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 ␣1(VI) and ␣2(VI) chains and a part of the carboxyl-terminal domain of the ␣3(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 ␣1(VI) was shown to interact with the carboxylterminal 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.Type VI collagen filaments are ubiquitous. They are present in all connective tissues that contain type I and type III collagen fibers and in cartilage, a tissue that contains predominantly type II collagen. The major functions that have been suggested for type VI collagen filamentous networks are as a substrate for cell attachment and as an anchoring meshwork that connects collagen fibers, nerves, and blood vessels to the surrounding matrix (1, 2). This implies that not only is there an interaction, either direct or indirect, with the type I/III collagen fibers but also that there is an interaction with components of endothelial basement membranes.Matrix components that have been shown to interact with type VI collagen in vitro include proteoglycans, collagens, hyaluronan, heparin, and integrins.Proteoglycans appear to be particularly important, since cell surface-, basement membrane-, and collagen fibril-associated proteoglycans have all been reported to bind to various forms of type VI collagen. Decorin is a small dermatan sulfate proteoglycan that binds to fibrillar collagens (3). It was shown that the leucine-rich module of the core protein bound to type VI and that the binding could be inhibited by the core proteins of fibromodulin and biglycan (4). The cell surface-associated membrane chondroitin sulfate proteoglycan NG2 was originally detected in cells from the rodent central nervous system but subsequently was also detected in blood vessels and cartilaginous structures of the head, neck, and spine. It interacts via its core protein with type VI collagen and is thought to provide machinery for transmembrane signaling (5). Since ␣1␤1 and ␣2␤1 integrins also bind type VI collagen (6, 7), the cell signaling potential of this molecule woul...

Abstract. An mAb was used in conjunction with immunoelectron microscopy to study the ultrastructure and distribution of the type VI collagen network. Type VI collagen in femoral head and costal cartilage was found distributed throughout the matrix but concentrated in areas surrounding chondrocytes. Threedimensional information gained from high voltage stereo pair electron microscopy showed that the type VI collagen network in skin was organized into a highly branched, open, filamentous network that encircled interstitial collagen fibers, but did not appear to interact directly with them. Type VI collagen was also found concentrated near basement membranes of nerves, blood vessels, and fat cells although in a less organized state. Labeling was conspicuously reduced close to the epithelial basement membrane in the region of the anchoring fibrils. No labeling of basement membranes was seen. Based on these observations it is suggested that the type VI collagen forms a flexible network that anchors large interstitial structures such as nerves, blood vessels, and collagen fibers into surrounding connective tissues.T YPE VI collagen forms a filamentous network in most extracellular matrices. The basic structural subunit of the filaments is a tetramer of type VI collagen molecules (for recent review see Timpl and Engel, 1987). The structure of the subunits has been described in some detail from electron microscope studies of rotary-shadowed molecules, initially on pepsin-solubilized and later intact type VI collagen tetramers (Furthmayr et al., 1983; vonder Mark et al., 1984;Engvall et al., 1986). The type VI collagen molecule consists of a short triple helix, '~105 nm in length, which has a very large globular domain at each end. From biosynthetic studies in fibroblasts, it is believed that dimers are first formed by two molecules aligning themselves in an antiparallel fashion so that their helices are staggered and overlap by 75 nm. Tetramers are formed by lateral association of two dimers with their ends in register . These structures are stabilized by disulfide bonds. The end-on-end aggregation of tetramers gives rise to illaments that have been observed in cell culture as beaded filaments, the beads being a structure formed by the interaction of globular domains from two tetramers (Bruns, 1984

Fibrillin is a large (relative molecular mass 350,000) glycoprotein which can be isolated from fibroblast cell cultures and is a component of the microfibrils that are ubiquitous in the connective tissue space. The microfibrils of the suspensory ligament of the lens as well as the elastic fibre microfibrils of the blood vessel wall are composed of fibrillin. The ocular and cardiovascular manifestations of the Marfan syndrome are consistent with a defect in the gene coding for a structural constituent of these connective tissues. Immunohistological experiments have recently implicated fibrillin microfibrils in the pathogenesis of the Marfan syndrome. Genetic linkage data localizing the Marfan gene to chromosome 15 and the in situ hybridization of fibrillin complementary DNA to 15q21.1 together support fibrillin as a candidate Marfan gene. As a first step towards investigating the function of fibrillin in the architecture and development of connective tissues and its relationship to the Marfan syndrome, we report the cloning and partial sequencing of fibrillin cDNA.