Type V collagen: heterotypic type I/V collagen interactions in the regulation of fibril assembly

Type V collagen is a quantitatively minor fibrillar collagen with a broad tissue distribution. The most common type V collagen isoform is α1(V)2 α2(V) found in cornea. However, other isoforms exist, including an [α1(V)α2(V)α3(V)] form, an α1(V)3 homotrimer and hybrid type V/XI forms. The functional role and fibrillar organization of these isoforms is not understood. In the cornea, type V collagen has a key role in the regulation of initial fibril assembly. Type I and type V collagen co-assemble into heterotypic fibrils. The entire triple-helical domain of the type V collagen molecules is buried within the fibril and type I collagen molecules are present along the fibril surface. The retained NH2-terminal domains of the type V collagen are exposed at the surface, extending outward through the gap zones. The molecular model of the NH2-terminal domain indicates that the short α helical region is a flexible hinge-like region allowing the peptide to project away from the major axis of the molecule; the short triple-helical regions serve as an extension through the hole zone, placing the tyrosine-rich domain at the surface. The assembly of early, immature fibril intermediates (segments) is regulated by the NH2-terminal domain of type V collagen. These NH2-terminal domains alter accretion of collagen molecules onto fibrils and therefore lateral growth. A critical density would favor the initiation of new fibrils rather than the continued growth of existing fibrils. Other type V collagen isoforms are likely to have an important role in non-cornea tissues. This role may be mediated by supramolecular aggregates different from those in the corneal stroma or by an alteration of the interactions mediated by tissue-specific type V collagen domains generated by different isoforms or aggregate structures. Presumably, the aggregate structure or specific domains are involved in the regionalization of fibril-associated macromolecules necessary for the tissue-specific regulation of later fibril growth and matrix assembly stages.