| text | Collagens IV, VI, VIII and X form open networks. Type IV networks are irregular. Type VIII and X form hexagonal networks. Type VI collagen forms tetramers which aggregate linearly to form beaded filaments, but also associates laterally through the globular domains so forming a network (Baldock et al. 2003, Knupp et al. 2006, ). Type IV collagen is the predominant collagen type in basement membranes (Parkin et al. 2011). It assembles into three distinct networks with differing combinations of alpha chains, namely alpha1.alpha1.alpha2, alpha3.alpha4.alpha5 and alpha1.alpha2.alpha5.alpha6, (Siebold et al. 1988, Gunwar et al. 1998, Borza et al. 2001), the last of these forms through the association of alpha5.alpha5.alpha6 triple-helical protomers and alpha1.alpha1.alpha2 protomers, interacting tail-to-tail at the retained NC1 domains. Further associations are formed by tetramerization of the 7S domain at the N terminus (Timpl et al. 1981, Siebold et al. 1987). These interactions are the most significant for network formation, but a third interaction occurs whereby type IV collagen dimers interact through lateral association (Yurchenco & Furthmayr 1984, Yurchenco & Ruben 1987, Yurchenko & Patton 2009). Collagen type VI forms tetramers and subsequently several types of higher-order structure (Ball et al. 2001, Beecher et al. 2011) that are probably influenced by the association of other matrix constituents such as hyaluronan (Kielty et al. 1992), fibrillin (Ueda & Yue 2003), biglycan and decorin (Wiberg et al. 2001).
Type VIII collagen forms a hexagonal lattice in Descemet's membrane (Shuttleworth 1997). These are thought to be derived from tetrahedral structures that form when 4 type VIII molecules associate via hydrophobic patches on their C-termini, which then associate via their N-terminals (Stephan et al. 2004). Type X collagen is very similar to type VIII and in vitro forms hexagonal arrays, believed to arise from interactions of the globular domains (Kwan et al. 1991, Jacenko et al. 2001). In vivo type X collagen is found associated with cartilage fibrils in the form of fine filaments (Schmidt & Linsenmayer 1990), which may represent hexagonal lattices that have collapsed during sample preparation (Gordon & Hahn 2010). |
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