mytilus edulis are composed of a type-l-Iike
collagen with two different subunits, but also contain a tyrosine-rich non-collagen protein. Collagens of type I, III and IV are immunologically
detectable in the Echinodermata. The sea urchin
Paracentrotus lividus possesses tissue-specific collagen fibrils with a much smaller periodicity than
vertebrate collagens I-III: 55 nm in the foot,
47 nm in the epidermis, and 44 nm in the peristome. A partial eDNA sequence from this sea
urchin encodes an uninterrupted triple helix of
480 amino acids and a C-terminal pro-peptide of
252 amino acids. The gene organization is apparently similar to that of the fibrillar vertebrate collagens [14, 24]. The collagens of the Tunicata and
the Acrania differ from vertebrate interstitial collagens by having a high content of glycosylated
hydroxylysines; they therefore resemble more
closely the basement-membrane collagens [150].
The cuticular collagens of the Annelida Lumbricus, Pheretima, Nereis and Neanthes are rather
unusual. The molecules are 950 nm long with a
mass of 1700-1900 kDa and are thus several
times larger than the classical vertebrate collagens. The cuticular collagen of Nereis is made up
of two different subunits of about 470 kDa with
6000 amino acids, i.e. five to six times larger than
the subunits of vertebrate collagens, and is one of
the largest known polypeptides. The cuticular collagens are not striped, in contrast to the collagens
of the internal organs of annelids, which in their
molecular size and periodicity are very similar to
the fibril-forming collagens of the vertebrates.
Furthermore, annelid cuticular collagens are unique in that 4-hydroxyproline is found not only in
position Y but also in position X of the sequence
Gly-X-Y. Thus the 4-hydroxyproline content is
particularly high at 14-16.5 % ; 3-hydroxyproline
is also present [Table 11.1 (4)]. The prolylhydroxylase of the earthworm is very similar to the vertebrate enzyme but, in contrast, is also able to
hydroxylate the synthetic substrate (Gly-ProAla)n. The cuticular collagens contain 5-15 % carbohydrate, mainly galactose; however, this is
bound not to hydroxylysine but to serine and
threonine [150].
The collagen from the cuticle of the nematode
Ascaris lumbricoides, with a length of 410 nm and
a mass of 900 kDa, is also significantly larger than
vertebrate fibrillar collagens. However, reduction
with, for example, mercaptoethanol increases the
solubility and releases three different chains of
52 kDa. The native collagen is thus cross-linked
by disulphide bridges and also by bis- and isotertyrosine residues (Fig. 11.4); cross-linking structures based on allysine or hydroxyallysine are not
detectable. The total proline content of cuticular
collagens is very high (22 % in Caenorhabditis,
23 % in Panagrellus and 34 % in Ascaris) but the
proportions of 4-hydroxyproline differ (1 in 2 prolines in the case of Caenorhabditis, 1 in 7 in
Panagrellus, and 1 in 20 in Ascaris). 3Hydroxyproline is not present and, as in the
annelid cuticle, hydroxylysine is also absent [Table 11.1 (3 a)]; the carbohydrate content is low.
Lysylhydroxylase shows no activity with the cuticular collagen of Ascaris, although this contains
45 lysines per chain. The collagen fibrils of the
connective tissues of internal organs are not
striped but have the typical X-ray pattern; they
are also much more similar to the vertebrate collagens in their amino acid composition than are
the cuticular collagens [Table 11.1 (3b)]. The
basement membrane of the gut epithelium con-