Biglycan organizes collagen VI into hexagonal-like networks resembling tissue structures

The ability of the leucine-rich repeat (LRR) proteins biglycan, decorin, and chondroadherin to interact with collagen VI and influence its assembly to supramolecular structures was studied by electron microscopy and surface plasmon resonance measurements in the BIAcore 2000 system. Biglycan showed a unique ability to organize collagen VI into extensive hexagonal-like networks over a time period of only a few minutes. Only the intact molecule, substituted with two dermatan sulfate chains, had this capacity. Intact decorin, with one dermatan sulfate chain only, was considerably less efficient, and aggregates of organized collagen VI were found only after several hours. Chondroadherin without glycosaminoglycan substitutions did not induce any ordered collagen VI organization. However, all three related LRR proteins were shown to interact with collagen VI using electron microscopy and surface plasmon resonance. Biglycan and decorin were exclusively found close to the N-terminal parts of the collagen VI tetramers, whereas chondroadherin was shown to bind close to both the N- and C-terminal parts of collagen VI. In the formed hexagonal networks, biglycan was localized to the intra-network junctions of the collagen VI filaments. This was demonstrated by electron microscopy after negative staining of gold-labeled biglycan in aggregation experiments with collagen VI.