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 t issue distribution. The most common type V collagen isoform is alpha1(V)(2) alpha2(V) found in cornea. However, other isoforms exist, including an for [alpha1(V)alpha2(V)alpha3(V)] form, an alpha1(V)(3) homotrimer and hybrid t ype V/XI forms. The functional role and fibrillar organization of these iso forms is not understood. In the cornea, type V collagen has a key role in t he regulation of initial fibril assembly. Type I and type V collagen co-ass emble into heterotypic fibrils, The entire triple-helical domain of the typ e V collagen molecules is buried within the fibril and type I collagen mole cules are present along the fibril surface. The retained NH2-terminal domai ns of the type V collagen are exposed at the surface, extending outward thr ough the gap zones. The molecular model of the NH2-terminal domain indicate s that the short or 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, placin g 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 mol ecules onto fibrils and therefore lateral growth. A critical density would favor the initiation of new fibrils rather than the continued growth of exi sting fibrils, Other type V collagen isoforms are likely to have an importa nt role in non-cornea tissues. This role may be mediated by supramolecular aggregates different from those in the corneal stroma or by an alteration o f the interactions mediated by tissue-specific type V collagen domains gene rated by different isoforms or aggregate structures. Presumably, the aggreg ate structure or specific domains are involved in the regionalization of fi bril-associated macromolecules necessary for the tissue-specific regulation of later fibril growth and matrix assembly stages, (C) 2000 Elsevier Scien ce Ltd. All rights reserved.