Chapter category: Adhesion Molecules
Cellular Signaling by Collagen-Binding Integrins
I Domains in Integrins
Edited by: Donald GullbergISBN: 0-306-47836-6
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Chapter authors:
Jyrki Heino
Collagens are structural proteins of extracellular matrix that typically have triple helical domains of variable length.1 Collagens form, for example, the large fibers of connec tive tissues and networks in basement membranes, while some collagens are transmembrane proteins. Metazoans from sea sponges2 to mammals express collagens and in man, twenty two structurally and functionally different collagen subtypes have been published.1,3,4,5,6 The subtypes have been named from type I to XXVII in the order they have been found. It is notable that the number of collagen genes is larger than the number of subtypes. For example, type I collagen is formed by two a1(I) proteins and one a2(I) protein and in the case of type IV collagen six genes coding for different a-subunits are known. The collagen family has been divided into several subgroups (Table 10.1). The fibril-forming collagens have a long, continuous triple helix that gives the molecule a rigid, rod-like structure. These collagens form large fibrils that give structural integrity and tensil-strength to the tissues. Type I collagen is the most abundant protein in the human body forming extracellular matrix in bone, tendons, dermis etc. Type II collagen has a similar function in cartilage. The network-forming collagens have interruptions in the triple helix. Type IV collagen belongs to this subfamily. It is an important structural component of basement membranes. Two collagen subtypes have unique functions and they alone form their own subgroups: beaded-filaments are formed by type VI collagen and anchoring fibers by type VII collagen. Fibril-associated collagens with interruptions in triple-helices (FACIT) form a large subgroup. Type IX collagen is a typical member of FACIT collagens. In cartilage it binds to surfaces of fibers formed by type II collagen. All FACIT collagens may not, however, be able to bind to fibers, and some of these collagens may have to be reclassified as FACIT-like collagens. Collagen types XIII and XVII were the first collagens shown to be transmembrane proteins. Type XVII is a structural component of hemidesmosomes, whereas type XIII is found, for example, in muscle, bone and skin. Multiplexins are collagens that are associated to basement membranes. Types XV and XVIII belong to this subfamily. The C-terminal cleavage product of type XVIII collagen has become known as angiogenesis-blocking endostatin.7
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