Structural Characteristics of Cross‐Linking Sites in type V Collagen of Bone: Chain Specificities and Heterotypic Links to Type I Collagen

To understand the role of type V collagen and its spatial interrelationship with type I collagen in bone matrix, the molecule's covalent intermolecular cross‐links were structurally characterized. Type V collagen containing α1(V), α2(V) and α1(XI) chains was isolated from bovine bone and reacted with NaB³H₄ to label the cross‐linking residues. Radiolabeled native molecules and isolated a chains were treated with sodium metaperiodate to cleave the divalent cross‐linking bonds. Sequence analysis of the periodate‐released peptides matched two of them to α1(V) and α1(XI) aminopropeptide domains. A third peptide was derived from the α1(I) carboxytelopeptide domain of type I collagen. This latter peptide, therefore, came from a site of heterotypic cross‐linking between types I and V collagens and accounted for about 15% of the total cross‐linked peptides. Sequence analysis of isolated cross‐linked tryptic peptides defined the helical sites of attachment of the periodate‐released telopeptides and revealed that the putative aminoproteinase‐cleavage sites in the α1(V) and α1(XI) chains are located in the molecule interior to the cross‐linking residue. These data imply that type V collagen molecules in the extracellular matrix are primarily cross‐linked to each other in a head‐to‐tail linear polymer that is linked laterally to type I collagen molecules in copolymeric fibrils.