328
Cell Junctions, Structure, Function, and Regulation
Tab. 1
Collagens. (Adapted from Prockop, D.J., Kivirikko, K.I. (1995)
Annu. Rev. Biochem.
64
,
403–434.)
Collagens
a
Subunit composition
b
Tissue distribution
Fibril-forming (±brillar)
Type I
[
α
1
(I)
2
α
2
(I)
Most connective tissues including
bone, skin, tendons, ligaments,
cornea, internal organs
Type II
[
α
1
(II)]
3
Cartilage, invertebral disc,
notochord, vitreous humor of
the eye
Type III
[
α
1
(III)]
3
Skin, blood vessels, internal
organs
Type V
[
α
1
(V)]
2
α
2
(V) and
α
1
(V)
α
2
(V)
α
3
(V)
Similar to Type I
Type XI
α
1
(IX)
α
2
(IX)
α
3
(IX)
Similar to Type II
Fibril-associated
Type IX
α
1
(IX)
α
2
(IX)
α
3
(IX)
Cartilage
Type XII
[
α
1
(IV)]2
α
2
(IV)
Tendons, ligaments, some other
tissues
Network forming
Type IV
[
α
1
(IV)]2
α
2
(IV)
Basement membranes
Anchoring ±laments
Type VII
[
α
1
(VII)]3
Beneath strati±ed squamous
epithelia
Transmembrane
Type XVII (BP180)
[
α
1
(XCII)]3
Hemidesmosomes present in
skin
a
Only a subset of the known collagens are presented.
b
Collagen subunits are also referred to as collagen chains.
contain a triple helical domain contain-
ing three collagen subunits referred to as
α
-chains or
α
-subunits. The triple heli-
cal structure is dictated by a stretch of
glycine-X-Y amino acid repeats in each
α
-chain, where X and Y are frequently
proline and 4-hydroxy-proline respectively.
Depending upon the type of collagen,
thet
r
ip
lehe
l
ica
ldoma
incanfo
rmf
rom
three identical or different
α
-chains. Colla-
gens also differ in structure and function.
Fibrillar collagens, which are major com-
ponents of connective tissue, have long
stretches of triple helical domains that pro-
mote the formation of ±brils and larger
±bers. Fibril-associated collagens medi-
ate the interactions of collagen ±brils
with each other and with other matrix
components. Network-forming collagens
form extended lattices that are believed
to have important scaffolding functions in
basement membranes. The triple helical
domains of ±bril-associated and network-
forming collagens are interrupted with
nonhelical domains, making these colla-
gen proteins more flexible. Other collagens
form anchoring ±brils that connect base-
ment membranes of strati±ed epithelia to
the underlying connective tissue.
Mutations
have
been
identi±ed
in
collagen genes that have been linked to
human genetic disorders. Mutations in
genes that encode type I ±brillar collagen
are responsible for the genetic disorder
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