Collagen VI - Molecular Assembly and Interactions

The extracellular matrix plays an integral role in tissue remodeling and development and it defines the different mechanical properties of a variety of connective tissues. Its complexity in composition and the details in organization has only been partly elucidated. This thesis focuses on collagen VI and the mutual interactions of a number of matrix proteins that lie within close proximity. Collagen VI is unique among the collagens in its molecular assembly. It forms filamentous networks distinct from those of the fibrillar collagens. During this assembly, the role of the individual domains of the collagen VI molecule in the initial intracellular assembly and subsequent extracellular organization was studied. The N5 domain of the N-terminal globular domain of collagen VI α3-chain was shown to have an essential role. We investigated how two small proteoglycans from the leucine-rich repeat (LRR) protein family, biglycan and decorin, interact with collagen VI and how they effect the supermolecular organization of collagen VI. Biglycan and decorin were shown to interact with collagen VI within the region spanning the N-terminal end of the triple helical domain to the first part of the globular domain. Binding was mediated by the core protein and isolated glycosaminoglycan chains were not able to prevent this interaction. However, biglycan showed a unique ability, not only to organize collagen VI into hexagonal networks but also to catalyze this event, where the two glycosaminoglycan chains were shown to play a crucial role. By extracting collagen VI microfibrils from chondrosarcoma tissue under mild conditions, we showed that collagen VI was attached to the major cartilage molecules, collagen II and aggrecan, via a bridge of the small LRR proteoglycans and matrilins. This bridge constituted collagen VI bound to biglycan or decorin, which in turn bound to matrilin-1, -2 or –3 associated with collagen II or aggrecan