Type VI collagen anchors endothelial basement membranes by interacting with type IV collagen

Type VI collagen filaments are found associated with interstitial collagen fibers, around cells, and in contact with endothelial basement membranes. To identify type VI collagen binding proteins, the amino-terminal domains of the α1(VI) and α2(VI) chains and a part of the carboxyl-terminal domain of the α3(VI) chain were used as bait in a yeast two-hybrid system to screen a human placenta library. Eight persistently positive clones were identified, two coding the known matrix proteins fibronectin and basement membrane type IV collagen and the rest coding new proteins. The amino- terminal domain of α1(VI) was shown to interact with the carboxyl-terminal globular domain of type IV collagen. The specificity of this interaction was further studied using the yeast two-hybrid system in a one-on-one format and confirmed by using isolated protein domains in immunoprecipitation, affinity blots, and enzyme-linked immunosorbent assay-based binding studies. Co- distribution of type VI and type IV collagens in human muscle was demonstrated using double labeling immunofluorescent microscopy and immunoelectron microscopy. The strong interaction of type VI collagen filaments with basement membrane collagen provided a possible molecular pathogenesis for the heritable disorder Bethlem myopathy.